Enthalpies of ligand substitution for [Mo(eta(5)-C5H5)(CO)(2)(NO)] producing [Mo(eta(5)-C5H5) Mo(CO)(L)(NO)] have been measured by solution calorimetry at 30 degrees C in THF for L = P(OMe)(3) < PMePh2 < SIPr < PMe2Ph < IPr < PMe < PnBu(3) (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(eta 5-C5H5)(CO)(NO)] has a vacant site containing strongly pi-accepting carbonyl and nitrosyl ligands and this is shown to influence the stability of the product complex. Infrared studies of both V-CO and V-NO 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(eta(5)-C5H5)(CO)(IPr)(NO)] and [Mo(eta 5-C5H5)(CO)(SIPr)(NO)] are reported. (C) 2014 Published by Elsevier Ltd.