Electron counting borane cluster compounds, 364

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. 

Finally, just as BH3 coordinates to CO, so do a number of the higher borane cages. For example, B2H4(CO)2 (63), B3H7CO (64), B4H8CO (65), and BioHio(CO)2 (66) are all well characterized compounds. Because each boron atom with a coordinated CO contributes 3 electrons to the cluster count, only a few such exo-ligands can be accommodated. Dispite this, these clusters also express the close relationship between boron and transition metal clusters. 

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 ... 

The electron counting schemes described for boranes can be extended to isoelec-tronic species such as carboranes,22 clusters containing both carbon and boron as framework atoms. The CH+ unit is isoelectronic with BH many compounds are known in which one or more BH groups have been formally replaced by CH+ (or by C, which has the same number of electrons as BH). For example, the replacement of two BH groups in closo-B6H62 with CH+ yields closo-CjB. ... 

The adherence to close-packed structural arrangements lends support to the idea that these compounds can be used as models for metal surface chemistry—with respect to chemisorbed species and their mobility and reactions of substrates on these surfaces. It also indicates a marked deviation from the behavior of boranes and their derivatives. Structures based upon some polyhedra favored by boron, such as the pentagonal bipyramid, triangulated dodecahedron, and especially the icosahedron, are absent so far in metal-carbonyl cluster chemistry. In this connection, it has been mentioned that [M(CO)3],g compounds should be the closest analogs to On skeletal electron counting... 

The compound [(NH)2(CH)2(BMe)2M] M = Cr(CO)3 [171] gives formally a nido-count. However, the hexagonal-pyramidal structure with a benzene-analogous hexagon (=NH-NH=BMe-CH=CH-BMe=) as the pyramidal basis does not present a borane-type cluster structure. The electron-deficiency is not concentrated at boron, but at the metal. 

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