Structure and bonding of group 4-nickel heterobimetallics supported by 2-(diphenylphosphino)pyrrolide ligands

Abstract

The synthesis of a full series of group 4/nickel complexes supported by a 2-(diphenylphosphino)pyrrolide (NP) ligand is reported. Treatment of the homoleptic, 8-coordinate M(NP)₄ monometallic precursors with Ni(COD)₂ (COD = 1,5-cyclooctadiene) yielded the heterobimetallic complexes (κ²-NP)M(μ₂-NP)₃Ni (M = Ti, Zr, Hf). Although X-ray crystallographic analysis reveals similarly short metal–metal distances in all three complexes, quantum chemical calculations indicate that ZrNi (5) and HfNi (6) contain only single Ni → M dative bonds while TiNi (4) has an additional Ti–Ni π-bond. All three complexes have quasireversible reductions by cyclic voltammetry, and 1-electron chemical reduction of 4 by Na(Hg) yields the anion, [Na][(κ²-NP)Ti(μ₂-NP)₃Ni] (7). X-ray and computational analysis indicate that the 1-electron reduction of 4 completely breaks the metal–metal bond, yielding a formally Ti^III–Ni⁰ complex. Ti–Ni bonding can also be disrupted by coordination of CO, wherein Ni → CO backbonding effectively outcompetes Ni → Ti dative bonding.