NEUTRONS & CATALYSIS - FAS

NEUTRONS & CATALYSISby Juergen Eckert and Phillip J. Vergamini

c atalysis—the ability of some ture of chemicals, could not functionsubstances to alter the rate of without catalysts. It has been estimated,chemical reactions without for example, that catalysts are involvedbeing consumed-was rec- at some point in the production of 60 to

ognized more than 150 years ago and 70 percent of all industrial chemicals.has been applied on an industrial scale Yet the store of knowledge about howsince the beginning of this century. catalysts work is surprisingly small. TheModem industrial chemistry, especially search for a catalyst for a particular re-petroleum processing and the manufac- action, or for ways to improve existing

catalysts, is still. as it always has been,largely empirical.

In the last few years, however, so-phisticated new analytical and compu-tational techniques have helped stimu-late a renaissance in catalysis research.Powerful economic forces have mo-tivated the study of catalysis as well:the need for new sources of energy and

Neutrons and Catalysis

chemicals, changes in the availabilityof’ raw materials, potential restrictionson the availability of noble-metal cat-alysts, and the desire for new productshave pointed up the need for a clearerunderstanding of catalytic processes.

Much of the research into cataly-sis is directed toward metals. becausethey catalyze many important reac-tions. Metals may be catalytically ac-tive in the form of finely divided parti-cles, organometallic compounds in so-lution, or ions bound to large biologi-cally active molecules, such as enzymes.The catalysis may be heterogeneous inthe sense of involving more than onephase (solid metal and gaseous reac-tants, for example) or it may be homo-geneous in the sense of involving onlyone phase (such as a solution). What-ever the form, when the metal binds toa reactant molecule, it almost always

alters the chemical bonding in the re-actant, If that alteration is favorable tosome particular reaction, then the metalis a catalyst for that reaction.

To understand catalytic activity, orto tailor a catalyst to do a specific job,we need to know the individual steps inthe catalytic process in great detail, Forexample, consider the hydrogenation ofethylene to ethane,

which can serve as a prototype of re-actions used in producing synthetic fu-els. The production of synthetic fuelfrom coal, for example, involves variousseries of reactions, including the step-wise hydrogenation of carbon to formacetylene (HCCH), ethylene, and ethane,as well as the stepwise hydrogenationof carbon chains with more than twocarbon atoms. The hydrogenation ofethylene shown above is a particularlyuseful react ion to study because it canbe carried out at moderate temperaturesin the presence of a metal catalyst. Thevarious steps to the reaction are repre-

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Unfortunately, these details of struc-ture and dynamics cannot easily be de-termined in a “real-world” situation—that is, during an actual catalytic reac-tion. Catalytic processes usually pro-ceed under conditions that preclude thedirect application of many powerful an-alytical techniques-or at least makesuch application very difficult. Consid-erable effort has therefore been devotedto the study of so-called model systems,which are designed to reproduce thecritical relationships as accurately aspossible. One useful model system isa single crystal of a metal for whichthe surface arrangement of atoms isknown. Others that have been widelyused are synthetic molecules consistingof a metal atom (or a cluster of metalatoms) surrounded by stabilizing lig-ands, usually carbonyl groups (CO) orother more complex organic groups.When a reactant molecule such as ethy-lene or benzene binds to such a syn-thetic molecule, we can assume that, tosome degree, the configuration of theresulting complex resembles that of thesame reactant adsorbed on a metal sur-face. The complex can be studied withseveral spectroscopic techniques, and itscrystalline form can be characterized byx-ray’ and neutron diffraction, which re-veal details of its architecture with greataccuracy.

The more closely the properties of themodel system approximate the proper-ties of the real-world system, of course,the better. As a result, model systemsare often structurally modified to refinetheir properties and bring them closer inline with the more complex system ofinterest. However. such modificationscan complicate the structural character-ization of the model system. For exam-ple, as the model system becomes largerand more complex. the chances increasethat some portions of the molecule willbe disordered or less easily defined. Thenecessity of modeling the disorder candecrease the precision of the results for

Los Alamos Science Summer 1990


the metal-hydrogen interaction, whichis the feature of most interest. In effect,the results become slightly fuzzy andless precise.

Besides being useful in the study ofcatalysis, metal complexes are highlysuitable for theoretical studies of chemi-cal bonding between the bound molecule(ligand) and the metal atoms. Theyare therefore of fundamental interestto researchers studying chemical bondsfrom first principles. Finally, metal-cluster complexes can stabilize cer-tain molecules that are unstable in pureform. For example, cyclobutadiene canbe stabilized by binding to iron car-bonyl, Fe(CO)3; and ethylidyne, CH3-C(a highly reactive intermediate formedin the hydrogenation of ethylene), canbe isolated by reacting with cobalt car-bonyl to form the metal-cluster complexCH3C–C03(CO)9.

The kind of information availablethrough the study of model compoundsis illustrated by the case of the clus-ter compound HFeCo3(CO) 12. Diffrac-tion studies show that the single hydro-gen atom is located at a site of three-fold symmetry, that is, just outsidethe triangle formed by the three cobaltatoms (Fig. 2). The vibrational spec-trum of hydrogen in this compound isvery similar to that of hydrogen atomschemisorbed on a nickel or a platinumsurface. Since the vibrational spectrumof a molecule or atom strongly reflectsthe way in which it is bound to otheratoms, the similarity here allows theinference that hydrogen chemisorbedon a catalyst surface is located at a siteof threefold symmetry. We can furtherinfer that the catalytically active sur-face is the so-called (111) plane of themetal, because that is the only crystalplane having threefold symmetry. Thisinformation could not have been easilyobtained in any direct way.

How does one then study the modelsystems? There are many experimen-tal techniques, each especially suited

for a particular aspect of the problem,and neutron scattering is one of these.However, even the most intense neu-tron sources produce fluxes far belowthose commonly available from sourcesof photons (x rays, ultraviolet, visiblelight, and infrared), and so neutronscattering is not one of the principaltools of surface science. Nevertheless,when the systems include hydrogen

A CLUSTER COMPOUND

Fig. 2. The HFeCo3(CO)12 complex, whichcontains a single hydrogen atom (blue)located against an equilateral triangle ofcobalt atoms (red), can serve as a modelsystem for hydrogen atoms chemisorbed ona metal surface. In particular, comparison ofvibrational spectra can help establish whetheror not the hydrogen on the metal surface isalso located at sites with threefold symmetry.(Adapted from a figure in an article by R. G.Teller, R. D. Wilson, R. K. McMullan, T. F.Koetzle, and R. Bau. Journal of the AmericanChemical Society 100: 3071, 1978.)

or molecules containing hydrogen—asdo the more important types of com-pounds involved in industrial catalyticprocesses—neutron scattering is ex-tremely useful.

The singular utility of neutron scatter-

ing is in locating the all-important hy-drogen atoms and highlighting the vibra-tional and rotational motions associatedwith them. This strength is a result ofthe fact that neutrons scatter as stronglyfrom hydrogen as from most other el-ements (see “Neutron Scattering—APrimer” by Roger Pynn). Although itis nearly impossible to “see” hydrogenatoms in the presence of heavy met-als using x rays, x-ray diffraction cansometimes implicitly locate hydrogenatoms bound to or interacting with metalatoms. If a site in a metal complex isusually filled, an apparent vacancy atthat site, together with other physicaland chemical evidence, can lead to theinference that hydrogen occupies the po-sition. Neutron scattering, however, isneeded to confirm the actual presence ofhydrogen. Thus, the structures of com-pounds of interest are typically deter-mined by first applying x-ray diffrac-tion to locate the heavier atoms andthen neutron diffraction to obtain pre-cise metal-hydrogen distances and bondangles.

