Download ppt - Organometallic M T Complexes. M T Organometallics Organometallic compounds of the transition metals have unusual structures, and practical applications

OrganometallicOrganometallicMMTT Complexes Complexes

MMT T OrganometallicsOrganometallics

Organometallic compounds of Organometallic compounds of the transition metals have unusual the transition metals have unusual structures, and practical structures, and practical applications in organic synthesis and applications in organic synthesis and industrial catalysis.industrial catalysis.

MMT T OrganometallicsOrganometallics

One of the earliest compounds, One of the earliest compounds, known as Zeise’s salt, was prepared in known as Zeise’s salt, was prepared in 1827. It contains an ethylene molecule 1827. It contains an ethylene molecule ππ bonded to platinum (II).bonded to platinum (II).

Zeise’s SaltZeise’s Salt

The bonding The bonding orbital of ethene orbital of ethene donates electrons donates electrons to the metal. The to the metal. The filled d orbitals (dfilled d orbitals (dxz xz

or dor dyzyz) donate ) donate electrons to the electrons to the antibonding orbital antibonding orbital of ethene.of ethene.

Square Planar ComplexesSquare Planar Complexes

The complexes of platinum(II), The complexes of platinum(II), palladium(II), rhodium(I) and iridium(I) palladium(II), rhodium(I) and iridium(I) usually have 4-coordinate square planar usually have 4-coordinate square planar geometry. These complexes also geometry. These complexes also typically contain 16 electrons, rather typically contain 16 electrons, rather than 18.than 18.

The stability of 16 electron The stability of 16 electron complexes, especially with complexes, especially with σσ-donor -donor ππ--acceptor ligands, can be understood by acceptor ligands, can be understood by examining a MO diagram.examining a MO diagram.


The electron pairs from the 4 ligands used in σ bonding occupy the bonding orbitals.


The dxy, dxz, dyz and dz2 orbitals are either weakly bonding, non-bonding, or weakly antibonding.


The dx2-y2

orbital is anti-bonding, and if filled, will weaken the σ bonds with the ligands.


As a result, 16 electrons will produce a stable complex.

Catalysis of Square Planar Catalysis of Square Planar CompoundsCompounds

Square planar complexes are Square planar complexes are often involved as catalysis for often involved as catalysis for reactions. The four-coordinate reactions. The four-coordinate complexes can undergo addition of complexes can undergo addition of organic molecules or hydrogen, and organic molecules or hydrogen, and then be regenerated as the organic then be regenerated as the organic product is released from product is released from coordination to the catalyst.coordination to the catalyst.

Catalysis – aldehyde Catalysis – aldehyde formationformation

Pd(II) Pd(II) undergoes addition undergoes addition of an alkene which is of an alkene which is subsequently subsequently converted to an converted to an alcohol. Addition of a alcohol. Addition of a hydrogen atom to the hydrogen atom to the metal with metal with subsequent migration subsequent migration to the alcohol to the alcohol produces an produces an aldehyde.aldehyde.

CatalysisCatalysis

Bonding of Bonding of HydrocarbonsHydrocarbons

Hydrocarbons can bond to transition Hydrocarbons can bond to transition metals via metals via σσ bonds or bonds or ππ bonds. bonds. Wilkinson’s catalyst, [RhCl(PPhWilkinson’s catalyst, [RhCl(PPh33)] is used )] is used to hydrogenate a wide variety of alkenes to hydrogenate a wide variety of alkenes using pressures of Husing pressures of H22 at 1 atm or less. at 1 atm or less.

During the hydrogenation, the During the hydrogenation, the alkene initially alkene initially ππ bonds to the metal, and bonds to the metal, and then accepts a hydrogen to then accepts a hydrogen to σσ bond with bond with the metal.the metal.

Wilkinson’s CatalystWilkinson’s Catalyst

Hydrogen AdditionHydrogen Addition

Square planar complexes are Square planar complexes are known to react with hydrogen, known to react with hydrogen, undergoing addition, and breaking undergoing addition, and breaking the H-H bond. the H-H bond.


The The hydrogen hydrogen bonding orbital bonding orbital donates electron donates electron density into an density into an empty p or dempty p or d

orbital on the orbital on the metal.metal.

M


M

The loss of The loss of electron electron density in the density in the bonding bonding orbital orbital weakens the weakens the H-H bond.H-H bond.


The metal can The metal can donate electron donate electron density from a density from a filled d orbital (dfilled d orbital (dxzxz or dor dyzyz))to the antibonding to the antibonding orbital on orbital on hydrogen, thus hydrogen, thus weakening or weakening or breaking the H-H breaking the H-H bond.bond.

