The reaction of M₃(CO)₁₂(M = Fe, Ru, or Os) with M′(CO)₅^–(M′= Mn or Re) in different solvents and at different temperatures has been studied. The reactions are exceedingly complex and the nature of the products obtained depends critically on the experimental conditions. Isolation of individual anionic species from the complex mixtures formed is difficult, and it has only proved possible to obtain three such species, Me₄N⁺M′Os₂(CO)₁₂^– and Me₄N⁺ReRu₃(CO)₁₆^– in an analytically pure condition. In addition to these, good evidence has been obtained for the formation of Et₄N⁺HRu₃(CO)₁₁^–. Acidification of the reaction mixtures obtained under various conditions leads to the formation of neutral hydride-containing species, and these may be separated more readily. Apart from a number of previously characterised products the following new complexes have been prepared in this way: HM′Os₂(CO)₁₂, HM′Os₃(CO)₁₆, HReOs₃(CO)₁₅, H₃M′Os₃(CO)₁₃, H₂Ru₆(CO)₁₈, H₂Re₂Ru₂(CO)₁₆ and H₂Re₂Ru(CO)₁₂. From the nature of the products obtained in these reactions it is possible to deduce some of the reaction paths which are probably involved in the formation of polynuclear carbonyl complexes by this route.