Monodentate σ-Accepting Boron-Based Ligands Bearing Square-Planar Ni(0) Centers

Transition metals are known to work as electron donors toward electron-accepting heavier-group-13 elements (Al, Ga, and In), called Z-type ligands. However, complexes with boron-based Z-type ligands are stable only in the presence of additional coordination units (the so-called "supported-ligand" strategy). Here, we report the synthesis and characterization of square-planar Ni(0) complexes that bear tris(perfluoroaryl)boranes as monodentate Z-type ligands, even though such coordination geometry has been traditionally associated with Ni(II) species based on the well-established ligand-field theory. A combined theoretical and experimental approach revealed a mixed covalent/dative character for the Ni-B bonds. This strategy uses frustrated L/Z-ligand pairs that combine sterically encumbered electron-donating (L-type) and electron-accepting ligands to form noncovalent interactions over L-M-Z units to achieve unprecedented low-valent transition metal species with monodentate Z-type ligands.