A Trans Effect on the Rate of Ligand Dissociation from Octahedral Organometallic Complexes. Dissociation of L′ from Cr(CO)₄LL′ (L, L′ = PBu₃, PPh₃, P(OPh)₃, P(OMe)₃, AsPh₃)

Rates of ligand displacement have been measured from the complexes trans-Cr(CO)₄LL′ (L, L′ = PBu₃, P(OMe)₃, P(OPh)₃, PPh₃, AsPh₃). The kinetic parameters indicate rate-determining dissociation of L′. For any given trans ligand the leaving-group order is the same, PBu₃ < P(OMe)₃ < P(OPh)₃ < PPh₃ < AsPh₃. The relative ground-state energies are shown to vary by 13 kcal/mol. For constant leaving groups the trans ligand is shown to have a remarkable effect on the rate. The trans effect order (PPh₃ > PBu₃ > P(OPh)₃ ≈ P(OMe)₃ > CO) is the same for different leaving groups. This trans effect can span 4 orders of magnitude. The similarity of this trans effect order to the cis labilization order and to rate accelerations in metal carbonyl dimers and clusters is noted. The data strongly suggest that the trans effect arises by a stabilization of the 16-electron intermediate/transition state by strongly donating ligands.