Enthalpies of ligand substitution for [Mo(η⁵C₅H₅)(CO)₂(NO)] – The role of π-bonding effects in metal–ligand bond strengths

Enthalpies of ligand substitution for [Mo(g5-C5H5)(CO)2(NO)] producing [Mo(g5-C5H5)Mo(CO)(L)(NO)] have been measured by solution calorimetry at 30 C in THF for L = P(OMe)3 < PMePh2 < SIPr < PMe2Ph < IPr < PMe < PnBu3 (SIPr = 1,3-bis(2,6-bis(diisopropylphenyl)imidazolinylidene; IPr = 1,3-bis(2,6-bis(diisopropylphenyl)-imidazol-2-ylidene)). The accepting metal fragment [Mo(g5-C5H5)(CO)(NO)] has a vacant site containing strongly p-accepting carbonyl and nitrosyl ligands and this is shown to influence the stability
of the product complex. Infrared studies of both mCO and mNO show that metal-to-ligand backbonding increases in the order P(OMe)3 < PMe3 < SIPr < IPr implying that both steric and electronic factors play a role in determining complex stability. The crystal structures of [Mo(g5-C5H5)(CO)(IPr)(NO)] and [Mo(g5-C5H5)(CO)(SIPr)(NO)] are reported.