The reaction of alkyl-substituted (h5-CpR)(CO)2 complexes with cobalt or nickel atoms in the presence of CpH or an arene has been shown to give homo- and heteronuclear M2- and M3-bridged metal carbonyl clusters. The crystal structures of the two homonuclear species obtained in the metal atom reactions, namely [(h6-mesitylene)Co]2Co(h5-CpR)(m3-CO)2 with R = Me5 or tBu2, have been detd. The electrochem. behavior of the clusters [(h6-mesitylene)Co]2Co(h5-CpR)(m3-CO)2 (R = Me5, tBu2,iPr2) was studied by cyclic voltammetry and the results are discussed in detail. The reaction of (h5-CpMe)Mn(CO)3 with nickel atoms and CpH does not give heteronuclear Mn-Ni cluster compds., but instead the compd. [(h5-Cp)Ni]2Ni(h5-Cp)(m3-CO)2.