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 d ianions 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 faste st direct electrochemical methods but was shown to have an approximate lifetime of 10 ns. Cr(CO)(6)(-) also could not be observed by fast sc an cyclic voltammetry, and its lifetime was estimated to lie in the ra nge 50 mu s to 10 ns. In the absence of an electrophile, the electroge nerated dianions further react slowly via a nucleophilic substitution reaction with the parent Fe(CO)(5) and Cr(CO)(6) to yield the dimers F e-2(CO)(8)(2-) and Cr-2(CO)(10)(2-), respectively. The corresponding r ate 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 consta nts 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 ele ctroreductions of Fe(CO)(5) and Cr(CO)(6) when the electrolyses are pe rformed under classical conditions (viz., batch electrolyses; t(elec) greater than or equal to 0.5 h). Conversely, when generated by fast ex haustive electrolysis in a percolating flow cell (t(elec) less than a few seconds), Fe(CO)(4)(2-) is the single electrolysis product that re mains stable for minutes in the dry electrochemical medium in the abse nce of Fe(CO)(5).