The kinetics of reactions of the high-nuclearity carbonyl cluster (HNCC), Ru5C(CO)(15), with 21 P-donor nucleophiles, L, to form Ru5C(CO)(14)L have been studied. The nucleophiles were chosen such that their electronic (pK(a)’ = -2.79 to 12.20) and steric (Tolman cone angles, theta = 101-182 degrees) properties are systematically varied. With 10 smaller nucleophiles (theta less than or equal to 133 degrees) the reactions occur via two well-separated steps : adduct formation and CO-dissociation from the adducts to form the monosubstituted products. The structures of the adducts formed are shown spectroscopically to be closely related to others reported and structurally characterized elsewhere. The rate equations for the two steps are effectively k(obs) = k(+L)[L] and k(obs) = k-(CO), respectively. With 11 larger nucleophiles (theta greater than or equal to 136 degrees) the reaction is a quite different, second-order one-step, process with no spectral evidence for adduct formation being observed. Quantitative analysis of the dependence of the various rate constants on the electronic and steric properties of the nucleophiles or ligands involved shows that adduct formation of this HNCC with the group of smaller nucleophiles is much more facile than any comparable nucleophile-dependent reactions of other metal carbonyls. The rates of loss of CO from the adducts are decreased both by increasing net electron donicity of the ligands involved and by increasing the size of those ligands. The rate constants for the single-step reactions with the group of larger P-donors depend on the latters’ electronic and steric properties in a way showing that major expansion of the cluster is required to form the transition states and this is only possible by virtue of an exceptionally high degree of Ru-nucleophile bond-making. The flexibility of the transition state, once formed, is exceptionally low.