REACTIVITY OF 17-ELECTRON ORGANOMETALLIC TUNGSTEN AND MOLYBDENUM RADICALS - A LASER FLASH-PHOTOLYSIS STUDY

Visible (460-490 nm) laser flash photolysis of [CpW(CO)3]2 or [CpMo(CO)3]2 induces homolysis of the metal-metal bond with formation of 17-electron radicals, CpM(CO)3. Radical dimerization results in quantitative recovery of the parent dimer and can be followed by the time-resolved increase in dimer absorbance. The reaction follows clean second-order kinetics, -d[CpM(CO)3]/dt = 2k(c)[CpM(CO)3]2; k(c)(W) = 6.2 x 10(9) and k(c)(Mo) = 3.9 x 10(9) L mol-1 s-1 in CH3CN at 23-degrees-C. The CpM(CO)3 radicals react with organic and inorganic halides and pseudohalides by an atom-transfer mechanism. In the presence of a large excess of the halide-containing substrate, the rate of loss of the radical, -d[CpM(CO)3]/dt, proceeds according to a mixed first- and second-order rate law. The pseudo-first-order rate constants for reactions with organic halides vary linearly with the concentration of the organic halide; bimolecular rate constants for CpW(CO)3 range from 3.9 x 10(2) L mol-1 s-1 with CH2Br2 to 1.34 X 10(9) L mol-1 s-1 for CBr4. The reactivity trends (RI > RBr > RCl) and (benzyl > allyl > 3-degrees > 2-degrees > 1-degrees > CH3) are observed. The 7 orders of magnitude variation in bimolecular rate constants is attributed to a highly selective atom abstraction process. The range of rate constants for atom abstraction from halo- and pseudohalopentaamminecobalt(III) and halobis(dimethylglyoximato)cobalt(III) complexes is smaller (2 orders of magnitude, from 1.6 x 10(7) L mol-1 s-1 for NCCo(NH3)52+ to >2 X 10(9) L mol-1 s-1 for BrCo(dmgH)2py), because of the upper limit imposed by diffusion. Transfer of the halogen atom from both organic and metal substrates to CpW(CO)3 was confirmed by the IR spectrum of the organometallic product, CpW(CO)3X (X = Cl, Br, or I). Dioxygen traps CpW(CO)3 with a rate constant k = 3.3 x 10(9) L mol-1 s-1. Light-initiated chain reactions were observed at high concentrations of RX, XCoL5n+, or O2. Hydroperoxides react with CpW(CO)3 by a radical mechanism. The reaction observed between CpW(CO)3 and (n-Bu)3SnH is not consistent with either outer-sphere electron transfer or a hydrogen atom abstraction mechanism; oxidative addition to the 17-electron radical is believed to occur in this case. The dimer [(C5H4COOCH3)W(CO)3]2 shows photoreactivity in organic solvents which is very similar to that of [CpW(CO)3]2.