Ab initio molecular orbital calculations have been carried out to determine the minimum-energy pathways and thereby to probe the mechanism of reactions between phosphanylnitrenes (R(1)R(2)P equivalent to N, R(1), R(2) = H, F) and boranes (H(2)XB, X = H, F, CH3, and C2H5). Geometries have been determined using the MP2/6-31G(d,p) model, while relative energies have been estimated using, depending on the size of the system, the quadratic configuration interaction model (QCISD and QCISD(T)) with various basis sets including 6-31G(d,p), 6-311G(d,p), and 6-311++G(d,p). The stability of the primary complex adduct is strongly dependent on the substituents of the boranes. When the borane bears a H atom, the primary adduct is not at all stable and readily collapses to an amine isomer via a II-shift from B to N. This shift becomes more difficult if the substituent is F or CH3. In the F case, a phosphorane isomer, owing to the strength of the P-F bond, turns out to be favored. When non-hydrogen boranes (BF3 and B(CH3)(3) for example) could be used, the primary adducts could be stabilized and even exist as discrete intermediates. F substituents on the nitrene show no significant qualitative effect. In the H2PN + H2BC2H5 reaction, a retro-ene reaction of the adduct directly gives rise to an amine product via a five-membered transition structure. In the reverse reaction of a HX molecule plus an iminoborane (RB equivalent to N-PR(1)R(2)), both 1,2-addition to B and N and 1,3-addition to B and P reactions are possible.