Historically, single-crystal neutrondiffraction has been more difficult thanx-ray diffraction. Neutrons can travellarge distances through material withoutbeing scattered, so neutron diffractionrequires a much larger crystal. Thisproblem has been partly alleviated bythe availability of more intense sourcesof neutrons. Furthermore, the time-of-flight wavelength measurements possi-ble at pulsed-accelerator-based neutronsources makes all neutrons in each pulseusable. Area detectors make it possi-ble to collect large volumes of data atone time and make feasible full struc-tural determination from polycrystallinematerial.

For the observation of molecular vi-brations, optical techniques (infraredabsorption and Raman scattering) arefar more common and much easier touse than neutron scattering. Once again,however. the difference in the nature of

Los Alamos Science Summer 1990 117


the interaction between the scatterer andthe probe makes neutron-scattering vi-brational spectroscopy advantageous incertain cases. First, absorption of pho-tons in optical spectroscopy depends onthe symmetry properties of the vibra-tional mode being excited in the sample,whereas no such symmetry-based se-lection rules apply to inelastic neutronscattering. (We use inelastic to refer tothe fact that the neutron loses or gainsenergy during the scattering process.The change in energy corresponds to achange in the vibrational energy of thescattering molecule. ) Hence, in principle(though not necessarily in fact), all vi-brations of a molecule can be observedby inelastic neutron scattering.

The factors determining the intensityof a given excitation are a second, andperhaps more important, difference be-tween neutron-scattering and optical vi-brational spectroscopy. Large-amplitudevibrations by nuclei with high neutron-scattering cross sections (such as hydro-gen) typically give rise to intense inelas-tic neutron-scattering bands; whole-bodyVibrations of molecules are typical ex-amples. Such motions, however, usuallyinvolve only small changes (if any) inthe polarizability or the dipole momentof the molecule, which are the factorsthat govern intensities in Raman scat-tering and infrared absorption. Thus,optical and neutron-scattering methodsare remarkably complementary.

The utility of inelastic neutron scatter-ing can be greatly enhanced by replac-ing certain atoms, whose vibrations areto be highlighted, with isotopes of dif-ferent neutron-scattering cross sections.Such isotopic substitution is particularlyvaluable for studying hydrogen, becausethe neutron-scattering cross sections ap-propriate to inelastic neutron scatteringfor hydrogen and deuterium differ bymore than an order of magnitude. Forexample, to distinguish the motions ofthe methyl group in toluene (C6H5CH3,a benzene derivative in which one of

the ring hydrogens is replaced by amethyl group), the remaining ring hy-drogens can be replaced by deuteriumatoms. Then, as far as neutron scatter-ing is concerned, the deuterium atomsare much less “visible” than the threehydrogen atoms on the methyl group.(In optical spectroscopy, isotopic substi-tution alters the frequency of vibrationbut leaves the intensity of absorbed orscattered photons virtually unchanged.)Another application of isotopic substi-tution in neutron scattering involves thedifferential spectra of isotopic species,examples of which will be described inthe following sections.

We have chosen to describe three ex-amples of neutron-scattering studies onmetal complexes, each of which mayserve as a model system for a particu-lar step in the hydrogenation reactionshown in Fig. 1. The first example isa hydride ligand in an octahedral clus-ter of metal atoms, a model system thatmay help us understand the motion ofhydrogen atoms between the surface andthe region just below the surface (oncethe H2 molecule has dissociated on thesurface). The other two examples—anethylene-diosmium complex and a setof molecular-hydrogen complexes—may be regarded as models for the bondactivation that precedes the actual re-action on the surface. The complexesthat bind molecular hydrogen are par-ticularly important in this context be-cause they represent a “capture” of thelong-sought intermediate in perhaps themost fundamental reaction, the disso-ciation of hydrogen molecules. As weshall show below, elastic and inelas-tic neutron-scattering studies of thesecompounds have provided remarkablydetailed information on the nature ofthe chemical bond between the dihydro-gen, or molecular hydrogen, ligand andthe metal center, including evidence forback-donation of electron density fromthe metal to the antibonding orbital ofthe hydrogen molecule.

The Hydride Ligand

The first example we want to dis-cuss in detail is the interstitial hydrideligand—a single hydrogen atom boundto a metal atom or atoms. Hydride lig-ands are usually formed on metal sur-faces when molecular hydrogen disso-ciates and are referred to as terminal,doubly bridging, triply bridging, andso forth. depending on whether theyare bonded to one, two, three, or moremetal atoms.

In large cluster complexes with manymetal atoms, hydride ligands may alsooccupy interior, or interstitial, sites.Among the large metal-cluster com-pounds of this type that have been syn-thesized, two—the octahedral clustersof cobalt and ruthenium—stand out fortheir remarkable simplicity. Both thesecompounds have a cluster of six metalatoms that form the octahedral site, asingle hydride ligand, and several car-bonyl groups outside the metal clusterthat serve to stabilize the molecule.

This kind of hydride coordinationlooks very much like that observed inbulk metals, where interstitial sites ofoctahedral or tetrahedral symmetry maybe occupied by hydrogen. A hydrogenatom in a metal is of course surroundedby many more metal atoms than thesix of a cluster compound. There aresix atoms as nearest neighbors in anoctahedral site, but further shells ofmetal atoms occur at ever increasingdistances. However, if the hydrogenatom is located just below the surfaceof the metal, that is, between the firsttwo layers of metal atoms, the numberof more distant neighbors is minimizedin one direction. The interstitial hydro-gen in the cluster may therefore be abetter model for “subsurface” hydro-gen than for hydrogen within the bulkof the metal. Such a system may helpanswer a question raised earlier in thediscussion of hydrogenation-that is,where the hydrogen is likely to move,

118 Los Alamos Science Summer 1990


(a) (b) The Central Octahedron

Site

co

AN OCTAHEDRAL CLUSTER

Fig. 3. (a) The location of the hydride ligand in the anion [HC06(CO)15]- has been determined with considerable certainty by neutron diffraction

from a single crystal containing the cation [(Ph3P)2N]+. The hydride ligand (blue) is located at the center of an octahedron of cobalt atoms (red);the cobalt atoms, in turn, are surrounded by twelve carbonyl ligands (green and yellow). The shapes at each atomic position are thermal ellipsoids,which indicate the extent and direction of the thermal motion of the atoms about their equilibrium positions. (The surface of each ellipsoid definesthe volume in which the atom is contained 50 percent of the time.) (b) There are a number of alternative sites close to the central octahedron ofcobalt atoms that may, in some compounds, serve as the location of the hydride ligand (two possibilities are shown in light blue). The alternativesites are either external or internal to the octahedron of cobalt atoms, and the hydrogen atom can be doubly or triply bridged to those atoms.However, such sites have considerably less symmetry than the central octahedral site, and the vibrational spectra of hydrogen when located atsuch a site would be quite different from the single excitation that is actually observed in the spectrum of [HC06(CO)15] . (Adapted from a figurein an article by D. W. Hart, R. G. Teller, C.-Y. Wei, R. Bau, G. Longoni, S. Campanella, P, Chini, and T. F, Koetzle, Journal of the American Chemica/Society 103: 1458, 1981.)

after dissociation, relative to the metalcatalyst’s surface.

