THERMOCHEMISTRY OF BIS-ARENE-CHROMIUM AND ARENETRICARBONYL-CHROMIUM COMPOUNDS CONTAINING HEXAMETHYLBENZENE, 1,3,5-TRIMETHYLBENZENE AND NAPHTHALENE

Microcalorimetric measurements at elevated temperatures of the heats of thermal decomposition and iodination have led to values of the standard enthalpies of formation of the following crystalline compounds (values given in kJ mol-1) at 298K: [Cr(η6-1,3,5-C6H3(CH3)3)2] = (63±12); [Cr(η6-C6(CH3)6)2] : -(88±12); [Cr(1,2,3,4,4a,8a-η-C10H8)2] = (407±11); [Cr(CO)3(1,2,3,4,4a,8a-η-C10H8)] = -(258±8). Separate measurements by the vacuum sublimation microcalorimetric technique gave the following values for the enthalpy of sublimation at 298K (kJ mol-1) : [Cr(η6-1,3,5-C6H3(CH3)3)2] = (104±1); [Cr(η6-C6(CH3)6)2] = (119±4); [Cr(CO)3(1,2,3,4,4a,8a-η-C10H8)] = (107±3). From these and other data, the bond enthalpy contributions of the metal-ligand bonds in the gaseous metal complexes were evaluated as follows: [(η6-C6(CH3)6)-Cr] (155±7); [(η6-C6H3(CH3)3)-Cr] (151±6); [(1,2,3,4,4a, 8a-η-C10H8)-Cr](145±6) kJ mol-1]. The question of the transferability of the enthalpy contributions of chromium-ligand bonds between organochronium complexes is discussed with aid of information from structural and spectroscopic investigation. The limitations of the procedure are defined. The thermodynamic data are used to discuss various substitution, redistribution and exchange reaction of Cr(η-arene)2 and [Cr(CO)3(η-arene)] compounds. © 1979.