Equilibration between a Phosphine−Cobalt Complex and an Analogous Complex Containing an N-Heterocyclic Carbene:  The Thermodynamics of a Phosphine−Carbene Exchange Reaction

The reaction of CpCo(PPh3)Me2 with the N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) does not result in complete substitution of the PPh3, as expected, but rather in an equilibrium mixture of CpCo(PPh3)Me2, IPr, CpCo(IPr)Me2, and PPh3. Determination of the equilibrium constants for this unusual reaction over the temperature range 30−50 °C shows that ΔH° = −24.4 ± 1.7 kJ mol-1, ΔS° = −65.8 ± 5.5 J mol-1 K-1, and thus that it is entropy factors arising from steric crowding in CpCo(IPr)Me2 that prevent complete substitution. Evidence for steric crowding is found in a comparison with the crystal structures of CpCo(PPh3)Me2, CpCo(IPr)Me2, and CpCo(IPr)(CO). The smaller, more basic PMe3 does displace IPr completely from CpCo(IPr)Me2.