Journal of Organometallic Chemistry vol:529 issue:1-2 pages:3-14
Ab initio molecular orbital calculations at the QCISD(T)/6-311G(d,p)//(U)MP2/6-31G(dp) + ZPE level have been applied to determine the relative energies between the species featuring double bond to phosphorus, X2C=PX, X2Si=PX, XP=O and XP=S, and their rearranged products XC-PX2, X3C-P, XSi-PX2, X3Si-P, POX and PSX. The energy ordering between isomers is strongly dependent on the halogen substituents. When P is bonded to a more electronegative element, such as C, O and S, fluorination at P enormously stabilizes the corresponding species. The molecular stability is overwhelmingly dominated by the strength of the P-F bond. When P is bonded to a more electropositive element, such as Si, a reversal of role occurs. The relative stability is now mainly determined by the Si-F bond strength. Perfluorination amplifies the effect in such a way that in the SiPX3 system, the P-SiF3 phosphinidene turns out to be the most stable isomer, making it an attractive target for a laboratory preparation. While the negative hyperconjugation effect in some cases plays a noticeable effect, the 'cis-effect' is not operative. The effect of chlorine has also been examined in both XPO and XPS cases. The singlet-triplet energy separations were also estimated for the carbenes, silylenes and phosphinidenes involved.