Abstract: ABSTRACT: A series of transition metal carbonyls and binuclear cyclopentadienylmetal carbonyls were studied theoretically. The studies show that some of the early experimental conclusion about these complexes are suspicious. The bonding rules and the structure character of transition metal carbonyls from same group metals were also studied. For unsaturated first row binuclear cyclopentadienylmetal carbonyls, it seems that structures with metal-metal bonds are preferred over structures with four-electron donor bridging carbonyl groups except for highly unsaturated derivatives of the early transition metal vanadium.