The first step, however, in under-standing the microscopic properties ofhydrogen in a metal cluster is to attemptto locate it by diffraction studies. Theonly reliable way to do this is by useof neutron beams, for the reasons dis-cussed earlier. Because hydrogen is aminor component of the rather largemetal-cluster molecules, single crystals

must be used for the diffraction stud-ies. Also, cluster compounds are mostcommonly ionic species, and a suit-able counter ion -a large, oppositelycharged ion—must be added to pro-duce sufficiently large single crystals. Inthe present case, the complex of inter-est is [HCO6(CO) 15]-, and the counterion used to produce the single crystal is[(Ph 3P)2N]+, in which Ph is the phenylgroup, C6H5.

A neutron-diffraction study of a sin-gle crystal of [(Ph3P)2N ][HCo6(CO)15],,carried out on the high-flux-beam reac-tor at Brookhaven National Laboratory,showed quite convincingly that the hy-drogen is located approximately at thecenter of a somewhat irregular octa-hedron of cobalt atoms (Fig. 3) eventhough other locations. such as three-fold coordination inside or outside oneof the triangular faces of the octahedron,

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are possible. The vibrational spectrumof hydrogen in a regular octahedral sitewould show a single excitation, a triplydegenerate hydrogen-metal stretchingmode. If, however, the hydrogen wereto also move significantly off center,additional peaks would appear in thevibrational spectrum. It is in just suchcases that vibrational spectra are ofgreat value in obtaining structural infor-mation. An inelastic neutron-scatteringstudy of the cesium salt of the samecobalt cluster showed primarily a singleexcitation at a frequency of 1056 recip-rocal centimeters (cm– ]), confirming thecentral location of the hydride ligand inthe octahedral site.

Prompted by the results of some in-frared spectroscopic studies that showedinteresting changes in the spectra of the[H C 06(C O)15]

- cluster as the crystallineenvironment was altered, we recentlyinvestigated the vibrational spectrumof the cluster combined with the verymuch smaller counter ion K+. The data,shown in Fig. 4, were obtained by thedifferential technique on the Filter Dif-ference Spectrometer at the ManuelLujan, Jr. Neutron Scattering Center(LANSCE) at Los Alamos. Two sam-ples were measured, one with hydrogenand one with deuterium as the ligand.Because the scattering cross section fordeuterium is much smaller than that forhydrogen, the vibrational spectrum ofthe deuterated compound serves essen-tially as a “blank” to be subtracted fromthat of the protonated compound. Theresulting differential spectrum is free ofall the many vibrational modes of thislarge molecule that do not involve mo-tion of the hydrogen and thus highlightsthe vibrational modes that do involvehydrogen.

The features shown in the differentialspectrum can be immediately identifiedwith hydrogen vibrations and suggestthat all the hydrogen atoms are by nomeans located at the center of the oc-tahedron of cobalt atoms. The broad

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cm -1 is not present in the spectra of crystalscontaining Cs+ as a counter ion and may bethe stretching vibration of doubly or triplybridged hydrogen located at an alternativesite (such as those shown in light blue inFig. 3b). If the latter assignment is correct,the high-frequency shoulder just above 1100cm

-1 would correspond to the asymmetricstretching vibration of hydrogen at a triplybridged site. These data were obtained byusing the Filter Difference Spectrometer atLANSCE.

band in the region between 1050 and1100 cm-1 may certainly be assignedto the stretching vibration of hydrogenat the interstitial site, but the band at950 cm–] must then be indicative ofhydrogen at a different site—one bridg-ing either two or three cobalt atoms. In

either case, a second vibrational line athigher frequency would be expected.The data are not conclusive in this re-spect, but if the band at 950 cm-1 is thesymmetric stretching vibration for thedoubly or triply bridged hydrogen, thenthe high-frequency shoulder just above1100 cm–1 has about the expected fre-quency for the asymmetric stretch oftriply bridged hydrogen.

The spectrum thus appears to revealan instance of the fluxionality of thehydride ligands in cluster compounds.Fluxionality-commonly detected innuclear-magnetic-resonance studies—refers to the movement of hydrogenfrom one site to another. Because themovement occurs on a time scale that ismany orders of magnitude greater thanthe time scale of a typical vibration,the hydrogen can be “caught” vibratingrapidly at more than one site. However,if the binding energy is much larger atone site than at others, such fluxionalityis unlikely.

In any case, the remarkable result ofour studies is that the position of thehydride ligand in these metal clustersapparently depends on the nature of thecounter ion used to crystallize the com-pound. This fact suggests that the bind-ing strengths for hydrogen at the vari-ous sites differ by only small amountsand may, in fact, be affected by thecharge balance between the complexion and its counter ion. Such a conjec-ture is needed to explain the observedchange in fluxionality of the hydrogenatom in the cluster. Moreover, the con-jecture is in agreement with nuclear-magnetic-resonance observations of the[HC06(CO)15]

- ion in solution, whichshow that the hydrogen can easily leavethe octahedron and exchange with pro-tons of the solvent molecules.

The factors that govern fluxionality ofthe hydride ligand in cluster compoundsmay, of course, differ considerably fromthose that determine the diffusion of hy-drogen between the metal surface and,



(a) Ethylene for example, the subsurface layer. Ifone wishes to hydrogenate, say, ethy-lene on a metal surface. it is important

(b) Ethane

dissociation to react with the ethylenerather than diffuse rapidly into the bulk

portant for understanding the diffusionof hydrogen along a metal surface orbetween the surface and the bulk.

The Ethylene-Metal Complex

GEOMETRY OF ETHANE AND ETHYLENE

Fig. 5. (a) All the atoms of ethylene lie in a single plane, and almost all of its electron orbitalsare concentrated close to that plane. The exception is the double bond between the two carbon

other hand, are arranged in two overlapping tetrahedral groups that surround each carbon atom

arrangement allows the two CH3 groups to rotate with respect to one another.


We now focus on another questionin the hydrogenation reaction, namelythe formation of a complex betweenethylene and the metal and the result-ing bond activation that is necessary forethylene to take up hydrogen. Hydro-genation changes the ethylene moleculefrom an arrangement in which all theatoms are coplanar and the carbon-carbon bond is a short double bond thatincludes n-bonding electrons (Fig. 5)to an ethane molecule in which theatoms are grouped tetrahedrally and thecarbon-carbon bond is the longer sin-

Here again, we shall examine a modelcompound in which the precise. arrange-ment of the ethylene molecule and themetal atoms can be studied by neutrondiffraction.

