The thermochemistry of carbonyl complexes of Cr, Mo, Wand Co with arenes, cycloheptatriene, and norbornadiene

Abstract

Microcalorimetric measurements at elevated temperatures of the heats of thermal decomposition and of iodination of a number of arenemetal carbonyls have led to values for the standard enthalpies of formation of the following crystalline compounds (values given in kJ mol−1) at 25°C: (C6H6)Co4(CO)9 = −(1313 ± 13); (C6H3Me3)Co4(CO)9 = −(1444 ± 13); (C6Me6)Co4(CO)9 = −(1555 ± 17); (C6H3Me3)Mo(CO)3 = −(533 ± 13); (C6H3Me3)W(CO)3 = −(477 ± 13); (C6Me6)Cr(CO)3 = −(671 ± 13); (C6Me6)Mo(CO)3 = −(631 ± 8); (cyclo-C7H8)Mo(CO)3 = −(297 ± 8); (cyclo-C7H8)W(CO)3 = −(236 ± 8); (nor-C7H8)Cr-(CO)4 ∼ −(400 ± 13); (nor-C7H8)Mo(CO)4 = −(428 ± 10).Separate measurements by the vacuum-sublimation microcalorimetric technique gave the following values for ΔHsub298 (kJ mol−1): (cyclo-C7H8)Cr(CO)3 = (87.9 ± 4); (cyclo-C7H8)Mo(CO)3 = (87.9 ± 4); (cyclo-C7H8)W(CO)3 = (92.0 ± 4); (nor-C7H8)Cr(CO)4 = (88.7 ± 4) and (nor-C7H8)Mo(CO)4 = (91.6 ± 4).From these (and other) data, the bond-enthalpy contributions of the various ligand—metal bonds (D(LM)) in the gaseous metal complexes were evaluated as follows (values in kJ mol−1): (C6H6)−Co = 270; (C6Me3H3)−Co = 285; (C6Me6)−Co = 310; (C6Me3H3)−Cr = 191; (C6Me3H3)−Mo = 279; (C6Me3H3)−W = 334; (C6Me6)−Cr = 205; (C6Me6)−Mo = 292; (cyclo-C7H8)−Cr = 150; (cyclo-C7H8)−Mo = 264; (cyclo-C7H8)−W = 311; (nor-C7H8)−Cr ∼ (80); (nor-C7H8)−Mo, ∼ 187.The bond-enthalpy contribution, D(ML), for a given ligand increases on ascending the series M = Cr → Mo → W, and for a given metal, D(ML) increases on changing L along the series L = benzene → mesitylene → hexamethylbenzene.Thermal stability, however, is not generally determined by the magnitude of the bond-enthalpy contributions, D(ML) and D(MCO), and complexes of Mo and W are frequently less stable to heat than the corresponding complexes of Cr. It is suggested that the thermal decomposition of some complexes may take place in the condensed state, and involve the initial formation of poly-nuclear metal carbonyl products, and of the metal hexacarbonyls.