Exploring the Forces That Control the P−C Bond Length in Phosphamides and Their Complexes: The Key Role of Hyperconjugation

The factors tuning the R(P−C) bond length of the labile P−C bond in phosphamides have been explored
by means of density functional theory, employing natural bond orbital (NBO) analysis. The second-order perturbation stabilization energy ΔE(2) due to hyperconjugation accounts well for the unusually
long R(P−C) bond lengths in a series of phosphamides and related species. Favorable n(O) → σ*(P−C)
interactions primarily affect the R(P−C) bond length through electron delocalization. Deleting these
interactions from our calculations results in significant shortening of the P−C bond, to a bond length
close to that of the P−C(sp2) bonds. This holds also true if n(O) is “screened” by protonation, while n(P)
does not affect the strength of the P−C bond. Moreover, the linear correlations of R(P−C) vs ΔE(2) and
of R(P−C) vs ΔEdel substantiate the validity of the concept that hyperconjugative interactions tune the
P−C bond length in phosphamides and provide a novel explanation of P−C bond lability. It was found
that the combination of electron-withdrawing substituents on the carbonyl C atom with electron-releasing
substituents on the P atom strengthen the P−C bond, thus stabilizing phosphamides. Phosphamides could
also be stabilized by coordination with early-transition-metal ions (e.g. [CpTiCl2{κ1O-H2PC(O)Me}]+)
but are destabilized upon coordination with late-transition-metal ions (e.g. [Ag{κ1O-MeC(O)PH2}]+ and
[Cu{κ2P,O-MeC(O)PH2}]+). The coordination of phosphamides with Rh(III) in [CpRhCl2{κ1P-H2PC(OH)CH3}]+ only marginally affects the R(P−C) bond length. Finally, the interaction of phosphamides
with the hard Li+, K+, and Tl+ cations does not affect significantly the n(O) → σ*(P−C) hyperconjugative
interactions, and therefore, the R(P−C) bond length is only slightly shortened.