Reactions Leading to Structural Changes in Clusters

Such changes may be realized by the reduction or oxidation reactions of clusters. Reduction and oxidation processes involving transition metal compounds are much more common compared to main group compounds. This is particularly true for certain clusters which may be considered as multistep redox systems. Among others, the cubic iron clusters Fe4cp4()U3-S)4 ( = +3, +2, -f 1, 0, —1) and Fe4cp4(/i-CO)4 ( = +2, +1, 0, systems. In these complexes the metal atoms lie at the vertices 

In clusters Co2(CO)6(PhCCPh) and Co3(/z3-CY)(CO)9 where Y = Ph or Cl, substitution of CO ligands by PR 3 groups is initiated by reduction of the cluster. Thermal substitution is very slow. The extra electron is placed in the antibonding molecular orbital formed by the metal-metal interaction. Such reduction causes reversible metal-metal bond breaking with subsequent fast addition of the ligand to the cluster. Oxidation of the complex may occur by electron transfer either to the reagent or to the electrode  

The metal-metal bond breaking may also take place as a result of ligand addition to the cluster, oxidative addition of XY molecule, or as a result of changes in bonding properties of the ligand itself, for instance, conversion of an n electron ligand into an fi 2 electron one, such as j -CO(2e)- f/ -CO(4e) [equations (3.54) and In contrast to mononuclear complexes, oxidative addition of an X —Y 

oxidative addition is accompanied by dissociation of a Lewis base. Thus, in such a case the M —M bond is not broken  

Often the skeletal change occurs after proton addition to the cluster. 

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