The electrochemical behavior. of a series of biscarbyne complexes, Tp＇(CO)(2)M=(CmHn)=M(CO)(2)Tp＇ (Tp＇ = hydridotris(3,5-dimethyl-1-pyrazolyl)borate; M = Mo, W; m = 3, n = 2; m = 4, n = 0, 2, 4), has been studied. These complexes exhibit two oxidation waves between 0 and 0.5 V (vs ferrocene). The difference in E-1/2 for the two oxidation waves is a function of the hydrocarbon spacer with the =C-C=C-C= spacers (m = 4, n = 0) exhibiting considerably larger Delta E-1/2 values than the corresponding =C-CH=CH-C= spacers (m = 4, n = 2), which were in turn more strongly coupled than the saturated analogues (m = 4, n 4). While the trend in electronic coupling was quantitatively as expected for the degree of unsaturation in the bridge, the absolute magnitude of the coupling as indicated by the comproportionation constants (K-c = 10(0)-10(4)) was much smaller than that in isologous dimers where the bridge is connected to the metal centers via single bonds (K-c = 10(8)-10(12)). A qualitative model based on relevant orbital occupancy as a function of metal oxidation state has been developed to account for the experimental results.