The influence of group 15 various substituents and effect of metal centers on metal–borane interactions and structural isomers of transition metal–borane complexes W(CO) 5 (BH 3 ·AH 3 ) and M(CO) 5 (BH 3 ·PH 3 ) (A=N, P, As, and Sb; M=Cr, Mo, and W), were investigated by pure density functional theory at BP86 level. The following results were observed: (a) the ground state is monodentate, η 1 , with C 1 point group; (b) in all complexes, the η 1 isomer with C S symmetry on potential energy surface is the transition state for oscillating borane; (c) the η 2 isomer is the transition state for the hydrogens interchange mechanism; (d) in W(CO) 5 (BH 3 ·AH 3 ), the degree of pyramidalization at boron, interaction energy as well as charge transfer between metal and boron moieties, energy barrier for interchanging hydrogens, and diffuseness of A increase along the series A=Sb<As<P<N; (e) in M(CO) 5 (BH 3 ·PH 3 ), interaction energy is ordered as M=W>Cr>Mo, while energy barrier for interchanging hydrogens decreases in the order of M=Cr>W>Mo.