Ethylene molecules can interact witha metal surface in several ways. Per-haps the most common configuration is

in Fig. 6a, in which the planar ethy-lene molecule is parallel to the surface

carbon-carbon double bond interact di-rectly with the electrons on a single

bonded complex (Fig. 6b), has also beenobserved: in this case the double bond

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between the carbon atoms is reduced to

formed between each carbon atom andone of two adjacent metal atoms. Ob-viously, this last complex could be anintermediate species in the hydrogena-tion reaction,

The OS2(CO)8(C2H4) complex (Fig. 7)does in fact have an ethylene ligand

try. Further, the vibrational spectrumof this complex is very similar to thatof ethylene chemisorbed on platinumat temperatures below 100 kelvins, andthe complex can serve as a model forthat system. X-ray diffraction studiesof OS2(CO) 8(C2H4) show both that thecarbon-carbon distance is longer than anormal ethylene double bond and thatthe four-membered ring formed by thetwo osmiums and two carbons is twistedand nonplanar. This last observationimplies that the hydrogens have proba-bly also twisted out of their plane withthe carbons. However, as we alreadypointed out, it is very difficult to di-rectly determine the positions of thehydrogen atoms with x rays. Spectro-scopic evidence also suggests unusualstructural features within the bridgingethylene ligand. This evidence may ormay not be consistent with the x-rayobservations but cannot be interpretedin an unambiguous way. Therefore, aknowledge of the detailed structure, par-ticularly the positions of the hydrogenatoms, is necessary to resolve questionsregarding the bonding in this compound.

Neutron-scattering measurementsshow that two hydrogens, an osmo-nium atom, and the other carbon atomare arranged approximately tetrahedrallyaround each ethylene carbon atom. Thisobservation is consistent with the elon-gation of the carbon-carbon bond, aswell as with the spectroscopic evidenceand theoretical calculations that allow

bon atom and an osmium atom. In thisexample, then, the bonding of ethylene

122

TWO KINDS OF ETHYLENE LIGANDS

Fig. 6. Ethylene can form two distinctly different types of ligand bonds with metal atoms. (a)

two sigma-like bonds between the carbon atoms and two adjacent metal atoms. As a result, thecarbon-carbon bond in the ethylene becomes a single u bond and the hydrogen atoms move outof the molecular plane and assume an approximate tetrahedral arrangement about the carbonatoms.

to a metal-cluster compound results in mium carbonyl cluster. Simply replac-rearrangement of the bonding electrons ing the two osmium atoms with hydro-in the smaller molecule. gen atoms would produce the geometry

The planar configuration of the ethy- found in ethane, the product of hydro-lene molecule is obviously drastically genation of ethylene. The twist ob-altered by its association with the os- served in the cluster-bonded ethylene



is tending toward the bond angles nor-mally found in ethane.

In this example, the osmium clustercan be considered a model of either anisolated fragment of a metal surface (asin heterogeneous catalysis) or an indi-vidual catalytic molecule (as in homoge-neous catalysis). In either case, osmiumis not necessarily unique in completingwith ethylene. Other metal atoms havedifferent electrons at different energylevels, so the degree of activation anddistortion may differ from one complexto another. However, all of them shouldhave a tendency to activate ethyleneto one degree or another by forming acomplex of this kind.

Binding of MolecularHydrogen to a Metal

In this final example we shall go backone step in the hydrogenation reactionand focus on the reaction of molecularhydrogen with a metal atom, the reac-tion that precedes its dissociation intohydrogen atoms. As mentioned ear-lier, most small molecules can bindchemically to complexes containingone or more metal atoms, often in waysthat roughly resemble the chemisorbedstate of the molecule. The coordinatedmolecule and the metal atom or atomsshare electrons to some extent; as a re-sult, some bond angles or bond lengthsin the bound molecule are changed.Molecular hydrogen has always beena notable exception; until recently it wasfound to bind only dissociatively, thatis, as two individual hydrogen atoms.Observation of chemically bound molec-ular hydrogen would offer enormouspotential for understanding on the basisof first principles the process that even-tually results in dissociative binding ofhydrogen.

A few years ago, G. J. Kubas andcollaborators isolated the tungsten com-plex W(CO)3(PCy 3)2H2 (where Cy iscyclohexyl, a 6-carbon alkane ring) that

AN ETHYLENE-BRIDGED COMPLEX

of ethylene ligand (Fig. 6b) may be foundin the osmium complex Os2(CO)8(C2H4).Not only has the carbon-carbon bond in theethylene ligand lengthened, but the ligand hastwisted, allowing the hydrogen atoms (blue)and the two osmium atoms (red) to assume amore tetrahedral grouping about each centralcarbon atom (green).

may represent the long-sought interme-diate in the oxidative addition of hy-drogen to a metal. Since then, manyadditional molecular-hydrogen com-plexes with central metal atoms otherthan tungsten and ligands other thantricyclohexylphosphine have been iden-tified. The hydrogen in these complexesis apparently reversibly bound to themetal, as can be demonstrated by pass-ing hydrogen gas into a solution of, forexample, the precursor of the abovetungsten-tricyclohexylphosphine com-plex at room temperature. The solution,which is originally purple. turns yellow,and light yellow crystals of the H2 com-plex can be precipitated from it. If thehydrogen stream is replaced by a chemi-cally inert gas such as argon, the purplecolor returns, implying the dissociationof H2 from the complex. An interest-ing feature of these compounds is thatformation of a stable hydrogen complexapparently requires organophosphineligands that are large and bulky. Thestructure of the purple precursor con-tains a clue to the role these ligands

may play: the P–W–P axis is distortedand the organic portion of one phos-phine appears to fill the hole left by theabsent molecule of hydrogen.

Neutron-scattering techniques haveplayed a decisive role in characterizingthe dihydrogen ligand of the complexesin terms of both its structural and dy-namical properties. This information hasthen been used to work out a detailedquantitative picture of the bonding ofthe hydrogen molecule to the metal, aswill be described in the following sec-tions.

A Sigma-Bond Complex. In the earlystages of the investigation of the com-plexes, it was absolutely essential to lo-cate the dihydrogen ligand and ascertainwhether, in fact, it retained its molecu-lar identity. Although some initial evi-dence for the molecular-hydrogen bind-ing came from x-ray diffraction, conclu-sive evidence required the use of neu-tron diffraction because of its sensitivityto scattering from hydrogen. The firststructure determined for a molecular-hydrogen complex is shown in Fig. 8.The complex is the same as the onewe have been discussing except thetricyclohexylphosphine ligands (PCy3)have been replaced with less bulky tri-isopropylphosphine ligands (P(i-Pr)3,where i -Pr represents CH(CH3)2). Prob-ably the most important features of thisstructure are the two equal W–H dis-

H–H distance by some 10 percent over

These facts clearly suggest the forma-tion of a three-center metal-dihydrogenbond (that is, some of the electrons areshared between the metal atom and thetwo hydrogen nuclei) and a substantiallyweakened H–H bond.

