Ab initio calculations at the SCF, MP2, CASSCF, and CASPT2 levels of theory with basis sets using atomic pseudopotentials have been carried out for the stretched eta (3)-hydridoborate sigma -complex of niobium, [Cl2Nb{H2B(OH)(2)}], in order to investigate the nature and energetics of the interaction between the transition metal and the eta (3)-hydridoborate ligand. The geometry of the complex [Cl2Nb{H2B(OH)(2)}] and its fragments [Cl2Nb](+) and [H2B(OH)(2)](-) were optimized at SCF and CASSCF levels. These results are consistent with [Cl2Nb{eta (3)-H2B(OH)(2)}] being a Nb(III) complex in which both hydrogen and boron of the [eta (3)-H2B(OH)(2)](-) ligand have a bonding interaction with the niobium preserving stretching B-H bond character. The calculated values of DEF (energy required to restore the fragment from the equilibrium structure to the structure it takes in the complex) for [Cl2Nb](+) are 5.35 kcal/mol at SCF, 3.27 kcal/mol at MP2, 4.80 kcal/mol at CASSCF, and 2.82 kcal/mol at CASPT2 and for [H2B(OH)(2)](-) 21.13 kcal/mol at SCF, 23.85 kcal/mol at MP2, 20.69 kcal/mol at CASSCF, and 23.48 kcal/mol at CASPT2. Values of INT (stabilization energy resulting from the coordination of distorted ligand to the metal fragment) for the complex [Cl2Nb{H2B(OH)(2)}] are -239.35 kcal/mol at SCF, -260.00 kcal/mol at MP2, -230.76 kcal/mol at CASSCF, and -252.60 kcal/mol at CASPT2. For the complex [(eta (5)-C5H5)(2)Nb{H2B(OH)2}], calculations at the SCF and MP2 levels were carried out. Values of INT for [(eta (5)-C5H5)(2)Nb{H2B(OH)(2)}] are -169.93 kcal/mol at SCF and -210.62 kcal/mol at MP2. The results indicate that the bonding of the [eta (3)-H2B(OH)(2)](-) ligand with niobium is substantially stable. The electronic structures of [Cl2Nb{H2B(OH)(2)}], [(eta (5)-C5H5)(2)Nb{H2B(OH)(2)}], and its fragments are analyzed in detail as measured by Mulliken charge distributions and orbital populations.