We have examined singlet-triplet energy separations in different phosphinidenes (RP) substituted by first- and second-row elements, making use of ab initio molecular orbital theory. Our main purpose is to find out the substituents that particularly favor the singlet electronic state. The QCISD(T)/6-311++G(3df,2p) + ZPE level has been applied to small molecules and the CISD(Q) and QCISD(T) with the 6-311G(d,p) basis set for all species considered. We have identified few factors that come into play rendering the singlet phosphinidene more stable than the triplet. The parent phosphinidene, PH, has a triplet ground state lying 28 kcal/mol below the closed-shell singlet excited state. The triplet ground state is mainly favored when negative hyperconjugation is involved. In the boryl-, alkyl-, and silyl-substituted phosphinidenes, the triplet state remains by far the ground state. When the substituents have pi-type lone pair electrons (i.e., -NX(2), -PX(2), -OX, -SX), the singlet state becomes stabilized by such an amount that both states have similar energies or even a change in ground state occurs. The most stabilized singlet ground states are attributed to PSF and PSCl. P and S have similar p-orbital sizes, making pi-delocalization easier. Implantation of alkyl and/or amino groups in the beta-position of amino- and phosphinophosphinidenes also contributes to a singlet stabilization. Bulky beta-groups also destabilize the triplet state by a steric effect. From a practical viewpoint, amino (P-NR(2)) and phosphino (P-PR(2)) derivatives bearing large alkyl groups (R) are the most plausible and feasible targets for preparing phosphinidenes possessing a closed-shell singlet ground state.

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&tbd1;N, R(1), R(2) = H, F) and boranes (H(2)XB, X = H, F, CH(3), and C(2)H(5)). 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 H-shift from B to N. This shift becomes more difficult if the substituent is F or CH(3). 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 (BF(3) and B(CH(3) )(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 H(2)PN + H(2)BC(2)H(5) 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&tbd1;N-PR(1)R(2) ), both 1,2-addition to B and N and 1,3-addition to B and P reactions are possible.