Density functional theory has been used to probe the bonding and electronic properties of the homo- and heterobimetallic sp carbon chain complexes [LmMC4M'L-n',](z+) (MLm, M'L-n', = (eta(5)-C5R5)(eta(2)-R2-PCH2CH2PR2)Fe, (eta(5)-C5R5)(NO)(PR3)Re; z = 0-4). All neutral complexes are best described by MCequivalent toCCequivalent toCM electronic structures, in accord with much experimental data. The singlet dications are best described by cumulenic M+=C=C=C=C=M+ valence formulations. However, the diiron and rhenium/iron dications are found to possess triplet states of nearly identical energy, clarifying experimental magnetic data. Their electronic structures have dominant (.+)MCequivalent toCCequivalent toCM(+.) character, with some spin delocalization onto the carbon chain. The mixed valence monocation radicals exhibit delocalized unpaired electrons, in accord with class III (strongly coupled) and II (weakly coupled) assignments made from experimental data earlier, with some spin density on the carbon chain. An isolable diiron trication has a doublet ground state, but some computational data suggest a close-lying quartet. For the unknown diiron tetracation, a bis(carbyne) or (2+)Feequivalent toCCequivalent toCCequivalent toFe(2+) electronic structure is predicted. Calculated adiabatic ionization potentials show the iron endgroup to be more electron-releasing than rhenium, in accord with electrochemical data. This polarizes the electronic structures of the rhenium/iron complexes. To help validate the computed model structures, crystal structures of {(eta(5)-C5Me5)Fe(eta(2)-dppe)}(2)(mu-C-4) and [{(eta(5)-C5Me5)Fe(eta(2)-dippe)}(2)(mu-C-4)](3+) 3PF(6)(-) are determined. Data are analyzed with respect to related diruthenium and climanganese complexes.