The Template EffectThe Template Effect

A metal ion can be used to assemble A metal ion can be used to assemble a group of organic ligands which then a group of organic ligands which then undergo a condensation reaction to form a undergo a condensation reaction to form a macrocyclic ligand. Nickel (II) is used in macrocyclic ligand. Nickel (II) is used in the scheme below.the scheme below.

MMTT Carbonyls Carbonyls

Metal carbonyl compounds were Metal carbonyl compounds were first synthesized in 1868. Although first synthesized in 1868. Although many compounds were produced, many compounds were produced, they couldn’t be fully characterized they couldn’t be fully characterized until the development of X-ray until the development of X-ray diffraction, and IR and NMR diffraction, and IR and NMR spectroscopy.spectroscopy.


Metal carbonyl compounds typically Metal carbonyl compounds typically contain metals in the zero oxidation contain metals in the zero oxidation state. In general, these compounds state. In general, these compounds obey the “18 electron rule.” obey the “18 electron rule.”

Although there are exceptions, this Although there are exceptions, this rule can be used to predict the structure rule can be used to predict the structure of metal carbonyl cluster compounds, of metal carbonyl cluster compounds, which contain metal-metal bonds.which contain metal-metal bonds.

The 18 Electron RuleThe 18 Electron Rule

Many transition metal carbonyl Many transition metal carbonyl compounds obey the 18-electron compounds obey the 18-electron rule. The reason for this can be rule. The reason for this can be readily seen from the molecular readily seen from the molecular orbital diagram of Cr(CO)orbital diagram of Cr(CO)66. The . The σσ donor and donor and ππ acceptor nature of CO acceptor nature of CO as a ligand results in an MO diagram as a ligand results in an MO diagram with greatest stability at 18 with greatest stability at 18 electrons.electrons.

The eg* orbitals are destabilizing to the complex. Since the 12 bonding orbitals are filled with electrons from the CO molecules, 6 electrons from the metal will produce a stable complex.


The CO stretching frequency is often The CO stretching frequency is often used to determine the structure of these used to determine the structure of these compounds. The carbon monoxide compounds. The carbon monoxide molecule can be terminal, or bridge molecule can be terminal, or bridge between 2 or 3 metal atoms.between 2 or 3 metal atoms.

The CO stretching frequency The CO stretching frequency decreases with increased bonding to decreases with increased bonding to metals. As the metals. As the ππ* orbital on CO receives * orbital on CO receives electrons from the metal, the CO bond electrons from the metal, the CO bond weakens and the weakens and the νν decreases. decreases.


As the As the ππ* orbital on CO receives * orbital on CO receives electrons from the metal, the CO electrons from the metal, the CO bond weakens and the bond weakens and the νν decreases. decreases.


MnMn22(CO)(CO)10 10 Fe2(CO)9


CoCo44(CO)(CO)12 12


ν for free CO = 2143 cm-1


ν for free CO = 2143 cm-1

MMTT Carbonyls CarbonylsThe CO stretching frequency will The CO stretching frequency will

also be affected by the charge of the also be affected by the charge of the metal.metal.

CompoundCompound νν (cm (cm-1-1))

[Fe(CO)[Fe(CO)66]]2+2+ 22042204

[Mn(CO)[Mn(CO)66)])]++ 21432143

Cr(CO)Cr(CO)66 20902090

[V(CO)[V(CO)66]]-- 18601860

[Ti(CO)[Ti(CO)66]]2-2- 17501750


The IR spectra of transition The IR spectra of transition metal carbonyl compounds are metal carbonyl compounds are consistent with the predictions consistent with the predictions based on the symmetry of the based on the symmetry of the molecule and group theory.molecule and group theory.

The more symmetrical the The more symmetrical the structure, the fewer CO stretches structure, the fewer CO stretches are observed in the IR spectra.are observed in the IR spectra.


If there is a center of symmetry, If there is a center of symmetry, with CO ligands with CO ligands transtrans to each other, to each other, a symmetrical stretch will not a symmetrical stretch will not involve a change in dipole moment, involve a change in dipole moment, so it will be IR inactive. An so it will be IR inactive. An asymmetric stretch will be seen in asymmetric stretch will be seen in the IR spectrum. As a result, the IR spectrum. As a result, trans trans carbonyls give one peak in the IR carbonyls give one peak in the IR spectrum.spectrum.


If CO ligands If CO ligands are are ciscis to each to each other, both the other, both the symmetric stretch symmetric stretch and the asymmetric and the asymmetric stretch will involve stretch will involve a change in dipole a change in dipole moment, and hence moment, and hence two peaks will be two peaks will be seen in the IR seen in the IR spectrum.spectrum.