The dihydrogen ligand was also foundto have a well-defined equilibrium ori-entation (Fig. 8c), one in which theH–H axis is parallel to the P–W–P axisof the complex. This fact might be ex-

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A DIHYDROGEN COMPLEX

Fig. 8. (a) W(CO)3(P(i-Pr) 3 )2H2, the firstmolecular-hydrogen complex to have itsstructure determined, has two bulky tri-isopropylphosphine ligands (orange, blue, andgreen) located on opposite sides of a tungstenatom (red). The central region betweenthe opposing phosphorus atoms (orange)contains three carbonyl ligands (green andyellow) and the molecular-hydrogen ligand(blue). The fact that the H-H bond length (0.82A) is longer than in free H2 (0.74 A) and the factthat the two W–H bond lengths are equal (1.89A) suggest a three-center metal-dihydrogenbond and a substantially weakened H–H bond.(b) The preferred orientation of the H2 ligandis parallel to P–W–P axis, suggesting thatthere is a barrier to rotation of the H2 ligandabout the W–H2 axis. (c) A potential-energycurve for rotation of the H2 ligand in a planeperpendicular to the W–H2 axis with one

minima for the identical orientations of O and180 degrees from the P–W–P axis. Becausethe ground-state wave functions (dashedlines) for each potential well overlap (shadedareas), there is tunneling between potentialwells and, as a result, the energy levels split.

plained on the basis of interactions be-tween the dihydrogen ligand and otherligands bound to the metal that wouldmake alignment perpendicular to theP–W–P axis (and parallel to the OC–W–CO axis) energetically less favorablethan alignment parallel to the P–W–Paxis. We also observed that the hydro-gen atoms of the bulky organophos-phines formed a pocket around the re-gion of the dihydrogen ligand, but theorientation of these organic groups isvery accommodating and would not beexpected to constrain the H2 molecule.

Theoretical analysis is necessary toderive a more quantitative picture of themetal-ligand bonding than that indicatedby the structural results. The most fun-damental types of calculations can, infact, derive structural parameters such

The Central Region

H 2

Tri-isopropylphosphineLigand

(b)

(c)

H2-Ligand Dynamics I

Potential-Energy Barrier to Rotation


as the H–H or W–H distances; com-parison with experimental values thenserves as a check on the validity of thetheory. The current problem, however,is sufficiently complex that structuralinformation is used as input to simpli-fied theoretical models. Whether or notthe theoretical model is derived fromfirst principles or from a combination ofstructural data and a theoretical model,it is highly desirable to have other ex-perimental information on the nature ofthe chemical bonding that can be usedto gauge the theoretical picture.

The nature of the bonding betweenthe dihydrogen ligand and the transitionmetal is of major significance becausethe complex represents the first exam-ple of a sigma-bond complex, that is,a complex in which the ligand bindsthrough interaction of a metal centerwith a o-bonding electron pair. The-oretical studies of this three-center,two-electron bond indicate that boththe bonding and antibonding orbitals ofhydrogen (Fig. 9) may be involved. The

MOLECULAR-HYDROGEN ORBITALS

Fig. 9. The usual theoretical picture ofH-H bonding has the two electrons in thehydrogen molecule occupying a low-energy

map (here pictured schematically in a cross-sectional view) generally occupies the spacebetween the two hydrogen nuclei. However,

orbital that is usually unoccupied and whoseelectron-density map has a node between thetwo hydrogen nuclei. (The plus and minuspatterns in the antibonding orbital are thereto indicate the antisymmetric nature of theorbital.)

Orbitals

BONDING OFMOLECULAR-HYDROGEN LIGAND

Fig. 10. Although the main bonding betweenthe dihydrogen Iigand and the metal atomis due to an interaction between an empty

hydrogen molecule (here shown in a cross-sectional view), there is evidence for somebackbonding, which is an interaction betweenan antisymmetric metal-atom orbital and the

these interactions weaken the H-H bondand strengthen the M-H bond. The formerinteraction donates electron density from theo-bonding orbital of H2 to the metal atom,whereas the latter interaction puts electrondensity from the metal atom into the H2

antibonding orbital.

primary interaction between H2 and themetal atom is donation of electron den-

orbital in the metal atom (Fig. 10); how-ever, the same studies indicate that, toa lesser degree, backbonding betweena metal orbital and the H2 antibonding

bonding stabilizes the side-on orienta-tion shown in Figs. 8 and 10 rather thanan end-on orientation (in which the H2

molecule would have its bonding axispointed straight at the tungsten atomwith one hydrogen atom much closerto the metal atom than the other). Theside-on coordination ultimately facili-tates cleavage of the H–H bond to givedihydride complexes in oxidative addi-

tion reactions. These theoretical predic-tions, of course, require experimentalconfirmation.

Rotational Dynamics. Hydrogen inthe side-on coordination mode can un-dergo a remarkably wide variety of lig-and dynamics, including torsional os-cillations, or librations, about its equi-librium orientation and much slower180-degree reorientations by tunnel-ing through the rotational barrier. Es-tablishing the presence of a significantelectronic energy barrier to rotationwould provide confirmation of metal-to-H2 backbonding. Such a barrier istoo small to be observed by standardnuclear-magnetic-resonance techniques.Inelastic neutron scattering, however,is highly sensitive to hydrogen mo-tions because of the very large neutron-scattering cross section of protons andthe typically large amplitude of the mo-tions. In fact, this technique is routinelyused to study rapid rotational motion(for example, of methyl groups and ofsolid or liquid hydrogen or molecularhydrogen in zeolites).

The nature of the rotational motion ofthe bound hydrogen molecule may bedescribed with the aid of a diagram thatshows the energy levels that the dihy-drogen ligand may occupy as a functionof the height of the barrier hindering therotation. These levels are the solutionsto the Schrodinger equation chosen torepresent the rotational motion of thebound hydrogen molecule. In particular.the equation includes only one angulardegree of freedom because we assumethat the relatively strong three-centermetal-dihydrogen bond keeps the hydro-gen ligand essentially in a plane duringits rotational motion. The complex may,in fact, be the first example of hydro-gen rotation with only one degree ofrotational freedom, a situation first de-scribed by Pauling as an approximationfor solid hydrogen. If any mixing withvibrational modes can also be neglected,



the Schrodinger equation for the rota-tional motion is

where B is the rotational constant (B =h2/21, where I is the moment of inertiaof the molecule for the rotation in ques-

the O–C–W–H2 axis, V2n represents thebarrier-height potential energy for a po-

are, respectively, the wave function andenergy of the allowed rotational states.

In the present case, as we’ve alreadypointed out. crystal-structure studiesas well as theoretical calculations haveshown the dihydrogen ligand to have awell-defined orientation parallel to theP–W–P axis. The hydrogen moleculein this complex should then have twoequivalent orientations located at po-tential minima that are 180 degreesapart (Fig. 8c), and we may assumethat the term with n = 1 (a simpledouble-minimum potential) will dom-inate. Equation 1 can then be reducedto the Mathieu equation, for whichsolutions are tabulated. The resultingenergy-level diagram as a function ofbarrier height V2 is shown in Fig. 11,in which both the energies and barrierheights are given in terms of B.

The energy levels corresponding toV2 = 0 (left axis in Fig. 11) are those ofa free rotor with one degree of freedom( Ej = B J2, where J is the rotationalquantum number, yielding levels at en-ergies of 0, B, 4B, 9B. . .). introduc-tion of a barrier to this rotation (V2 > O)changes the level spacing drastically andremoves some degeneracies. In the limitof very high barriers (suggested by thearrows on the right side of Fig. 11), thestates approach a set of equally spacedenergy levels characteristic of essen-tially harmonic torsional oscillations.

