By using CASSCF, MR-SDCI+DC, and various DFT methods, we have studied the electronic and geometrical structures of MBH(2), MBH(2)(+) (where M = Sc, Co, Rh, and Ir), and RhB(OH)(2), and RhB(OH)(2)(+) complexes. The ground states of CoBH2+ and IrBH2+ are nearly degenerate quarter states derived from the F-5(s(1)d(7)) state of M(+) and the (2)A(1) state of BH2, while the ground states of RhBH2+ are doublet states which are results of the interaction of (RhF)-F-+ 3(s(0)d(8)) with BH2((2)A(1)). The ground states of ScBH2+ are doublet states from Sc+ D-3(s(1)d(1)) and BH2((2)A(1)) For neutral species MBH(2), where M = Co, Rh, and Ir, the triplet states are always the ground states, while for ScBH2 the (1)A(l), (3)A(1), and (3)A(2) states are clustered within a 0.5 kcal/mol energy range. The MR-SDCI+DC binding energies (BEs) are 49.3, 52.8, 64.3, and 87.5 and 37.8, 51.0, 74.1, and 84.5 kcal/mol for ScBH2+, CoBH2+, RhBH2+, and IrBH2+ and ScBH2 CoBH2. RhBH2, and IrBH2 respectively. M(+)-BH2 bonds are stronger than M-BH2 bonds for M = Sc, Co, and Ir, while the Rh+-BH2 bond is weaker than Rh-BH2. The BE of the metal-boryl bonds increases via Co < Rh < Ir and Co+ < Rh+ < Ir+. The M-B bond is stronger than the M-H and M-CH3 bonds for the late metals Co, Rh, and Ir, while for the early metal Sc/Sc+, the M-B bond is weaker than the M-H and M-CH3 bonds. These results can be explained in terms of the importance of pi-interaction between metal occupied d(pi) orbitals and the BR(2) empty p(pi) orbital. MR-SDCI+DC BEs calculated at the DFT-optimized geometries are larger than those obtained at the CASSCF-optimized geometries, indicating that the DFT geometries are more reliable than the CASSCF geometries.