Thermochemistry of η 3-Allyltricarbonyliodoiron, Tricarbonyl (η 4-cyclooctatetraene)iron and Tetracarbonyldiiodoiron. The Enthalpy Contributions of Allyliron and Cyclooctatetraene-iron Bonds

Abstract

Microcalorimetric measurements at elevated temperatures of the heats of thermal decomposition and iodination have led to values for the standard enthalpies of formation of the following crystalline compounds (values given in kJ mol −1) at 298K : [(η 3-C 3H 5) Fe(CO) 3I] = −(428±10); [(η 4-C 8H 8)Fe(CO) 3] = −(237±12); [Fe(CO) 4I 2] =−(722±8). Separate measurements by the vacuum-sublimation microcalorimetric technique gave the following values for the enthalpy of sublimation at 298K (kJ mol −1: [(η 3-C 3H 5)Fe(CO) 3I] = (84.5±4); [(η 4-C 8H 8)Fe(CO) 3] = (87±4); [Fe(CO) 4I 2] = (86.0±4). From these and other data, the bond enthalpy contributions of the various metal-ligand bonds in the gaseous metal complexes were evaluated as follows: [(η 3-C 3H 5)-Fe] 176; [(η 4-C 8H 8)-Fe] 180; Fe-I 177 kJ mol −1. The question of the transferability of the enthalpy contributions of iron-ligand bonds between organoiron complexes is discussed with the aid of structural and thermochemical data. It is concluded that the transfer of bond enthalpy contributions is valid within the scheme which relates iron-carbon bond lengths to iron-carbon bond enthalpies.