126

Barrier Height (V2/B)

ROTATIONAL ENERGY-LEVEL DIAGRAM

Fig. 11. A dumbbell molecule (such as hydrogen) constrained to rotate in a plane has onerotational degree of freedom and rotational states J = O, 1, 2, 3, . . at energies of 0, B, 4B, 9B,...

if there is no barrier to the rotation (that is, if V2 = O). On the other hand, if V2 is very high (thatis, beyond the right side of the figure), the molecule will occupy a set of equally spaced torsionaloscillator levels. For intermediate barrier heights we find a series of split Vibrational states. Theobserved transitions (indicated by arrows) are of two types: transitions within the Vibrational

ground to the first excited Vibrational state called torsional transitions. Because photons do notcouple with nuclear moments, optical spectroscopy cannot be used to observe the tunnelingtransitions directly and can be used to observe only the torsional transitions between levels

= O). The observed values for the transitionenergies are scaled by a value for B of 49.5 cm–’ rather than the 60 cm -1 value for free H2 toreflect the increased H-H bond length (0.82 A) relative to free H2 (0.74 A). The transitions shownare for a complex with a barrier height V2 equal to 15.7 B.

The molecular hydrogen complexes (Fig. 8c). To satisfy the Pauli principle,being discussed here have intermediate the degenerate states corresponding tobarriers, and for these, we find a series these two orientations must split intoof vibrational states, each of which is two states, each with a slightly differentsplit relative to the torsional oscillator energy (Fig. 11 inset). The splitting islevels. This splitting arises from the fact called tunnel splitting because it is duethat the barrier is not overly high, al- to the overlap of wave functions throughlowing the amplitude of the libations a potential barrier. The size of the split-of the hydrogen molecule to be rela- ting decreases rapidly with increasingtively large—large enough, in fact, for barrier height and is thus an extremelythe wave functions that correspond to sensitive measure of barrier height.the molecule’s being located in either The two resulting states are charac-of the two potential minima to overlap terized by their symmetry. For exam-



(a) W-PCY3 Complex (b) W-P(i-Pr)3 Complex

300 400 500 600 400 600 800 1000

Frequency (cm-’) Frequency (cm-’)

VIBRATIONAL-ROTATIONAL SPECTRA

Fig. 12. The high-frequency transitions associated with torsional, or rotational, motion ofthe dihydrogen Iigand have been identified for the two complexes (a) W(CO)3(PCy3)2H2 and(b) W(CO)3(P(i-Pr)3)2H2 by using the Filter Difference Spectrometer at LANSCE. Unrelatedfrequencies in the spectra were eliminated by taking the difference between the scatteringspectrum for the complex with a dihydrogen ligand and that for the complex with a dideuteriumligand. The deformation modes include rocking and wagging of the dihydrogen ligand withrespect to the complex. One piece of evidence that the assignment are correct is the fact that,for inelastic neutron scattering, the modes with the largest-amplitude motions of the hydrogenatoms have the highest intensity and the rotational modes involve more motion of the hydrogenatoms than the rocking modes.

pie, 180-degree rotation corresponds toan odd permutation of identical spin-½particles (the protons), with respect towhich the total ground-state wave func-tion must be antisymmetric. The low-temperature wave function can be con-structed from linear combinations ofnuclear-spin and rotational wave func-tions. Thus, a symmetric nuclear-spinwave function (I = 1, where I is thenuclear-spin-state quantum number)combines with an antisymmetric rota-tional wave function (J odd) and viceversa. These two cases correspond forzero barrier height to the two kinds ofH2 molecules referred to as ortho- andpara-hydrogen, respectively. For finitebarrier heights, J is no longer a “good”quantum number to describe the en-ergy levels. The total nuclear spin ofthe molecule, however, must still changein a transition between the two low-est energy levels, that is, in a tunnelingtransition.

We note that transitions in whichthe total nuclear spin of the moleculechanges cannot be observed in opticalspectroscopy because photons do notcouple to the nuclear spin. Neutrons,


however, do couple and are quite use-ful for studying rotational transitionsof this type. The neutron has a nuclearspin of ½, and a flip of the neutron spinduring the scattering process will causethe total nuclear-spin state of the H2

A spin-flip neutron-scattering processthen allows direct observation of theortho-para transition in hydrogen—forexample, para-hydrogen (with I = Oand J even) changing to ortho-hydrogen(with I = 1 and J odd). For a free hy-drogen molecule with two rotationaldegrees of freedom, the transition thatchanges the rotational state from J = Oto J = 1 has an energy of 2B, whereB is the rotational constant. However,if the molecule is constrained to rotatein a plane with only one degree of ro-tational freedom, as is the case for ourcompounds, the transition has an en-ergy of B for zero barrier height, that is,for free rotation. Moreover, as we dis-cussed above, the energy for a tunnelingtransition rapidly becomes smaller withincreasing barrier height until, at infinitebarrier height, the splitting disappearsand the two states become degenerate.

Experimental Confirmation. Nowthat we have selected an appropriatemodel for the rotational dynamics of thedihydrogen ligand in our system, it isa simple matter to relate the observedrotational transitions to the height ofthe rotational barrier. To observe boththe high-frequency transitions to the ex-cited Vibrational state (the longer arrowsin Fig. 11) and the very-low-frequencytransitions associated with rotationaltunneling (the two short arrows in theexploded portion of Fig. 11), we hadto perform experiments on two spec-trometers, each located at a differentneutron source. The high-frequencytorsional transitions were measuredon the Filter Difference Spectrometerat LANSCE by using two samples foreach complex, one of which had dideu-terium instead of dihydrogen ligands.Vibrational modes involving mainly thedideuterium ligand cannot be “seen” inthe presence of the many more modesthat include hydrogen motion (that is,those of the organophosphine ligands).The deuterium-substituted sample thusserved as a “blank” for subtracting allthe various vibrational modes exceptthose of interest—the motions of thedihydrogen ligand. Figure 12 showsthe results for two tungsten complexes:one with tricyclohexylphosphine ligands(PCy3) and one with the less bulky tri-isopropylphosphine ligands (P(i-Pr) 3).

The low-frequency rotational tunnel-ing spectra for three complexes (Fig. 13)were obtained on a cold-neutron time-of-flight spectrometer at the High FluxReactor of the Institut Laue-Langevinin Grenoble, France. No “blank” sam-ple was necessary in this case, sincethe other ligands were not expected tohave observable excitations in the fre-quency range of interest for this experi-ment (which is less than 10 cm-1).

For the two tungsten complexes withPCy3 and P(i-Pr)3 ligands, this typeof analysis yielded a significant bar-rier height—one that was roughly 15

127


ROTATIONAL-TUNNELING SPECTRA

Fig. 13. The spectra for the low-frequencytransitions associated with rotational tunnel-ing are shown here for the three complexes (a)

peak in each spectrum is an elastic-scatteringline, whereas the peaks to both sides of thatline are the inelastic-scattering transitionsassociated with rotational tunneling. The factthat the inelastic peaks have a doublet natureis most likely due to structural disorder in thecrystals.

times our derived rotational constantfor bound H2. Using the barrier heightsand the energy-level diagram (Fig. 11),we were able to calculate the frequen-cies expected for the high- frequencytransitions associated with the torsionalmotion of both complexes. The cal-culated values are in good agreementwith the experimental values measuredwith the Filter Difference Spectrometer(Fig. 12), which suggests that the sim-ple model of planar reorientation in adouble-minimum potential is a reason-able description for the hydrogen motionin these systems.

