ELSEVIER

THEO CHEM

Journal of Molecular Structure (Theochem) 393 (1997) 189- 196

[Pt2(PPh&(p-S)Z] as a metalloligand toward theoretical study of intermetallic complexes

and Ga(III)

main-group Lewis acids: with Tl(I), Pb(II), In(II1)

A.L. Tana,*, M.L. Chiew”, T.S.A. Herb

“Computational Science Programme, Faculty of Science, National University of Singapore, Kent Ridge, Singapore, I 19260 Singapore

bDepartment of Chemistry, Faculty of Science, National University of Singapore, Kent Ridge, Singupore, 119260 Singapore

Received 16 February 1996; accepted 30 July 1996

Abstract

This paper describes theoretical studies on adducts of Ptz(PPh3)&S)z with TI(I), Pb(II), In(II1) and Ga(II1). Using one of the Pb compounds as an example, the fundamental principle of competition for interactions (M-ligand vs. M-S; M-S vs. Pt-S) is illustrated; in particular, any ligand bound trans to an M-S bond will cause the latter to be weakened more than the other M-S bond. The polarizable phosphine ligands also play a role in moderating changes in the Pt-S interactions. The general weakening of the M-S bonds upon ligand coordination, which is also demonstrated by binding energies of Pt2(PPh3)&-S)? to various heterometal fragments, could explain why Ga, Pb and Tl do not accept additional ligands. In (PPh3)4Pt2(pL2-S)zInC13, the In atom is square pyramidally coordinated. Calculations indicate strong pr-pr and dp--pp bonding between In and the Cl atom at the apex of the square pyramid, thus accounting for the unusually short In-Cl bond length. With regard to the trend of increasing Pt-S-S-Pt dihedral angle with increasing heterometal size, it is found that in the parent compound (without the heterometal M), the dihedral angle determines the character of the sulfur lone pairs; it is inferred that the “optimal” orientation of the lone pairs for interactions with the heterometal depends on the size of the latter. This result has implications for all binuclear Pt(I1) or Pd(I1) compounds with bridging sulfur ligand(s). 0 1997 Elsevier Science B.V. All rights reserved.

Keywords: Platinum; Sulfur; Intermetallic; Fenske-Hall calculations; Metalloligand

1. Introduction photophysical [4] and electrochemical [5] properties.

We have recently explored the reactivity of 1 with

Ever since Briant et al. [I] discovered that main-group metals and demonstrated that it can func- Pt2(PPh3)4(p-S)2 [2] (1 in Fig. 1) is a versatile metal- tion as a bidentate ligand in a series of intermetallic

loligand toward a variety of transition metal frag- species [6]. The complexes so obtained exhibit unu- ments, a host of addition reactions based on the sual coordination geometries of the heterometal. For

Lewis basicity of 1 have been established [3]. Struc- example, the coordination of 1 to Tl+ produces an

tural analogs of 1 have also emerged with targeted unusual compound with two-coordinated V-shaped Tl(I) which is coordinatively exposed [I] (2 in Fig. 1). In contrast, Pbzf, which is isoelectronic to Tl+, forms

* Corresponding author. the adducts [(PPh~>JPt2(C13-S>,Pb(NO~)]X (X = NO3

0166-1280/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII SOl66-1280(96)04814-Z

190 A.L. Tan et al.Nournal of Molecular Structure (Theochem) 393 (1997) 189-196

Z O* N,-0. oyN

40

x 0, .,.., j;o

“““.b?b ..S..,

,:’ s ‘._, ,$I

P.._;PY \Ptqp ,ss‘;

P P P_.;;Pt ‘Pt;;;_.p

1 2 P P

3

Cl 1 +

&Cl GaC14- Cl Cl

‘&Cl

5 6

Fig. 1. Structural representation of the intermetallic complexes formed from Ptz(PPh&,(p-S)z (1) with TI(I) (Z), Pb(II) (3 and 4), Ga(III) (5) and

In(II1) (6). (For clarity, PPh? is represented by P.)

