The ligand hydrotris(indazol-1-yl)borate, Tp(4Bo), has been shown to possess abnormal regiochemistry, i.e. one in which boron is bonded to the most hindered N. This was established from the NMR of paramagnetic home- and heteroleptic Co(II) complexes and from the crystallographic characterization of Tp(4Bo)Mo(CO)(2)(eta(3)-methallyl). Tp(4Bo) represents a unique case of a 4,5-disubstituted Tp ligand, where the boron atom is sterically protected and yet the metal side is an unhindered as that of the parent Tp. By contrast, 7-substituted indazoles exhibit normal regiochemistry, i.e. B bonding to the less hindered N.