The crucial question at this point be-comes what interactions give rise to thebarrier to rotation. Two possible sourcesfor the hindrance potential are electronicand steric effects. By an electronic ef-fect we mean that the dihydrogen ligandmay be constrained in its orientation be-cause of the way the chemical bond isformed with the metal. In other words,the electron orbitals on the metal shar-ing electrons with those of the dihydro-gen ligand have a symmetry that deter-mines the orientation of the ligand.

Steric effects refer to the interactionsof the dihydrogen ligand with the sur-rounding atoms of the other ligands.These are nonbonding interactions thatmay be described by van der Waalsforces between pairs of atoms. Theymay be summed up for all pairs formed

I

I II

-2 0 2 4-4

(c) W-P(i -Pr)3 Complex

0.73 cm-’

I I I I I

o 1 2-2 -1

Energy Transfer (Cm-1)


128


by using either one of the H atoms onthe dihydrogen ligand and any one ofthe surrounding atoms. As the dihydro-gen ligand is rotated, the sum of theseinteractions shows an angular variation,which gives rise to an effective “steric”barrier.

In an attempt to sort out the relativeeffects of these two types of interac-tions, we performed separate measure-ments on the two tungsten complexeswith PCy3 and P(i-Pr)3 ligands; thenwe replaced the tungsten atom in thePCy 3 complex with a molybdenum atomand took measurements on this thirdcomplex. Thus, we hoped to gauge theeffects of changing the central metalatom and of replacing the large, bulkyPCy3 ligand with the less bulky P(i-Pr)3

ligand.The peaks in the spectra of Fig. 13

to the left and right of the strong elasticline represent the rotational-energy split-ting associated with the H2 moleculetunneling through the barrier from one180-degree orientation to the other. Theposition of these lines is extremely sen-sitive to the height of the barrier. Acomparison of the three spectra showsthat replacement of tungsten (Fig. 13b)with molybdenum (Fig. 13a) changesthe tunneling frequency by a factor ofjust over 3, from 0.89 to 2.82 cm-]. Onthe other hand, replacing the PCy3 lig-and in the tungsten complex with theless bulky P(i–Pr)3 ligand (Fig. 13c)changes the frequency by less than 20percent, from 0.89 to 0.73 cm-1.

The change that occurs when the cen-tral metal atom is replaced may be takenas reflecting the metal-dihydrogen bond-ing directly; that is, it is essentially anelectronic effect. Replacing the PCy3

ligands with P(i–Pr)3 ligands, on theother hand, probably has little effecton the electronic state of the metal andtherefore on the metal-dihydrogen bond-ing. These ligands most likely producea steric component of the barrier to H2

rotation through direct, nonbonded in-


Table 1

Barrier Heights to Rotation for the Dihydrogen Ligand

Complex Theoretical (keal/mole)Observed

(keal/mole)

Metal Atom Ligand Molecular ab initio SumMechanics (pH 3)

W PCy30.6 1.8 2.4 2.2

W P(i-Pr) j 1.4 1.8 3.2 2.4Mo PCy3

0.6 0.6 1.2 1.5

tractions. Thus, the experimental evi-dence, at least in these cases, stronglysuggests that the barrier to H2 rota-tion is determined more by electronicthan steric effects. To test this conclu-sion, Jeff Hay, John Hall, and CarolineBoyle of the Theoretical Division at LosAlamos earned out two sets of calcula-tions: an ab initio calculation—that is,from first principles—and a molecular-mechanics calculation.

The ab initio calculation treats pri-marily the electronic effects becausea full set of one-electron wave func-tions for the whole molecule is used tocompute the relative energy of a givenconfiguration. The barrier to rotationwas obtained from the difference intotal energies for the structure withthe H2 aligned along the P–M–P axisand the structure with the H2 alignedalong the OC–M–CO axis. The calcu-lation is rather extensive, and the bulkyorganophosphine ligands must be sim-plified to make it possible at all. Whenunsubstituted phosphine (PH3) is usedas a ligand instead of tricyclohexylphos-phine or tri-isopropylphosphine, the cal-culation yields a barrier height of 1.8kilocalories per mole for the tungstencomplex and 0.6 kilocalorie per molefor the molybdenum complex.

The second type of calculation—themolecular-mechanics type—may be

viewed as representing mainly stericeffects. In this case, the pairwise, non-binding interactions between the hydro-gen atoms of the dihydrogen ligand andeach of the other atoms of the moleculeare summed. The summation is repeatedfor each orientation of the ligand, gen-erating a curve of potential energy asa function of orientation. This calcula-tion is not sensitive to the type of metalatom at the center of the molecule. Theresults show a barrier height of 0.6 kilo-calorie per mole for the complexes withPCy3 ligands and 1.4 kilocalories permole for those with P(i -Pr)3 ligands.

If one makes the assumption thatmolecular mechanics treats only thesteric effects and that the ab initio the-ory accounts primarily for the directelectronic interaction between H2 andthe metal, barrier heights from the twocalculations may be added to arrive atan estimate of the effective total bar-rier. These assumptions are not unrea-sonable, because replacement of tung-sten by molybdenum has no effect onthe results of the molecular-mechanicscase, whereas the ab initio theory usesthe very small PH3 ligands, rather thanP(i-Pr) 3 or PCy3 ligands, and thus es-sentially ignores steric effects. The sumof the two calculations for each of thecomplexes is shown in Table 1 alongwith the corresponding barrier height

129


calculated from the observed inelasticneutron-scattering data.

The calculated and observed barriersto H2 rotation appear. at first glance,to agree only qualitatively. If, how-ever. one takes into account the variouslimitations of the theoretical calcula-tions, the agreement with experimentis remarkably good. For example, thenecessary structural information is notknown in detail for all three complexesin this experiment, and both types ofcalculations are normally used for bar-rier heights that are a factor of ten or sohigher than the one in this study. Fur-thermore. comparison with experimentaldata does suggest that the molecular-mechanics calculation overestimates thesteric part of the barrier, since replace-ment of the PCy3 with P(i-Pr)3 is foundto change the barrier height by 0.8 kilo-calorie per mole. which is four timesthe experimentally observed change. Inview of these considerations, we canclearly conclude that the direct elec-tronic binding of the dihydrogen ligandto the metal contributes significantlyto the barrier, at least one-half to two-thirds of’ the experimentally determinedvalue.

The rotational tunnel splitting is anextremely sensitive measure of the bar-rier height—in fact, it depends exponen-tially on the value of the barrier. Suchsensitivity has clear advantages. Forexample, the observation of a higherbarrier in the tungsten complex than inthe molybdenum complex is an indica-tion of stronger binding of the hydro-gen molecule to the metal atom. Thisconclusion can be reached both by ob-serving the higher M–H2 infrared stretchfrequency in the tungsten complex orby observing the difference in rotationaltunnel splitting. However. the changein infrared stretch frequency is only 10percent, whereas the change in rota-tional tunnel splitting is more than 50percent. It is therefore clear that rota-tional tunneling spectroscopy of side-on

H 2 by inelastic neutron scattering can beused as a probe of the details of metal-to-H 2 binding.

Given the fact that the directionalproperties of the electron wave functionsthat help optimize the electron flow be-tween the dihydrogen ligand and themetal atom also seem to be largely re-sponsible for the barrier to H2 rotation,we feel that establishment of a signif-icant electronic component to the bar-

backbonding between the metal atomand molecular hydrogen.

