NATURE AND STRENGTH OF THE LAMBDA(5)-P=C DOUBLE-BOND

The strength of the ''second bond'' in H3P=CH2 has been obtained in di fferent ways at the MP4/6-311G*//MP2/6-311G**+ZPE level of theory. Th e calculated bond strengths of 33.7 and 38.4 kcal/mol are comparable t o those obtained for the lambda 3-P=C bond (38-49 kcal/mol). The simil arity in bond strengths could explain the similar bond lengths of the two systems. The rotational barrier about the P-C bond in H3P-CH2 obta ined at the QCISD/6-311+G*+ZPE level of theory is 0.9 kcal/mol. The l ow barrier is, however, not necessarily an indication of a weak double bond but is probably the consequence of the similar bond strength of the ''second bond'' for both rotational conformers. The bonding MO-mod el evaluated here for the description of the less stable conformer can be derived from the pi-MO of the lambda 3-P=C bond; the resulting ele ctronic structure is similar to an allylic ct-system. Topological elec tron density analysis has indeed shown that the electron density and e llipticity at the bond critical point of the somewhat less stable conf ormer can be related to those of HP=CH2 and similarly to those of ally l ion and ethane.