(3h PF6 (4)) PI> where one nitrate ligand is coordinated to Pb, and X is weakly bound (Fig. 1). These structural peculiarities are difficult to explain based on the crystallographic and spectro- scopic data.

The experimental study was extended to In(III) and

Ga(II1) [3]. In (PPh3)4Pt2(~3-S)21nCl~ (5), the coordi- nation geometry of In is square pyramidal; most adduct complexes of InC13 are trigonal bipyramidal. This contrasts with the tetrahedral Ga found in the ion-pair [(PPh3)4Pt2(~3-S)2GaC121[GaC141 (6). Another unexpected feature is0 the strength of the axial In-Cl bond (2.131(5) A) compared to the basal counterparts (2.666(7) and 2.528(7) A). Such a large disparity is not found in square-pyramidal InCl:-

[71. The dihedral angle between the two platinum

coordination planes is also a mystery. Although both

planar [8] and hinged [9] forms in similar Pt(I1) (and other d* complexes) dimers are well established, the electronic and steric importance of the dihedral angle is a subject of much speculation [lo]. Recent ab initio calculations [l I] on 1 and its mono- and dialkylated derivatives demonstrated the possibility of obtaining optimized geometries consistent with most experimental data; an electronic rationale,

namely S-S antibonding interactions, was also proposed.’

In view of the current interest in the structure and bonding of interpolymetallics [ 121 and ligand-bridged dinuclear complexes [13], it is pertinent to gain a better insight into these unusual structural character- istics. We herein report some findings of our theore- tical calculations on these complexes using the Fenske-Hall method.

2. Details of calculations

The models of the molecules used for the calcula- tions are based on their crystal structures [6]. All the phenyl rings on the phosphine ligands were replaced by hydrogen atoms, and the P-H bond lengths adjusted to the sum of covalent radii. The models were aligned such that the S..S line is parallel to the

y-axis and S-M-S (M = Tl, Pb, In, Ga) is parallel to

’ However, the latter is only marginally relevant to the present work since we are interested in the structural trend whereby the dihedral angle increases with the size of the heterometal. Calculated S-S overlap populations did not show any correlation with the above-mentioned trend.

A.L. Tan et al./Jmrnal of Molecular Structure (Theochem) 393 (1997) 189-196 191

the yz-plane, such that the Pt...Pt line is approximately parallel to the x-axis.

Calculations (other than those described in Section 3.3) were performed using the self-consistent Fenske- Hall approximate molecular orbital method [ 141. The

basis functions were obtained by fitting the results of Xa! (Herman-Skillman) [ 151 calculations to Slater orbitals; [ 161 double-c functions were used for transi- tion metal d orbitals and main-group p orbitals. An exponent of 1.2 was used for hydrogen. Mulliken population analyses [ 171 were employed to calculate atomic charges and overlap populations.

Models for the hypothetical compounds [LTlN03],

W’WOhl and [LGaCld CL = Ptdp-ShWhdd, relevant to Section 3.3, were constructed as follows: the LM”+ cores of 2, 3 and 5 were retained. NO; was added to LTl+ based on the geometry of the ligand in

4. LPb(NOj)z was based on the structure of 3 with Pb-S bond lengths equalized, with the bound nitrate copied and rotated 180” about the z-axis. LGaC13 was based on the structure of 5. All bond lengths were adjusted based on comparison of M-S bond lengths with those of the relevant analog.

Ab initio calculations relevant to Section 3.3 were done at the Hartree-Fock level with GAUSSIAN 94 [ 181 using the LANLlDZ [ 191 basis set. Binding energies were calculated by subtracting the energies of the

heterometal fragment and [Pt,(p-S)2(PPh3)2] (calcu- lated separately) from the energy of the adduct.

