Skeletal electron counting rules

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... 

Use ihe polyhedral skeletal electron-counting rules and show that they are consistent with the nido I l-vencx structure shown below.1 1... 

Whereas in ligand bridged dinuclear complexes, removal or addition of two electrons makes or breaks one metal-metal bond (15) this does not seem to be the case for clusters, presumably because of their delocalized bonding. At least for one case, however, two-electron reduction can induce a significant change in cluster shape (18,42) the 84-electron cluster Os6(CO),g with framework 1 is easily reduced to the 86-electron anion Os6(CO) g with framework 2, in accordance with skeletal electron counting rules. 

In essence the alkyne has inserted across a metal-metal bond to give a closo-FeRu3C2 cluster. Such a structure is fully consistent with Wade s skeletal electron counting rules for a closo structure with 6 vertices (23,24). The three isomers of FeRu3(CO)i2(lfeCECPh) arise because in the equatorial isomer the methyl substituent can occupy a position cis or trans to the Fe atom. 

Give an account of the basis and use of the skeletal electron counting rules for clusters. 

The polyhedral skeletal electron-counting rules do have some theoretical underpinnings as outlined in Section 22.2. This is a simple and very useful model to rationalize molecular structures that cannot be framed within the standard two-electron two-center paradigm. Similar to the valence shell electron repulsion model and the isolobal analogy there are exceptions and these exceptions are quite interesting in and of themselves. We shall restrict our coverage here to the most common patterns where Wade s rules predict a different shape or instances where the molecules are not predicted to be stable. 

This chapter summarizes recent developments in the expanding field of electron-deficient compounds having from three up to 13 skeletal boron and carbon atoms. In particular, the focus will be on the transition of classical organoboranes into non-classical compounds. Therefore, we first want to briefly review electron counting rules and bonding characteristics of these classes. For a more thorough discussion see Chapter 1 by King and Schleyer. 

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

Wade electron counting rules borane-like cluster nomenclature. On initially studying compounds such as boranes (boron hydrides) and carboranes (or carbaboranes boron—carbon hydrides), Wade (1976) proposed a number of rules which have then been extended to several compounds and which relate the number of skeletal electrons with the structure of deltahedral clusters. A polyhedron which has only A-shaped, that is triangular, faces is also called a deltahedron. 

In both [Fe4X4(N0)4](X = S or Se) and [Fe4S2(NO)4(NCMe3)2] the total valence electron count is 60. This is the number characteristic of tetrahedral tetranuclear metal clusters, such as [Ir4(CO)12], in the Wade and Mingos skeletal-electron counting schemes (76, 77) and, furthermore, each iron atom in these clusters obeys the 18-electron rule, provided that it forms single Fe-Fe bonds to each of the other iron atoms in the tetrahedron. 

Historically, a scheme of skeletal electron-counting was developed to rationalize the structures of boranes and their derivatives, to which the following Wade s rules are applicable. 

First some simple calculational examples are presented to illustrate the changes in skeletal bonding as protons are moved about as well as to make connections with the electron counting rules. This is followed by a contemporary example illustrating how attempts to reconcile differences between molecules that are isoelectronic only in the sense of the electron count-... 

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