| Treating (eta5-C5H5)Cr(CO) 3H or (eta5-C5H5)Cr(CO) 3D with an excess of olefin containing the opposite isotope generally leads to H/D exchange, although hydrogenation is also observed in some cases. Application of an appropriate statistical correction to the observed exchange rate gives kH and kD, the rate constants for H· (D·) transfer from (eta5 -C5H5)Cr(CO)3H (D) to various olefins. The rate constants for H· transfer are influenced by steric factors as well as by the stability of the resulting carbon-centered radicals.;The pKa values of HV(CO)4(P-P) ((P-P) = Ph2P(CH2)nPPh 2: n = 1-4) in acetonitrile have been determined. These data and the redox potentials of the corresponding anions have allowed the BDE values of these V-H bonds to be estimated. The resulting BDE values of HV(CO)4(P-P) are 57.9 for dppm, 57.5 for dppe, 56.0 for dppp and 54.9 kcal/mol for dppb. These are exceptionally weak M-H bonds.;The rate constants for H· transfer from HV(CO)4(P-P) to styrene have been measured at 285 K. The rate constants for H· transfer show that the HV(CO)4(P-P) are excellent H· donors to olefins. The X-ray structures of the HV(CO)4(P-P) complexes have been obtained. The decrease in pKa with increasing P-V-P angle can be attributed to the strain these chelates impose on the octahedral structure remaining after deprotonation.;A series of V(CO)4(P-P) radical complexes have been tested as chain transfer catalysts for MMA polymerization. The chain transfer constants Cs (= ktr/k p), determined by Mayo plots from data at 70°C show that V(CO) 4(dppe) is the most efficient chain transfer catalyst (C s = 750 at 70°C). |
