Tetrakis (trephenylphosphine) Palladium (O) [Pd(PPh(3))(4)] and cis 1,6-bis(phenyl) hexa-1,5-diyn-3-ene (PhC=C-CH=CH-C=CPh):[L] have been synthesized and characterized. Addition of the diacetylene across the metal-metal bonds of previously prepared complexes of the general formula [Cp(2)M(2)(CO)(4)] (M=Mo, W) in toluene or dichloromethane at room temperature form 1:1 and 1:2 adducts of stable complexes (a-d). The IR of these complexes shows a semi-bridging carbonyl. The simplicity of the NMR spectra of H-1 and C-13{H-1} of these complexes shows a rapid interconvertion between CO groups. In all of the complexes, the alkyne bridges the two metal atoms, forming a quasi-tetrahedral M(2)C(2) moiety. The metal d-orbitals as well as an acceptor from carbonyl group ligands, act as donors to the carbonyl *pi-acceptor orbitals in a back-bonding fashion. The metallated products have been characterized by means of analytical data, IR, H-1, C-13{H-1}NMR, and mass spectroscopy.