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.