Compounds with Bond Order Two

Most of the compounds with metal-metal double bonding do not arise directly from the analogue of higher bond order discussed above. Thus for instance in the chemistry of rhenium compounds the best known and also most interesting species are the Re (III) trinuclear clusters that will be discussed in the next chapter. In this sense the chemistry of compounds with metal-metal double bonds appears as a transition from the chemistry of dinuclear metal compounds to that of higher metal aggregates. 

Group 5 elements in an oxidation state -f 3 form a series of dinuclear compounds which, according to their metal-metal internuclear distances and paramagnetic behavior, correspond to species with a double metal-metal bond and the electron configuration a n. These 7r-orbitals are not always degenerate. Thus for instance, the compound Ta2Cl6(THF)3 (Fig. 1.42) possesses a Ta-Ta internuclear distance corresponding to a double bond but it shows however 

The compound of ruthenium(II) with azulene RU2L2 constitutes an example of electron-rich species with a metal-metal double bond. Thus for this compound, which has the same structure as its oxidized derivatives (Ru2L2)BF4 and (Ru2L2)(BF4)2, an interatomic distance of 2.379 A agreeing with a double metal-metal bond and an electron configuration is observed. 

The number of organometallic compounds with a double metal-metal bond is notoriously higher than those with higher bond orders. This feature agrees with the relatively lower oxidation states of the metal in these compounds and it also points out to the idea of a transition toward the behavior observed in cluster compounds. 

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