The electrochemistry and spectroelectrochemistry of the substituted group V carbonyl complexes XTa(CO)(4)(dppe) (X = I, Br) and XM(CO)(2)(dppe)(2) (X = I, Br, Cl, H; dppe = 1,2 bis(diphenylphosphino)ethane; M = Nb, Ta) have been investigated. The XTa(CO)(4)(dppe) tetracarbonyl species exhibit irreversible oxidations at E(a) = +868 mV (X(-) = Br-) and E(a) = +1276 (X(-) = I-) mV. Spectroelectrochemical oxidation of these complexes at E(app) = +1000 mV indicates that CO is evolved with decomposition to non-carbonyl containing products. The reduction chemistry of ITa(CO)(4)(dppe) showed an irreversible cathodic process at E(c) = -1541 mV in TBA(+)-PF6-/THF with two coupled oxidation processes (at E(a) = -530 mV and E(a) = +100 mV). Spectroelectrochemical reduction of ITa(CO)(4)(dppe) generates Ta(CO)(4)(dppe)-; oxidation of the anion produces ITa(CO)(4)(dppe) at the first coupled process and HTa(CO)(4)(dppe) at the second, respectively. The more highly substituted XM(CO)(2)(dppe)(2) compounds such exhibit a one-electron reversible oxidative process in CH2Cl2/TBA(+)PF(6)(-) (E(o) ca. -270 to +110 mV) that generate stable (electrochemical time scale) 17-electron species, (XM(CO)(2)(dppe)(2)(+)) that were also characterized by IR spectroelectrochemical techniques. EPR spectra observed for these radical cations exhibit ten line signals at g = 2.04 (X(-) = Cl-) and g = 2.05 (X(-) = H-) with characteristic coupling to Nb-93 (I = 9/2) (A = 115 G for X(-) = Cl-; A = 113 G for X(-) = H-), Attempts to isolate salts of the XM(CO)(2)(dppe)(2)(+) species were unsuccessful.