3. Results and discussion

3.1. Structural aspects of Tl and Pb compounds

3.1.1. The lone pair

Since Tl+ and Pb2+ both possess two valence elec- trons, a lone pair is expected to be present in the Pt,S,

adducts. Calculations on [(PH3)4Pt&j-S)2Tl]+ and the isoelectronic [(PH3)qPt2(pL-S)2Pb]2+ (based on the structure of 4) reveal that the lone pair constitutes the highest occupied molecular orbital, and is oriented in the z-direction tram to the two sulfur donors. The character of the lone pair is listed in Table 1. The

greater total contribution from orbitals on Tl can be explained by the M-S antibonding nature of the lone pair: since Pb-S bonds are more covalent, the extent of mixing between the Pb and S orbitals in bonding

Table 1

Character (%J) of the lone pair orbital in [(PHj),Pt2(pq-S)2M], M =

Tl+ (2), Pb’+ (4)

Orbital Tl Pb

s 54.1 39.0

P; 24.4 28.6

Total 79.0 68.4

molecular orbitals is greater, and hence the lone pair orbital has higher S and correspondingly less Pb character.

When other ligands are coordinated to Pb (in 3 and 4), the lone pair apparently exerts some influence on the geometry, in that all NO3 and PFC moieties avoid the z-direction. Not unexpectedly, coordination of nitrate alters the character of the lone pair: in 4, the associated MO becomes 35.4% s, 3.9% p*, 3.0% p, and 12.1% p_.

3.1.2. Coordination of nitrate to Pb

In the lead compound (without NO3 or PF& the lowest unoccupied molecular orbital is 73.4% pr, while the LUM02 is 40.2% pY and 20.4% dYZ. When Pb formally accepts a nitrate ligand in 3 and 4, one of the oxygen atoms is bound approximately in the x- direction, while the other Pb-0 bond is roughly in the yz-direction, i.e. reflecting the character of the two LUMOs. The oxygen along the x-axis uses the Pb d_x:_yZ in addition to the P.~; hence in principle Pb may be able to accept another ligand bound along the x-axis. However, since the LUM02 has already been

used, the second nitrate in 3 cannot bind in a geometry similar to that of the first nitrate, which could account for the former being weakly bound [ 181.

3.1.3. Competition for interactions

In both 3 and 4, it is interesting to note that one of the Pb-S bonds is longer than the other (6 = 0.052 A for NOI; 0.125 A for PF;). A closer inspection of the structures did not reveal any steric reasons for this asymmetry. To discern any electronic effects, calcu- lations were carried out on [(PH3)qPt2(p3-S)2Pb]2+ (based on 4) with and without nitrate; the Pb-S over- lap populations obtained (Table 2) demonstrate that

coordination of nitrate causes the longer Pb-S bond to be weakened more than the other. This asymmetry can be rationalized in terms of one of the oxygen atoms

192 A.L. Tan et al./Journal of Molecular Structure (Theochem) 393 (1997) 189-196

Table 2 Table 3

For [(PHj)4Pt2(n.1-S),Pb(NO?)lt (4), Pb-S overlap populations

with and without nitrate ligand” For (PH3)4Pt2(~?-S)21nCI? (5), ?r and total overlap populations

between In and the apical Cl atomd

Pb Without nitrate

Sl S2

With nitrate

Si s2

In

P1 0.08726 0.09807 0.07550 0.07565

d,; 0.03 109 0.02977 0.03640 0.03939

Total 0.26898 0.23048 0.243 16 0.19213

’ Note that the Pb-Sl distance is shorter than Pb-S2.

PI d,;

P> d,: All

* The local coordinate axes for both atoms have been set such that

the z-axis is along the In-Cl bond. (Note that this is the only in-

stance in this paper where the local coordinate system is changed.)

being approximately trans to the longer Pb-S bond, and the relevant 0 and S atoms thus utilize similar Pb orbitals (p,, d,,) for interaction. Since orbitals must be “conserved’ ‘, competition for these orbitals results, i.e. the Pb-0 interaction is forged at the partial expense of the Pb-S bond. When the calculation was repeated with Pb and NO3 moved such that the two Pb-S bonds are of equal length, a similar result, i.e. higher reduction in the overlap population of the trans Pb-S bond (15.1% vs. 10.0%) was obtained; thus the foregoing result is not an artifact of the higher sensitivity of the longer Pb-S bond.

phosphines may also account for the versatility of the Pt,S, moiety as a ligand.