Metal carbonyls with a center of Metal carbonyls with a center of symmetry typically show only 1 C-O symmetry typically show only 1 C-O stretch in their IR spectra, since the stretch in their IR spectra, since the symmetric stretch doesn’t change symmetric stretch doesn’t change the dipole moment of the compound. the dipole moment of the compound. Combined with the Raman spectrum, Combined with the Raman spectrum, the structure of these compounds the structure of these compounds can be determined.can be determined.

Nomenclature for Nomenclature for LigandsLigands

The The hapticity hapticity of the ligand is the number of of the ligand is the number of atoms of the ligand which directly interact with atoms of the ligand which directly interact with the metal atom or ion. It is indicated using the the metal atom or ion. It is indicated using the greek letter greek letter ηη (eta) with the superscript (eta) with the superscript indicating the number of atoms bonded.indicating the number of atoms bonded.

Cyclopentadienyl Cyclopentadienyl CompoundsCompounds

The ligand CThe ligand C55HH55 can bond to can bond to metals via a metals via a σσ bond (contributing 1 bond (contributing 1 electron), or as a electron), or as a ππ bonding ligand. bonding ligand. As a As a ππ bonding ligand, it can donate bonding ligand, it can donate 3, or more commonly 5 electrons to 3, or more commonly 5 electrons to the metal. the metal.

Cyclopentadienyl Cyclopentadienyl CompoundsCompounds

W(W(ηη33-C-C55HH55)()(ηη55-C-C55HH55)(CO))(CO)22 has two has two ππ bonded cyclopentadienyl rings. One bonded cyclopentadienyl rings. One donates 3 electrons, and the other donates 3 electrons, and the other donates 5.donates 5.

Counting ElectronsCounting Electrons

There are two common methods for There are two common methods for determining the number of electrons in determining the number of electrons in an organometallic compound. an organometallic compound. One method views the One method views the cylcopentadienyl ring as Ccylcopentadienyl ring as C55HH55

--, a 6 , a 6 electron donor. CO and halides such electron donor. CO and halides such as Clas Cl-- are viewed as 2 electron donors. are viewed as 2 electron donors. The oxidation state of the metal must The oxidation state of the metal must be determined to complete the total be determined to complete the total electron count of the complex.electron count of the complex.


The other method treats all The other method treats all ligands as neutral in charge. ligands as neutral in charge. ηη55-C-C55HH5 5

is viewed as a 5 electron donor, Cl is is viewed as a 5 electron donor, Cl is viewed as a chlorine atom and a 1 viewed as a chlorine atom and a 1 electron donor, and CO is a 2 electron donor, and CO is a 2 electron donor. The metal is viewed electron donor. The metal is viewed as having an oxidation state of zero as having an oxidation state of zero in this method.in this method.


In either method, a metal-metal In either method, a metal-metal single bond is counted as one single bond is counted as one electron per metal. Metal-metal electron per metal. Metal-metal double bonds count as two electrons double bonds count as two electrons per metal, etc.per metal, etc.

FerroceneFerrocene

Fe(Fe(ηη55-C-C55HH55))22 , , ferrocene, is known as ferrocene, is known as a “sandwich” a “sandwich” compound. In the compound. In the solid at low solid at low temperature, the temperature, the rings are staggered.rings are staggered.

The rotational barrier The rotational barrier is very small, with is very small, with free rotation of the free rotation of the rings.rings.

FerroceneFerrocene

The cyclopentadienyl rings The cyclopentadienyl rings behave as an aromatic electron behave as an aromatic electron donor. They are viewed as Cdonor. They are viewed as C55HH55

-- ions donating 6 electrons to the ions donating 6 electrons to the metal. The iron atom is considered metal. The iron atom is considered to be Fe(II). to be Fe(II).

Bonding of FerroceneBonding of Ferrocene

Group theory is used to simplify Group theory is used to simplify the analysis of the bonding. First, the analysis of the bonding. First, consider just a single Cconsider just a single C55HH55 ring. ring. Determine Determine ΤΤππ by considering only the by considering only the ppzz orbitals which are perpendicular orbitals which are perpendicular to the 5-membered ring.to the 5-membered ring.


