The isolobal principle and application of Wades rules

In a condensed polyhedral cluster, two or more polyhedral cages are fused together through atom, edge or face sharing. 

The syntheses of high-nuclearity metal carbonyl clusters are not readily generalized, and we focus only on the structures 

In Section 13.11, we introduced Wade s rules to rationalize the structures of borane and related clusters. This method of counting electrons can be extended to simple organome-tallic clusters by making use of the isolobal relationship between cluster fragments. 

Two cluster fragments are isolobal if they possess the same frontier orbital characteristics same symmetry, same number of electrons available for cluster bonding, and approximately the same energy. 

Os5(CO)i8, in which the Os atoms form a planar raff [W. Wang et al. (1992) J. Chem. Soc., Chem. Commun., p. 1737]. Colour code Os, yellow C, grey O, red. 

Worked example 23.3 Application of Wade s rules (PSEPT) 

Using Wade s approach, rationalize why Co4(CO)j2 has a tetrahedral core. 

The cluster Co2(CO)jC2H2 has a tetrahedral C02C2 core. How many electrons does each CH unit contrihute to cluster bonding lAns. 3  

V = number of valence electrons from the metal atom  

This example illustrates an important point about the use of such electron-counting schemes no information about the positions of the ligands can be obtained. Although Wade s rules rationalize why Rh4(CO) 2 and lr4(CO)i2 both have tetrahedral cores, they say nothing about the fact that the ligand arrangements are different (Figs. 24.11a and b). 

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