A THEORETICAL-STUDY OF SINGLET AND TRIPLET PHOSPHINIDENES PAX(3) (A=C, SI, X=H, F) AND THEIR REARRANGEMENT TO PHOSPHAETHENES XPAX(2)

Calculated (MP4/6-31G*//MP2/6-31G** and/or CASSCF/6-31G**) energies a nd equilibrium geometries of the lowest triplet and singlet states of phosphinidenes PAX(3) (A = C, Si; X - H, F) and phosphaethenes XP = AX (2) are reported. As previously noted for PCH3, singlet phosphinidenes exhibit a slight C-s distortion from C-3v symmetry; the triplet-singl et separation is rather insensitive to the nature of the substituent. For phosphaethenes, of C-s symmetry, the singlet-triplet separation re flects the for FPSiF2. Singlet phosphaethene is the minimum on the cor responding strength of the n-bond and drops to 14.1 kcal mol PES, exce pt for PSiF3, for which both triplet and singlet phosphinidenes are fo und below singlet phosphaethene. The transition states for the phosphi nidene-phosphaethene transpositions have been determined as well as th e singlet and triplet states. PSiH3 is expected to undergo a fast tran sposition. For both compounds, fluorine substitution significantly rai ses the transposition barrier.