Electron counting, extended

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. 

More elaborate electron-counting procedures have been established, and these are usually more appropriate for the rationalisation of high-nuclearity clusters. An approach which was originally applied to complex boranes has been extended to carbonyl clusters and a number of topological theories have been described. Although there is some way to... 

A division into Wade s systems, belonging the 2n + 2 rule and non-Wade s clusters has been discussed by Struchkov125b with the aim to extend the electron counting procedure to conjuncto-clusters. An additional concept of structural rules for transition metal clusters has recently been presented by Xu125. Lauher126,127 used extended... 

A simple, but nevertheless very interesting electron counting rule for close-packed high nuclearity clusters has been described by Teo127a. The formula N = (mb + rms)n establishes a connection between the total number of electrons (N), the bulk atoms mb, and the surface atoms ms. The rule is based on an extended Hume-Rothery rule and works quite well for all kinds of close-packed clusters up to the Pt38 cluster of Chini61. ... 

Answer. The metal count is 59 (9 x 9 (Co) — 11x2 ([Se]2—) = 59). This is 25 more than for [Mo9Sen]2 Indeed a large gamut of electron counts is possible for these condensed octahedral clusters, either molecular or part of extended solids. They are nice examples of compounds which can display variable electron count with constant shape. This situation is somewhat reminiscent of the transition-metal elements which exhibit a metallic d band which is gradually filled as we go from the left to the right of the periodic table without substantial change in their structure. Also review the discussion of cubic clusters in Section 5.2.5. 

Many carbonyl-type clusters consist of a central polyhedron to which one or more additional metal atoms are appended by placement over a triangular face. These appended metal atoms are called capping atoms. The electron counting rules we have discussed can be extended to cover such structures by the rule that the addition of a capping atom does not change the skeletal electron requirement for the central cluster. Thus, the addition of a capping atom simply increases the total electron count by 12 electrons. 

The electron counting method of choice is a matter of individual preference. Method A includes the formal oxidation state of the metal Method B does not. Method B may be simpler to use for ligands having extended tt systems for example, ligands have an electron count of 5, ligands an electron count of 3, and so on. Because neither description describes the bonding in any real sense, these methods should, like the Lewis electron-dot approach in main group chemistry, be considered primarily electron... 

The electron-counting schemes can be extended to isoelectronic species such as the carboranes (also known as carbaboranes). The CH" unit is isoelectronic with BH many compounds are known in which one or more BH groups have been replaced by CH (or by C, which also has the same number of electrons as BH). For example, replacement of two BH groups by CH" " in yields closo-C- Bayif, a neutral compound. 

Although it may not be surprising that the same set of electron-counting rules can be used to describe satisfactorily such similar compounds as boranes and carboranes, we should examine how far the comparison can be extended. Can Wade s rules, for example, be used effectively on compounds containing metals bonded to boranes or carboranes Can the rules be extended even further to describe the bonding in polyhedral metal clusters ... 

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