PHOTOMICROCALORIMETRY - PHOTOSUBSTITUTION OF CARBONYL BY PHOSPHITES IN [MN(ETA(5)C5H4CH3)(CO)(3)]

A photomicrocalorimeter is described for studies of reactions involvin g oxygen and moisture-sensitive compounds in organic solvents. This ca lorimeter was used to investigate the thermochemistry of photochemical substitutions of a carbonyl in [Mn(eta(5)-C5H4CH3)(CO)(3)] by P(OR)(3 ) (R = Ph or Pr-i). The measured reaction enthalpies in heptane (lambd a = 400 nm), Delta H-r = - 15.7 +/- 0.5 and - 18.6 +/- 0.9 kJ mol(-1), respectively, were identified with the bond dissociation enthalpy dif ferences D(Mn-CO)-D[Mn-P(OPh)(3)] and D(Mn-CO)-D[Mn-P((OPr)-Pr-i)(3)] in solution. These results are discussed in terms of the basicity of t he phosphites, spectroscopic data of the complexes (CO stretching freq uencies), and the available thermochemical data for other transition m etal-phosphorus(III) ligand interactions.