## Properties of Cationic Pnicogen-Bonded Complexes F_{4–n}H_{n}P^{+}:N-Base
with F–P···N Linear and *n* =
0–3

journal contribution

posted on 04.06.2015, 00:00 by Janet E. Del Bene, Ibon Alkorta, José ElgueroAb
initio MP2/aug′-cc-pVTZ calculations were performed to investigate
the pnicogen-bonded complexes F

_{4–n}H_{n}P^{+}:N-base, for*n*= 0–3, each with a linear or nearly linear F–P···N alignment. The nitrogen bases include the sp^{3}bases NH_{3}, NClH_{2}, NFH_{2}, NCl_{2}H, NCl_{3}, NFCl_{2}, NF_{2}H, NF_{2}Cl, and NF_{3}and the sp bases NCNH_{2}, NCCH_{3}, NP, NCOH, NCCl, NCH, NCF, NCCN, and N_{2}. The binding energies vary between −20 and −180 kJ·mol^{–1}, while the P–N distances vary from 1.89 to 3.01 Å. In each series of complexes, binding energies decrease exponentially as the P–N distance increases, provided that complexes with sp^{3}and sp hybridized bases are treated separately. Different patterns are observed for the change in the binding energies of complexes with a particular base as the number of F atoms in the acid changes. Thus, the particular acid–base pair is a factor in determining the binding energies of these complexes. Three different charge-transfer interactions stabilize these complexes. These arise from the nitrogen lone pair to the σ*P–F_{ax}, σ*P–F_{eq}, and σ*P–H_{eq}orbitals. The dominant single charge-transfer energy in all complexes is N_{lp}→ σ*P–F_{ax}. However, since there are three N_{lp}→ σ*P–F_{eq}charge-transfer interactions in complexes with F_{4}P^{+}and two in complexes with F_{3}HP^{+}, the sum of the N_{lp}→ σ*P–F_{eq}charge-transfer energies is greater than the N_{lp}→ σ*P–F_{ax}charge-transfer energies in the former complexes, and similar to the N_{lp}→ σ*P–F_{ax}energies in the latter. The total charge-transfer energies of all complexes decrease exponentially as the P–N distance increases. Coupling constants^{1p}*J*(P–N) across the pnicogen bond vary with the P–N distance, but different patterns are observed for complexes with F_{4}P^{+}and complexes of the sp^{3}bases with F_{3}HP^{+}. These initially increase as the P–N distance decreases, reach a maximum, and then decrease with decreasing P–N distance as the P···N bond acquires increased covalent character. For the remaining complexes,^{1p}*J*(P–N) increases with decreasing P–N distance. Complexation increases the P–F_{ax}distance and^{1}*J*(P–F_{ax}) relative to the corresponding isolated ion.^{1}*J*(P–F_{ax}) correlates quadratically with the P–N distance.