The latter conclusion, in particular, isa truly remarkable result of our neutron-scattering studies and illustrates the veryfundamental details to which these cat-alytic model systems can be studiedwith such techniques. Apart from ourmodel systems, many more realistic cat-alytic materials are being investigatedby the same techniques—studies that areoften greatly aided by previous work onmodel compounds. These more realisticsystems include molecules adsorbed ondispersed metal particles, inside cavitiesof zeolites, or attached to many otheractive substrates. Although the level ofdetail that can safely be inferred fromthe “real” catalytic systems is somewhatlower than for the simpler model sys-tems, significant progress can nonethe-less be expected in understanding thecatalytic function of these materials onan atomic scale. Neutron scattering willcertainly play an important role in thesestudies. ■

Acknowledgments

It is a great pleasure to thank Greg Kubas, theperson who first demonstrated the existence ofmolecular-hydrogen complexes, for our ongoing,fruitful collaboration. Of the many others whohave made significant contributions to the workand the ideas discussed in this article, we wouldin particular like to mention Larry Dahl, WernerPress, Alberto Albinati, Guiliano Longoni, TomKoctzle, Oren Anderson, and Jeff Hay).

Juergen Eckert earned his B.S. at Yale Uni-versity and his Ph.D. at Princeton Universityin 1975. Most of the research for his doc-toral thesis, which involved neutron-scatteringstudies of the lattice dynamics of solid neon.was carried out as a member of the neutron-scattering group in the Physics Department ofBrookhavcn National Laboratory. After earn-ing his doctorate, he remained at Brookhavenuntil 1979, when he accepted a staff member-ship atl Los Alamos to initiate neutron-scattermgresearch on the newly commissioned pulsed neu-tron source (WNR) at LAMPF. The focus ofhis work has steadily shifted towards applica-tions of neutron scattering to chemistry, albeitfrom a physicist’s perspective. He recentlyspent one year as a visiting scientist at the In-stitut Laue Langevin in in Grenoble, France. wheresome of the work described here was carried out.

130 Los Alamos Science Summer 1990


Phillip J. Vergamini received his M.S. in in-organic chemistry from the University of Min-nesota, Minneapolis, in 1968 after earning hisB.S. at the University of Wisconsin, Superior.He completed his Ph.D. in inorganic chemistryat the University of Wisconsin, Madison, in thefall of 1971 and then joined the Isotope and Nu-clear Chemistry Division at Los Alamos. Hisresearch there involved application of spectro-scopic and x-ray crystallographic techniques tostudying the synthesis and structure of inor-ganic and organometallic compounds. In thespring of 1980, he joined the neutron-scatteringgroup at Los Alamos and took responsibility forthe construction and use of the Single-CrystalDiffractometer, one of the first two LANSCEinstruments to be placed into the internationalusers program. With that diffractometer he hasstudied the structures of materials ranging frommolecular-hydrogen complexes to the crystallinemineral in dinosaur bones.

Further Reading

R. K. Thomas. 1982. Neutron scattering fromadsorbed systems. Progress in Solid State Chem-istry 14: 1 –93.

C. J. Wright. 1985. Surface characterizationby the inelastic scattering of neutrons from ab-sorbates. In The Structure of Surfaces, edited byM. A. Van Hove and S. Y. Tong, pp. 210-218.Springer Series in Surface Science, volume 2.Berlin: Springer-Verlag.

T. J. Udovic and R. D. Kelley. 1988. Neutronscattering studies of hydrogen In catalysts. InHydrogen Effects in Catalysis, edited by Z. Paaland P G. Menon, pp. 107-182 New York: Mar-cel Dekker.

G. A. Somorjai. 1986. Surface science andcatalysis. Philosophical Transactions of theRoyal Society of London A318:81–100.

Norman Sheppard. 1988. Vibrational spectro-scopic studies of the structure of species derivedfrom the chemisorption of hydrocarbons on metalsingle-crystal surfaces. Annual Review of Physi-cal Chemistry 39:589–644.

E. L. Muetterties, T. N. Rhodin, Elliot Baud, C.F. Brucker, and W. R. Pretzer. 1979. Clustersand surfaces. Chemical Review 79: 91–137.

R. R. Cavanagh, J. J. Rush, R. D. Kelley, and T.J. Udovic. 1984. Adsorption and decompositionof hydrocarbons on platinum black: Vibrationalmodes from N15. Journal of Chemical Physics80: 3478–3484.

R. Whyman. 1980. Metal clusters in catalysis.In Transition Metal Clusters, edited by BrianF. G. Johnson, pp. 545–606. New York: JohnWiley and Sons.

Juergen Eckert, Gregory J. Kubas, John H. Hall,P. Jeffrey Hay, and Caroline M. Boyle. 1990.Molecular hydrogen complexes. 6. The barrier

(M = W, Mo: R = Cy, i-Pr): Inelastic neutronscattering, theoretical, and molecular mechan-ics studies. Journal of the Americal ChemicalSociety 112: 2324.

Gregory J. Kubas. 1988. Molecular hydrogen

sition metals. Accounts of Chemical Research21: 120–128.

J. Eckert, A. Albinati, and G. Longoni. 1989. Inel-astic neutron-scattering study of K[HCO6(CO)15]:Implications for the location of the hydride. In-organic Chemistry 28: 4055.

Pier Luigi Stanghellini and Guiliano Longoni.1987. Vibrational studies of interstitial hydro-gen in metal carbonyl clusters. Journal of theChemical Society Dalton Transactions 685–690.

Jacques Roziere and Antoine Potier. 1982. Li-aison metal-hydrogene-metal et spectroscopic devibration. Bulletin de la Societe Chimique deFrance. Partie I. 1-339–346.

Werner Press. 1981. Single-Particle Rotations inMolecular Crystals. Springer Tracts in ModemPhysics, volume 92. Berlin: Springer-Vcrlag.

P. Jeffrey Hay. 1987. Ab initio theoreticalstudies of dihydrogcn coordination vs. oxida-tive addition of H2 to five-coordinate tungstencomplexes. Journal of the American ChemicalSociety 109:705-710.

Gregory J. Kubas, Robert R. Ryan, Basil I.Swanson, Phillip J. Vergamini, and Harvey J.Wasserman. 1984. Characterization of the firstexamples of isolable molecular-hydrogen com-

Cy, i-Pr). Evidence for a side-on bonded H2 li-gand. Journal of the American Chemical Society106:451-452.

George Edmund Bacon. 1977. Neutron Scatter-ing in Chemistry. London: Butterworths.

R. E. Lechner and C. Riekel. 1983. Applicationsof neutron scattering in chemistry. In NeutronScattering and Muon SpinTracts in Modern Physics.Springer-Verlag.

Robert Bau, editor. 1978.drides. Washington, D.C.:Society.

H. Ibach and D. L. Mills.

Rotation. Springervolume 1 () 1. Berlin:

Transition Metal Hy-American Chemical

1982. Electron En-ergy loss Spectroscopy and Surface Vibrations.New York: Academic Press.

J. Eckert. 1986. Neutron vibrational spec-troscopy: The use of hydrogen as a structuraland dynamical probe. Physica B+C 136: 150-155.


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