RATE AND MECHANISM OF THE REDUCTIONS OF IRON PENTACARBONYL AND CHROMIUM HEXACARBONYL TO THEIR METALATE COMPLEXES

The electrochemical reductions of Fe(CO)(5) and Cr(CO)(6) in THF were shown to proceed by an ECE mechanism leading to the electrogenerated dianions Fe(CO)(4)(2-) and Cr(CO)(5)(2-), respectively. The initial 19-electron anion radical Fe(CO)(5)(-) could not be observed by the fastest direct electrochemical methods but was shown to have an approximate lifetime of 10 ns. Cr(CO)(6)(-) also could not be observed by fast scan cyclic voltammetry, and its lifetime was estimated to lie in the range 50 mu s to 10 ns. In the absence of an electrophile, the electrogenerated dianions further react slowly via a nucleophilic substitution reaction with the parent Fe(CO)(5) and Cr(CO)(6) to yield the dimers Fe-2(CO)(8)(2-) and Cr-2(CO)(10)(2-), respectively. The corresponding rate constants were estimated at 120 (Fe(CO)(4)(2-) + Fe(CO)(5)) and 0.95 M(-1) s(-1) (Cr(CO)(5)(2-) + Cr(CO)(6)). Although these rate constants are rather modest, their magnitudes are sufficient to explain why Fe-2(CO)(8)(2-) and Cr-2(CO)(10)(2-) are the major products of the electroreductions of Fe(CO)(5) and Cr(CO)(6) when the electrolyses are performed under classical conditions (viz., batch electrolyses; t(elec) greater than or equal to 0.5 h). Conversely, when generated by fast exhaustive electrolysis in a percolating flow cell (t(elec) less than a few seconds), Fe(CO)(4)(2-) is the single electrolysis product that remains stable for minutes in the dry electrochemical medium in the absence of Fe(CO)(5).