Six BH5 structures were examined in detail using the self-consistent field (SCF), configuration interaction including single and double excitations (CISD), and coupled cluster including single, double, and perturbatively included connected triple excitations [CCSD(T)] methods in conjunction with a double-zeta plus polarization (DZP), a triple-zeta plus polarization (TZ2P), and an augmented TZ(3d1f1g,2p1d) basis set. The C-4v and the D-3h isomers are high in energy [23 and 45 kcal mol(-1), respectively, relative to the C-s(I) structure at DZP CCSD]. Although structure C-s(I) is the global minimum, both C-s structures, where BH5 is comprised of nearly planar monoborane (BH3) and a hydrogen molecule, are essentially equal in energy and allow virtually free rotation of the hydrogen moiety. The global minimum was characterized by vibrational frequency analyses at the TZ2P CCSD(T) level. Final energies were obtained with the TZ(3d1f1g,2p1d) basis set and the CCSD(T) method. At room temperature, the borane-hydrogen complex BH5 is unstable toward dissociation by 6.8 kcal mol(-1) However, at the absolute zero (0 K), the complex forms exothermically (-1.4 kcal mol(-1)). The theoretical D-e value (6.3 kcal mol(-1)) appears to be effectively converged with respect to basis set and theoretical method. The inclusion of perturbative triple excitations accounts for 36% of the total binding energy, a remarkable effect. Hydrogen scrambling via a C-2v structure seems unlikely since the activation barrier for this process is at least 2.8 kcal mol(-1) higher than the dissociation barrier of BH5 into BH3 and H-2.