Bonding and Electron Counting

An important consequence of the nonutilization of tangential orbitals is that platinum clusters often do not obey the normal electron counting rules and appear to be electron deficient (19,21,29,58,75,76). Electron counts are usually intermediate between those found in normal transition metal clusters (58-68) and those observed in gold clusters (58,78), but no satisfactory general electron counting theory has been developed for Pt-containing clusters. In small Pt clusters constructed from PtL2 units, theoretical studies have shown that the total electron count depends on the relative orientation of the 

Fe5Pt(/i6-C)(CO)15(PPh3) (12) (see later, Fig. 12, for 12) (82). In all these clusters the coordination geometry about the Pt center is nonplanar, assuming nonbridged Pt-M vectors are included in the coordination sphere. Several observations about electron counts in hetero-Pt clusters can be made  

In tetrahedral clusters the Pt atoms have, of necessity, nonplanar coordination geometries and usually obey SEPT. Exceptions include cluster 1 (40,40a), which is formally electron deficient, and FePt3( -H)(/i-PnPr2)-(/i-CO)2(PfBu2Ph)3, a 56-CVE tetrahedron that is Pt rich (30). On the other hand, butterfly clusters with planar Pt centers give rise to 60 CVEs for clusters with one Pt atom, e.g., 2a (47), and 58 CVEs for those with two Pt atoms, e.g. Os2Pt2(/i-H)2(CO)8(PR3)2 (15) (41,84). 

In high-nuclearity clusters containing deltahedral or condensed polyhedral skeletons, the CVE count predicted by SEPT is often observed. Some unusual core geometries may be found, e.g., in Os6Pt2(/i-H)2(/i-CsH10)(CO)16(COD) (85), which are less straightforward to rationalize. 

EHMO interaction diagram for the hypothetical dianion [Fe3Pt(CO)n]2.  

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B7.11 Structure, bonding and electron counts in cubane-type clusters having M4S4, M2M 2S4 and MM 3S4 cores... 

Metal-Metal Bonding and Electron Counting in Polynuclear Complexes... 

The most comprehensive theoretical insight into the problem of aromaticity of cyclic transition metal systems was discussed in detail in a very recent review article published in Phys Chem Chem Phys by Boldyrev and coworkers [17], In this article the multifold nature of aromaticity, antiaromaticity, and even conflicting aromaticity (a-, Tt- and 8-aromaticity) has been analyzed in terms of the chemical bonding and electron counting rules. Therefore, we will discuss and comment herein on the latest developments in the field of stable aromatic three-membered rings of transition metal atoms. 

Clusters. The bonding and electron counting in polyaluminium clusters analogous to polyboranes and carbaalanes has been reviewed. ... 

Probably the most common detachment step in late transition metal catalyzed processes is reductive elimination. In this transformation two groups, that are both attached to the same metal centre, will be released and form a covalent bond, with the concomitant formation of a metal whose formal oxidation state, coordination number and electron count are decreased by two. Figure 1-9 presents a general order of the ease of reductive elimination for the most common complexes. 

As you become more familiar with transition metal dusters (no nonmetals in the framework) you will come to associate doso structures with numbers of electrons. A trimer will have 48 electrons, a tetrahedron will have 60 electrons, a trigonal bipyramid will have 72 electrons, and an octahedron will have 86. Some care is required, however, as can be illustrated with Os,H2(CO),0. An electron count gives us 46 electrons rather than 48. If, however, we allow for one Os—Os double bond, the electron count is as expected. In accord with this expectation, one osmium-osmium bond is found to be shorter than the other two and the complex shows the reactivity expected for an unsaturated complex. 

The dimeric species. Mn (CO) n and Co COI. also are stable and are diamagnetic. If it is assumed Dial each compound has a metal-metal single Itwiyelectron) bond, the electron count yields a total of 18 for each metal ... 

The types of compounds formed by gold(I) and gold(III) often differ from those of other metals due to the constraints imposed by coordination number and electron count at the metal. Thus, for example, whereas 7r-bonded cyclopentadienyl complexes of palladium and platinum are numerous (336), and a copper(I) species of this type is known (337), cyclopentadienyl complexes of univalent (94, 96, 97) and trivalent (228) gold have invariably been found to be fluxional behavior, similar to that in dicyclopentadienylmercury, was involved (228). 

MLX plots and electron count, 1, 36 MLX plots and ligand bond number, 1, 42 MLX plots and reactivity patterns, 1, 42 MLX plots and valence, 1, 40 multicenter bonding, 1, 32 pi- and delta-backbonding, 1, 29 pi-donation considerations, 1, 29 Covalent character, dn configuration compounds, 1, 20 Covalent interaction studies, and bis(arene)chromium, 5, 345 CPPL, see Circularly polarized photoluminescence Crabtree s catalyst, for iridium dihydride complexes,... 

The tetrahedral intermediate proposed is an attractive one as rupture of one M-M bonding interaction (and some ligand rearrangement) takes one to one isomer or the other depending on the M-M bond chosen. Observe that the cluster HPtOs3(CO)io(dppm) Si(OMe)3 and its isomer have planar butterfly structures and electron counts of 60. The typical butterfly structure is not planar however, bulky ligands can readily flatten the butterfly (see discussion of [Re4(CO)i6]2 in Section 3.2.5). The typical eve count for a butterfly structure is 62 however, with Pt clusters,... 

Situation 4 corresponds to a valence tautomeric rearrangement between the homoconjugative form 42a and the open monocyclic form 42c as already discussed in connection with equation 1. The transition state of the rearrangement may be stabilized by no-bond homoconjugation. This has been discussed in various ways using orbital symmetry and electron counting models (see Section Situation 4 represents an example of an... 

The dimeric species. MnjlCOim hd Co iCOli,. also arc stable and are dianuig-neiic. If tl IS as-sumed that each compound has a meuil-meta) single Itwo-electron) bond, the electron count yields a total oT 18 for each metiil ... 

Structure and bonding in the tellurium subhalides can be well rationalized, either employing valence and electron-counting rules, in-... 

FIGURE 15-10 Bond Order and Electron Count in Dimetal Clusters. (From A. Bino and F. A. Cotton, Inorg. Chem., 1979, 18, 3562 andF. A. Cotton, Chem. Soc. Rev., 1983, 12, 35.)... 

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