Three-centre bonds cluster complexes

A number of electron counting schemes have been developed to rationalize or even predict the metal framework geometries adopted by metal cores in cluster complexes. The simplest of these is the Effective Atomic Number (E.A.N.) rule which is based on the 18 electron rule , and assumes that each metal uses its five d, three p and one s valence orbitals for bonding, and that all metal-metal bonds are two centre - two electron bonds. Clusters which obey this rule are said to be electron precise . However, often there may be a number of different structures, with the same electron count, which conform to these rules and there is no discrimination... 

The requirement that the links somehow follow the molecules of the structure does not necessarily mean that the nodes themselves are centred on the building blocks or parts of them. In Figure 3.16 we see the net formed by the cocrystallisation of Co(III)(2,2 -biimidazole)3 complexes with phthalates. One biimidazole and two phthalates are joined together, and the natural choice in this case is to assign one of the three nodes to the centre of this cluster of hydrogen bonds. 

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