Heteroleptic scorpionate complexes of the type [(Tp)M(Bp)] (Tp = hydrotris{pyrazolyl)borate; Bp = dihydrobis{pyrazolyl)borate; M = Mn - Ni) are nominally 5-coordinate, with a pentaza ligand set in a distorted square-pyramidal geometry. An additional pseudo-axial bonding interaction of the endohedral B–H bond with the chelated metal ion is possible. The distance of the M–H–B interaction can be controlled by varying the substituents on the 3-position of the Tp and Bp ligands (i.e., 3R = Me, Ph) proximal to the metal. The resulting bond distances are quantified in X-ray crystal structures and the relative strength of the interaction is assessed by trends in highly sensitive paramagnetic contact shifts and in m(B–H) energies. DFT calculations suggest a monotonic decrease in the interaction with increasing distance, strikingly up to a van der Waals limit of ca. 3.3 Å. This trend is reflected in experimental data.

Length vs. strength of three-center M–H–B bonding in heteroleptic scorpionate complexes

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