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 are measured for Mo(C5H5)(CO)2(NO).
• Phosphines and N-heterocyclic carbenes are stronger ligands and displace CO.
• Backbonding to π∗ orbitals is an important part of complex stability.
• FTIR studies show shifts to lower wavenumbers of ν-CO and ν-NO.
• Structural studies show lengthening of the CO and NO bonds.

Enthalpies of ligand substitution for [Mo(η5-C5H5)(CO)2(NO)] producing [Mo(η5-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(η5-C5H5)(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 νCO and νNO 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(η5-C5H5)(CO)(IPr)(NO)] and [Mo(η5-C5H5)(CO)(SIPr)(NO)] are reported.

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