Skeletal electrons

Os4Pd6(CO)8(//-CO)8(/x-dppm)2] in low yields, while the reaction of [Os5(/u5-C)(CO)i5] with [Pd2(/u-dppm)2Cl2] afforded [Os5Pd4(/u6-C)(CO)12(/u-CO)3(/u-dppm)2] and [Os5(/X5-C)(CO)13(/r-dppm)] in moderate yields.289 The electron counts found in these osmium-palladium clusters do not always agree with those predicted by skeletal electron counting rules. This may simply be ascribed to the ability of Pd to be satisfied with both 16- and 18-electron counts.289... 

T. Baker, DuPont Central Research In reference to your so-called iso-closo iridaborane complex, H(PPh3)2IrB9H9, we have recently published two papers (1,2) dealing with isoelec-tronic, isostructural ten-vertex ruthenacarborane complexes and have demonstrated that these structures are related to the common closo bicapped square antiprismatic structure by the removal of two electrons (i.e. 2N skeletal electrons for an N vertex polyhedron). Such complexes have been referred to as hyper-closo to imply that the electronic unsaturation is not primarily metal-based (as in, for example, nido-(PPh3)2RhC2B8-Hx 2 (3) or closo-(PPh3)ClRh(1,7-C,BQH,) (4), but is delocal-... 

Polyhedral Skeletal Electron Pair Theory The Wade-Mingos Rules... 

The next important contribution in this area was made shortly thereafter by Wade [16], who recognized that this structural relationship could be related to the number of valence electrons associated with skeletal bonding in the boranes. Thus deprotonation of all of the bridging hydrogens from the related series of boranes B H 2T B iH(n 1)+4, and Bn 2H( 2)+6 gives the ions B H 2 , B H,- 4-, and B -2H 26-. All of these ions can readily be seen to have the same number of skeletal electron pairs, namely it +1, corresponding to 2it + 2 skeletal electrons. 

Balakrishnarajan and Jemmis [32, 33] have very recently extended the Wade-Mingos rules from isolated borane deltahedra to fused borane ("conjuncto ) delta -hedra. They arrive at the requirement of n I m skeletal electron pairs corresponding to 2n + 2m skeletal electrons for such fused deltahedra having n total vertices and m individual deltahedra. Note that for a single deltahedron (i.e., m = 1) the Jemmis 2n + 2m rule reduces to the Wade-Mingos 2n I 2 rule. 

Rules for counting the number of skeletal electrons provided by each vertex atom need to be established in order to determine the number of skeletal electrons in polygonal and polyhedral clusters of the post-transition elements. The rules discussed above for polyhedral boranes can be adapted to bare post-transition metal vertices as follows ... 

Application of this procedure to post-transition metal clusters indicates that bare Ga, In, and Tl vertices contribute one skeletal electron bare Ge, Sn, and Pb vertices contribute two skeletal electrons bare As, Sb, and Bi vertices contribute three skeletal electrons and bare Se and Te vertices contribute four skeletal electrons in two- and three-dimensional aromatic systems (see Chapter 1.1.3). Thus, Ge, Sn, and Pb vertices are isoelectronic with BH vertices and As, Sb, and Bi vertices are isoelectronic with CH vertices. 

Square. Bi42-, Se42+, and Te42+ are isoelectronic and isolobal with the delocalized planar cyclobutadiene dianions. There are 14 skeletal electrons [e.g., for Bi42- (4)(3) + 2 = 14] corresponding to 8 electrons for the 4 c-bonds and 6 electrons for the 71-bonding (see Chapter 2.7.2.1). 

Butterfly. While Tl2Te22- has a (2)(1) + (2)(4) + 2 = 12 skeletal electron count isoelectronic and isolobal with neutral cyclobutadiene, it undergoes a different Jahn-Teller-like distortion to the butterfly structure discussed by Bums and Corbett [81]. 

Tetrahedron. Sn2Bi22- and Pb2Sb22 have (2)(2) + (2)(3) + 2 = 12 skeletal electrons for tetrahedra analogous to P4 and organic C4R4 tetrahedrane derivatives (see Section 1.1.3.4). 

Seven-vertex Structures. As73- and Sb73- have the C3v structure depicted in Figure 1-7 and the correct (4) (3) + (3)(1) + 3 = 18 skeletal electron count for... 

Bicapped Square Antiprism. TlSn93 has the (1)(1) + (9)(2) + 3 = 22 = 2n + 2 skeletal electrons required for an n = 10 vertex globally delocalized D4( deltahedron (cf. the bicapped square antiprism in Figure 1-7) analogous to that found in the BioHio2- anion [84]. 

Bare group 13 metal vertices (e.g., Ga, In, Tl) provide, as noted above, only one skeletal electron each to polyhedral cluster structures. Thus it is not surprising that the bare metal cluster ions Enz (E = group 13 element) found in homonuclear alkali-metal/group 13 intermetallic phases [86-89] (mainly for In and Tl) have charges less negative than the — (n + 2) (i.e., z [Pg.21]

Mingos rules. This hypoelectronicity or electron poverty (fewer than the Wade-Mingos 2n + 2 skeletal electrons) in the bare metal cluster anions Enz (z < n + 2) leads to deltahedra not only different from those in the deltahedral boranes but also different from those in hypoelectronic metal carbonyl clusters of metals such as osmium. 

Electron counting in these supraicosahedral gallium clusters consisits of ambiguities since it is not clear which of the bare vertex atoms of the core polyhedra provide the usual three internal orbitals and which vertex atoms provide four internal orbitals. Typically the Wade-Mingos [16-18] or the Jemmis [32, 33] skeletal electron rule is obeyed if about half of the bare vertex gallium atoms use all four orbitals of their sp3 manifolds as internal orbitals, and thus are donors of three skeletal electrons, and the other half of the bare vertex gallium atoms use only three orbitals of their sp3 manifolds and thus are donors of only a single skeletal electron each. 

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