3.2. Square-based pyramidal ln(lU)

Building on the concept of competition for inter- actions, one would also expect that Pb and NO; bind- ing would affect the Pt-S and indirectly the Pt-P

bonds. Based on the structure of 4 (all without PFJ for the three cases (i) without Pb or NO;, (ii) with Pb but without NO;, and (iii) with both Pb and NO;, the average Pt-S overlap populations are 0.2563, 0.2458 and 0.2547 respectively. While the changes are fairly small, they suggest a slight weakening of the Pt-S bonds when the sulfur ligands bind to Pb, and since coordination of nitrate weakens the Pb-S bonds, much of the Pt-S overlap population is regained in the process. More telling are the effects on the phosphine ligands: the average P charges of +0.391, +0.5 18 and +0.488 for the three respective cases indi- cate that the polarizable phosphines can donate and accept electron density as the situation demands (when the S atoms coordinate to Pb there is a flow of electron density toward Pb; when NO; coordinates to Pb the flow is in the opposite direction). This suggests that the phosphines absorb a significant pro- portion of the effects of adduct formation, and thus may be responsible for the relatively small variation in Pt-S overlap populations. This behavior of the

Calculations indicate pa-pn and d7r-pa bonding between In and the apical Cl atom. The relevant over- lap populations are listed in Table 3. The percentage of the total overlap population attributable to x-bond- ing, 43.6%, is high, given the significantly smaller magnitude of a-overlap compared to u-overlap. One other Cl atom also participates somewhat in a-bond- ing, although to a significantly lesser extent. The sul- fur atoms do not participate because the necessary orbitals are used in bonding to platinum. The amount

of r-bonding in each atom at the base of the square pyramid is reflected in the apical Cl-In-X angles (X = Cl: 103.0”, 109.4”; X = S: 110.2”, 119.7”), in that an angle closer to 90” indicates more r-bonding.

The likely reason for strong In-axial Cl r-bonding is that since the sulfur orbitals cannot n-bond to In, two of the In d orbitals which could have been used

for r-bonding to the sulfur atoms also have the correct geometry to r-bond with the apical Cl (see Fig. 2). The strong dr-pn interaction which ensues is rein- forced by pr-pr interaction made possible by distor- tion of the square pyramid. (The distortion is probably

Fig. 2. dn-pa bonding in one of the bisecting planes of the square

pyramid of 6.

Cl Overlap population

PC 0.04290

Pr 0.06607

PV 0.04423

Pb 0.06656

All 0.50420

A.L. Tan em al.Nournal of Molecular Structure (Theochem) 393 (1997) 189-196 193

caused by lack of n-bonding to sulfur, and absence of

a ligand or lone pair at the sixth coordination site). Such interactions result in the unusually short In-Cl bond

length and, along with moderate r-bonding in one of the other Cl atoms, probably accounts for the stability of the square-pyramidal coordination geometry.*

3.3. Binding of additional ligands

In this section we address the issues pertaining to the non-binding of additional ligands to the hetero- metal in 2, 3, 4 and 6, i.e. (i) why Tl’ in 2 is coordi- natively exposed; (ii) why only one NO; is formally coordinated (in an asymmetric manner) to Pb(I1) in 3 and 4; and (iii) why Ga(III) in 6 is four-coordinate and not five-coordinate like In in 5. The calculations in Section 3.1.3 demonstrated that coordination of ligands to the heterometal weakens the M-S bonds. For hypothetical compounds in which additional ligands were coordinated to M (see details of calcu- lations for how these were constructed), the calculated (Fenske-Hall) overlap populations similarly decreased (see Table 4). However, there was appar- ently no consistent trend that may account for the non- existence of the hypothetical compounds.’

