Bonding and electronic structure in consanguineous and conjugal iron and rhenium sp carbon chain complexes [MC4M'](n)+: computational analyses of the effect of the metal.

Density functional theory has been used to probe the bonding and electronic properties of the homo- and heterobimetallic sp carbon chain complexes (ML(m), = (eta(5)-C(5)R(5))(eta(2)-R(2)PCH(2)CH(2)PR(2))Fe, (eta(5)-C(5)R(5))(NO)(PR(3))Re; z = 0-4). All neutral complexes are best described by MCtbd1;CCtbd1;CM electronic structures, in accord with much experimental data. The singlet dications are best described by cumulenic (+)M=C=C=C=C=M(+) valence formulations.