Pnicogen-Bonded Complexes HnF5–nP:N-Base, for n = 0–5
Ab initio MP2/aug′-cc-pVTZ calculations have been carried out on the pnicogen-bonded complexes HnF5–nP:N-base, for n = 0–5 and nitrogen bases NC–, NCLi, NP, NCH, and NCF. The structures of these complexes have either C4v or C2v symmetry with one exception. P–N distances and interaction energies vary dramatically in these complexes, while Fax–P–Feqangles in complexes with PF5 vary from 91° at short P–N distances to 100° at long distances. The value of this angle approaches the Fax–P–Feq angle of 102° computed for the Berry pseudorotation transition structure which interconverts axial and equatorial F atoms of PF5. The computed distances and Fax–P–Feq angles in complexes F5P:N-base are consistent with experimental CSD data. For a fixed acid, interaction energies decrease in the order NC– > NCLi > NP > NCH > NCF. In contrast, for a fixed base, there is no single pattern for the variations in distances and interaction energies as a function of the acid. This suggests that there are multiple factors that influence these properties. The dominant factor appears to be the number of F atoms in equatorial positions, and then a linear Fax–P···N rather than Hax–P···N alignment. The acids may be grouped into pairs (PF5, PHF4) with four equatorial F atoms, then (PH4F, PH2F3) with Fax–P···N linear, and then (PH3F2 and PH5) with Hax–P···N linear. The electron-donating ability of the base is also a factor in determining the structures and interaction energies of these complexes. Charge transfer from the N lone pair to the σ* P–Aax orbital stabilizes HnF5–nP:N-base complexes, with Aax either Fax or Hax. The total charge-transfer energies correlate with the interaction energies of these complexes. Spin–spin coupling constants 1pJ(P–N) for (PF5, PHF4) complexes with nitrogen bases are negative with the strongest bases NC– and NCLi but positive for the remaining bases. Complexes of (PH4F, PH2F3) with these same two strong bases and H4FP:NP have positive 1pJ(P–N) values but negative values for the remaining bases. (PH5, PH3F2) have negative values of 1pJ(P–N) only for complexes with NC–. Values of 1J(P–Fax) and 1J(P–Hax) correlate with the P–Fax and P–Haxdistances, respectively.