Electrochemistry and Infrared Spectroelectrochemistry of the Substituted Phosphine Complexes, XTa(CO)₄(dppe) (X = I, Br) and XM(CO)₂(dppe)₂ (X = H, I, Br, Cl; M= Nb, Ta)

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 each exhibit a one-electron reversible oxidative process in CH₂Cl₂/TBA⁺PF₆⁻ (E° 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 ⁹³Nb (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.