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We have studied the metallosite selectivity of substitution reactions at heterometallic tetranuclear clusters of the type HMCo3(CO)12 (M = Fe, Ru). Monosubstitution with PCyH2 occurs with a different metalloselectivity as a function of M. When M = Fe, substitution of a Co-bound CO ligand occurs whereas when M = Ru, the phosphine ligand is bound to Ru. Introduction of a second substituent (PCyH2 or NMe3) occurs in both cases at cobalt and, in the case of HFeCo3(CO)11(PCyH2), at a cobalt that does not carry the PCyH2 substituent. The clusters HMCo3(CO)11(PCyH2) (2a, M = Fe; 2b, M = Ru) transform in solution to give the corresponding μ3-phosphinidene-capped heterotrinuclear clusters MCo2(μ3-PCy)(CO)9 (4a, M = Fe; 4b, M = Ru). The μ4-phosphinidene-capped intermediate RuCo4(μ4-PCy)(μ-CO)2(CO)11 (5b) could be fully characterized, whereas the analogous species could not be isolated when M = Fe. The transformation of 2a,b was accelerated by addition of Me3NO. This work demonstrates that a reaction which represents a partial cluster fragmentation, with a nuclearity change from 4 to 3, may occur via the intermediacy of a larger cluster, of nuclearity 5. Reactions and products were studied by IR and 1H, 31P{1H}, and 59Co NMR spectroscopic methods, and the clusters 4b and 5b have been characterized by X-ray diffraction.

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