To resolve the matter, we decided to use ab initio [18] methods to calculate the binding energies of the metalloligand 1 to the various heterometal fragments, for which both the coordinated and loosely bound ligands are included. The results are listed in Table 5. It should be noted that there is apparently no variant of the LANLlDZ [19] basis set (one of two in which sets for Pt, Tl and Pb are defined, the other being LANLl MB) which includes d orbitals for the hetero- metal. Fenske-Hall calculations indicate that the heterometal uses its d orbitals in all the compounds. Nevertheless, the absence of these functions is consistent in all the calculations, and thus the latter should be of some value. The results indicate that for all the adducts that have been isolated, the binding energy is above a certain cut-off (about 0.1 a.u. for this set of calculations). It can thus be inferred that

2 It should be mentioned that the square-pyramidal geometry of InCl:. can also be attributed to r-bonding. However, since all the Cl atoms can x-bond, the disparity in the In-Cl bond lengths is less severe.

’ Note that in the hypothetical Ga analogue of 5, the bonding is similar, i.e. T-bonding to the axial Cl is also present.

Table 4

Calculated (Fenske-Hall) M-S overlap populations for different

heterometal fragments bound to Pt(PHJ),(p-S);

Fragment Overlap Fragment Overlap population population

Tl+ 0.1686 TINO, 0.1660 0.1686 0.1524

[Pb(NOj)]NOI 0.1925 Pb(NOJ)l 0.2207 0.2284 0.2209

GaCI; 0.2997 GaCI, 0.2364 0.2975 0.2410

InCl ; 0.2408 InCl , 0.1923 0.2408 0.1901

a The second of each fragment pair has an additional ligand co-

ordinated to the heterometal.

M-S bonding is an important factor in the hetero- metal being able to accept additional ligands.

3.4. The dihedral angle

Based on the crystallographic data of the inter- metallics, the Pt-S-S-Pt dihedral angle increases in the order Ga < In < Pb < Tl, i.e. according to the size of the heterometal (Table 6). To explain this trend, calculations were carried out on (PH3)4Pt2(p-S)2 based on the structures of the Ga, In, Pb and Tl com-

pounds, as well as a compound with a flat Pt,S2 core. The results demonstrate the dependence of the orien- tation of the two sulfur lone pair orbitals on the dihe- dral angle (see Table 7): it is observed that in the flat structure, the sulfur contribution to the lone pair orbi- tals is purely pz; some pY surfaces as the dihedral angle moves away from 180”, the py contribution increasing as the dihedral angle becomes smaller. The lone pairs

Table 5

Calculated (ab imtio) binding energies of Pt(PH,),(p-S)Z to various

heterometal fragments”

Fragment Binding energy/a.u.

Tl’ O.Il2807

TIN03 0.010786

[WN0~)1N0~ 0.109168

Pb(NOI)z 0.057882

GaCI; 0.3 I 1569

GaCll 0.091826

InCl 3 0.145891

a The LANLI DZ basis set does not contain P and heterometal d

orbitals.

194 A.L. Tan et d/Journal of Molecular Structure (Theochrm) 393 (1997) 189-196

Table 6 Structural details of the Pt&-based intermetallics

Compound LPt-S-S-Ptideg LM-S-Sldeg M-S/A S...SlA

Ga 123.1 48.0 2.296 3.066

48.1 2.292

In 128.3 54.6 2.614 3.030

54.6 2.614

Pb (NO?) 132.1 55.0 2.693 3.061

56.5 2.743

Pb (PF6) 133.5 53.5 2.463 3.072

57.3 2.761

TI 135.7 55.6 2.764 3.127

55.6 2.764

are directed “inwards”, i.e. toward where the hetero- metal would be situated if it were present. (One of the molecular orbitals is an additive combination of the two lone pairs while the other is a subtractive

combination). The relationship of the above result to the observed

structural trend is as follows: since the variation in S.3 distance is relatively small (but there is no clear trend with heterometal size), if M is symmetri-

cally disposed with respect to the two sulfur atoms, as with Ga, In and Tl, then the M-S-S angles increase with the size of the heterometal (Table 6). Leaving the Pb compounds aside for the moment, the M-S-S angles determine the “optimal” combination of sulfur orbitals used in the bonding to M. For example, in the thallium compound, the M-S-S angle is the largest of the three; this necessitates a larger

proportion of sulfur pC relative to pJ (refer to details of alignment) being used to interact with Tl. Because orbitals must be “conserved”, such an orientation of the sulfur lone pairs can only be achieved if less S pZ is used to bind to platinum, i.e. if the dihedral angle opens up.

