Suppressed Phosphine Dissociation by Polarization Effects on the Donor-Acceptor Bonds in [Ni(PEt₃)_{4- n}(ECp∗)_n] (E = Al, Ga)

A series of heteroleptic complexes [Ni(PEt₃)_4-n(ECp∗)_n] (E = Al, Ga, Cp∗ = pentamethylcyclopentadienyl, n = 0-4) was prepared and characterized by experimental as well as computational means. The series of compounds was studied with respect to ligand dissociation processes which are fundamental for reactivity. In contrast to the homoleptic complexes [Ni(PR₃)_n] phosphine dissociation is remarkably suppressed in the heteroleptic title complexes. Single crystal X-ray structures as well as density functional theory calculations reveal a gradual decrease of the Ni-PEt₃ distances with increasing number of coordinated group-13 ligands ECp∗. Accordingly, variable-temperature UV-vis studies of [Ni(PEt₃)_4-n(AlCp∗)_n] in solution indicate no ligand dissociation equilibrium for n ≥ 2. Energy decomposition analysis with the natural orbital for chemical valence extension shows higher Ni-P interaction energies for [Ni(PEt₃)_4-n(AlCp∗)_n] than for [Ni(PEt₃)₄] which is mainly a result of an increase in columbic attraction forces induced by Ni-PEt₃ bond polarization upon ECp∗ coordination.