In Search of Singlet Phosphinidenes

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Abstract
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 π-type lone pair electrons (i.e., −NX2, −PX2, −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 π-delocalization easier. Implantation of alkyl and/or amino groups in the β-position of amino- and phosphinophosphinidenes also contributes to a singlet stabilization. Bulky β-groups also destabilize the triplet state by a steric effect. From a practical viewpoint, amino (P−NR2) and phosphino (P−PR2) derivatives bearing large alkyl groups (R) are the most plausible and feasible targets for preparing phosphinidenes possessing a closed-shell singlet ground state.

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