DD5h5h EE 2C2C55 2C2C5522

5C5C22 σσhh 2S2S55 2S2S5533

5 5 σσvv

ΤΤππ



5C5C22 σσhh 2S2S55 2S2S5533

5 5 σσvv

ΤΤππ 55



5C5C22 σσhh 2S2S55 2S2S5533

5 5 σσvv

ΤΤππ 55 00



5C5C22 σσhh 2S2S55 2S2S5533

5 5 σσvv

ΤΤππ 55 00 00



5C5C22 σσhh 2S2S55 2S2S5533

5 5 σσvv

ΤΤππ 55 00 00 -1-1



5C5C22 σσhh 2S2S55 2S2S5533

5 5 σσvv

ΤΤππ 55 00 00 -1-1 -5-5



5C5C22 σσhh 2S2S55 2S2S5533

5 5 σσvv

ΤΤππ 55 00 00 -1-1 -5-5 00



5C5C22 σσhh 2S2S55 2S2S5533

5 5 σσvv

ΤΤππ 55 00 00 -1-1 -5-5 00 00



5C5C22 σσhh 2S2S55 2S2S5533

5 5 σσvv

ΤΤππ 55 00 00 -1-1 -5-5 00 00 11



5C5C22 σσhh 2S2S55 2S2S5533

5 5 σσvv

ΤΤππ 55 00 00 -1-1 -5-5 00 00 11

ΤΤππ reduces to: A′ reduces to: A′11, E′, E′11 and E′ and E′22

Group theory can be used to generate Group theory can be used to generate drawings of the drawings of the ππ molecular orbitals. molecular orbitals.

Bonding of FerroceneBonding of FerroceneΤΤππ reduces to: A′ reduces to: A′11, E′, E′11 and E and E

′′22

E′E′22

E′E′11

A′A′11


The totally The totally bonding orbital (Abonding orbital (A′′11) has no nodes, ) has no nodes, and is lowest in and is lowest in energy.energy.


The middle The middle set of orbitals (E′set of orbitals (E′11) ) are degenerate, are degenerate, with a single node. with a single node. These orbitals are These orbitals are primarily bonding primarily bonding orbitals.orbitals.


The upper set The upper set of orbitals (E′of orbitals (E′22) are ) are degenerate, with degenerate, with two nodes. These two nodes. These orbitals are orbitals are primarily anti-primarily anti-bonding orbitals.bonding orbitals.


Once the molecular orbitals of Once the molecular orbitals of the cyclopentadienyl ring has been the cyclopentadienyl ring has been determined, two rings are combined, determined, two rings are combined, and matched with symmetry and matched with symmetry appropriate orbitals on iron.appropriate orbitals on iron.


The A′The A′11 orbitals on the orbitals on the two cyclopentadienyl two cyclopentadienyl rings have the same rings have the same symmetry as the dsymmetry as the dzz2 2

orbital on iron. orbital on iron.

Since the metal Since the metal orbital is located in the orbital is located in the center of the Ccenter of the C55HH55 rings, this is rings, this is essentially a non-essentially a non-bonding orbital.bonding orbital.


The E′The E′11 orbitals orbitals on the rings have on the rings have the same the same symmetry as the symmetry as the ddxzxz and d and dyzyz orbitals orbitals of the iron.of the iron.


The E′The E′22 orbitals orbitals on the rings have on the rings have the same the same symmetry as the symmetry as the ddxyxy and d and dxx22-y-y22 orbitals of the iron.orbitals of the iron.


These are the bonding orbitals These are the bonding orbitals of ferrocene. If the upper of ferrocene. If the upper cyclopentadienyl ring is flipped over, cyclopentadienyl ring is flipped over, a set of antibonding orbitals results.a set of antibonding orbitals results.

MO DiagramMO Diagram

The frontier The frontier orbitals are orbitals are neither strongly neither strongly bonding nor bonding nor strongly strongly antibonding. As antibonding. As a result, metallo-a result, metallo-cene compounds cene compounds often diverge often diverge from the 18 from the 18 electron rule.electron rule.

MO DiagramMO Diagram

If the If the complex has more complex has more than 18 electrons, than 18 electrons, the ethe e1u1u orbitals, orbitals, which are slightly which are slightly antibonding (the antibonding (the ddxzxzand dand dyzyz), ), become occupied. become occupied. This lengthens the This lengthens the M-C distance.M-C distance.

Electron Count and Electron Count and StabilityStability

((ηη55-Cp)-Cp)22MM ee-- count count M-C(pm)M-C(pm)ΔΔHdissoc.*Hdissoc.*

FeFe 18 18 206.4206.4 1470 kJ/mol1470 kJ/mol

CoCo 19 19 211.9211.9 14001400

NiNi 20 20 219.6219.6 13201320

* * ΔΔHdissoc refers to the complex Hdissoc refers to the complex dissociating to Mdissociating to M2+2+ and 2C and 2C55HH55

--