ENTHALPIES OF REACTION OF (DIENE)IRON AND (ENONE)IRON TRICHARBONYL COMPLEXES WITH MONODENTATE AND BIDENTATE LIGANDS - SOLUTION THERMOCHEMICAL STUDY OF LIGAND SUBSTITUTION IN THE L2FE(CO)3 COMPLEXES

The enthalpies of reaction of (BDA)Fe(CO)3 (BDA = (C6H5)CH--CHO(CH3), benzylideneacetone) with a series of mono- and multidentate ligands, l eading to the formation of (eta4-L)Fe(CO)3, (L')2Fe(CO)3, and (L'')Fe( CO)3 complexes (L = diene, enone; L' = monodentate arsines; L'' = bide ntate ligands), have been measured by solution calorimetry in THF at 5 0-degrees-C. The range of reaction enthalpies spans some 44 kcal/mol. The overall relative order of stability established is as follows: for monodetate ligands, AsPh3 < AsEt3 < tertiary phosphine; for bidentate phosphine and arsine ligands, dppm < arphos < dmpm < dppb < dppv < dp pp < dppe < dmpe < dcpe < depe; for dienes and enones, benzylideneacet one < chalcone < cyclooctatetraene < cyclohexadiene. These thermodynam ic data help to establish a relative order of complex stability for th ese compounds in the iron tricarbonyl system. These data allow the cal culation of the enthalpy associated with the geometric isomerization p rocess (axial-equatorial/ diaxial) present in the (L')2Fe(CO)3 system (5.4 +/- 0.5 kcal/mol) as well as for a quantitative analysis of ring strain energies in the(L'') Fe(CO)3 system. The four-membered metallac ycle is the only cyclic structure exhibiting significant strain energy (12.6 kcal/mol). Comparisons with other organometallic systems and in sight into factors influencing the Fe-L bond disruption enthalpies are also discussed.