Enthalpies of ligand substitution for [Mo(η 5 -C 5 H 5 )(CO) 2 (NO)] producing [Mo(η 5 -C 5 H 5 )Mo(CO)(L)(NO)] have been measured by solution calorimetry at 30°C in THF for L=P(OMe) 3 <PMePh 2 <SIPr<PMe 2 Ph<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(η 5 -C 5 H 5 )(CO)(NO)] has a vacant site containing strongly π-accepting carbonyl and nitrosyl ligands and this is shown to influence the stability of the product complex. Infrared studies of both ν CO and ν NO show that metal-to-ligand backbonding increases in the order P(OMe) 3 <PMe 3 <SIPr<IPr implying that both steric and electronic factors play a role in determining complex stability. The crystal structures of [Mo(η 5 -C 5 H 5 )(CO)(IPr)(NO)] and [Mo(η 5 -C 5 H 5 )(CO)(SIPr)(NO)] are reported.