In extending the argument to the Pb compounds,

one must take the unequal Pb-S bond lengths into consideration. The larger M-S-S angle is associated with the shorter (stronger) Pb-S bond. Both M-S-S angles are expected to influence the dihedral angle; in view of the asymmetric weakening of the Pb-S bonds described earlier, it is reasonable to expect the Pt2S2 moiety to compensate for the Pb-S bond that is affected to a larger extent, i.e. the weaker bond may have an influence on the dihedral angle comparable to the stronger bond.

Table 7

Per cent contribution from sulfur p, and p_ orbitals in the two lone pair HOMOs of Pt2(PH3),(p-S)L, based on the structures of the Ga (6), In (5). Pb(PF,) (4) and Tl (2) compounds, as well as one with a flat Pt?S? core based on the Tl compound but with the Tl removed

Compound HOMO2

S PI S P;

HOMO

S P> S P:

Ga 7.84 30.23 19.66 26.69

8.04 31.50 18.99 25.70 In 7.06 30.50 17.44 27.87

7.07 30.5 I 17.44 27.87 Tl 4.99 32.80 12.83 31.95

4.99 32.79 12.83 31.95 Pb (PF,) 5.66 29.65 14.76 31.82

5.75 33.26 13.93 28.82 (PH~)~P~z(cL-S)Z (Flat) 0.0 37.20 0.0 44.00

0.0 37.19 0.0 43.97

A.L. Tan et al./Journal of Molecular Structure (Throchem) 393 (1997) 189-196 195

The above results have implications for all Pt,S,

(and Pd2S2) based compounds with substituents attached to the sulfur ligands. First, if a heteroatom

M is bound to both sulfur atoms, the Pt,S, moiety must be hinged. Second, if M is attached to one sulfur atom only, the lone pairs (and orbital conservation) impose some constraints on the position of M with respect to the Pt,S, core. Finally, since the M-S and Pt-S bonds share sulfur p, and p_ only when hinged, in this geometry the M-S and Pt-S bonds are directly coupled. This coupling was first pointed out qualitatively by Padilla et al. [20].

4. Summary

It has been shown, using 4 as an example, that binding of the heterometal M to Pt2(PPhj)&j-S)* results in weaker Pt-S bonds; coordination of addi- tional ligands to M renders the M-S bonds weaker but the Pt-S stronger. The possible role of the polarizable phosphine ligands has also been demonstrated. Any ligand bound trans to a particular M-S bond will cause the latter to be weakened more than the other M-S bond. The general M-S weakening upon ligand coordination, as supported by binding energy calcula- tions, may explain the lower coordination numbers of

the Tl, Pb and Ga compounds. The calculations indicate p*-pa and dr-pa

bonding between In and the Cl atoms in (PPh3)sPt2(~3-S)zlnCl~, notably with the Cl at the apex of the square pyramid. The latter is undoubtedly due to the inability of the sulfur atoms to engage in a-bonding. The a-bonding probably accounts for the stability of the square-pyramidal coordination

geometry. Finally, the S-M-S angle determines the “optimal”

combination of sulfur orbitals with which M interacts. It has been shown that the Pt-S-S-Pt dihedral angle governs the orientation of the lone pair in a manner consistent with the observed trend of increasing dihedral

angle with increasing heterometal size.

Acknowledgements

A.L.T. would like to thank the National Super- computing Research Centre for use of its SGI Onyx

computer and MS1 Cerius2 software, and the National

Science and Technology Board for financial support in the form of a Postdoctoral Fellowship. T.S.A.H.

acknowledges the university for financial support (RP950695) and Y.P. Leong for stenographic help.

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