Unusual Isomeric Lability in Both Oxidation States of the Redox Systems fac-/mer-[M(CO)₃(η³-P₂P′)]⁺/M(CO)₃(η³-P₂P′) (M = Cr, Mo, W; P₂P′ = Bis(2-(diphenylphosphino)ethyl)phenylphosphine): The First Examples Where the 17-Electron fac⁺ and mer⁺ Isomers Are of Comparable Stability

Oxidative cyclic voltammetric experiments have been carried out in dichloromethane solution on the sterically strained 18-electron M(CO)₃(η³-P₂P′) complexes (M = Cr, Mo, W; P₂P = Ph₂PCH₂CH₂P-(Ph)CH₂CH₂PPh₂). A wide range of scan rates (0.01–8000 V s⁻¹) and temperatures (+20 to −60 °C) have been used at both conventional and micro platinum-disk electrodes to establish the mechanism of the one-electron oxidation process to form a 17-electron cation. At room temperature fac-Cr(CO)₃(η³-P₂P′) gives a single chemically reversible one-electron oxidation response, but as the temperature is lowered (conventional electrodes) or the scan rate increases (microelectrodes) the process splits into two responses. At still faster scan rates (microelectrodes) only one reversible response is observed. These observations are consistent with the square reaction scheme but the kinetics of the isomerization steps are faster than in all other similar metal carbonyl systems previously studied. At room temperature the potential of the observed response is a function of E°_fac+/_fac0, E°_mer+/_mer0, E°_fac+/_mar+, and K_fac0+/_mer0. At low temperatures (conventional electrodes) or high scan rates (microelectrodes) the individual redox couples are observed. At the fastest scan rates the electrochemical experiment outruns the time scale of the fac⁺ → mer⁺ isomerization so that only the fac⁺/fac⁰ couple is observed. Digital simulation of the voltammograms has allowed the determination of all the equilibrium and rate constants in the square scheme for the chromium and molybdenum compounds. Of particular interest is the fact that the equilibrium constant (K_fac+/_mer+) only weakly favors mer⁺ for chromium, is close to unity for molybdenum, and weakly favors fac⁺ for tungsten. Normally the mer⁺ isomer is heavily favored. Additionally, the kinetics of all the isomerization steps are much faster than for all the other group 6 metal carbonyl derivatives previously studied. The voltammetric oxidations of Mo(CO)₃(η³-P₂P′) and W-(CO)₃(η³-P₂P′) at room temperature and slow scan rates are complicated by an additional nonisomerization chemical step after electron transfer. However, at faster scan rates (>5 V s⁻¹) the influence of the kinetics of this additional reaction is minimized and the systems then show behavior analogous to that of Cr-(CO)₃(η³-P₂P′), although all rate constants for all isomerization steps are even greater than for chromium. The unusual position of equilibria and the fast kinetics of the isomerization steps in these compounds are attributed to steric strains within the tridentate ligand in both isomeric forms of the complexes, which lead to a delicate structural energy balance being achieved in both oxidation states.