Pnicogen-Bonded Anionic Complexes

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the pnicogen-bonded complexes H2YP:X, for X,Y = Cl, NC, F, CCH, and CH3. Of the 36 possible complexes, only 21 are unique equilibrium structures. All substituents form (H2XPX) complexes with symmetric X–P–X bonds. The P–A ion–molecule pnicogen bonds in these and some additional complexes have partial covalent character, while some P–A′ covalent bonds have partial ion–molecule character. A and A′ are the atoms of X and Y, respectively, which are directly bonded to P. Complexes with these types of bonds include the symmetric complexes (H2XPX), H2(CH3)­P:F, H2(CCH)­P:F, H2FP:NC, H2FP:Cl, H2FP:CN, and H2(NC)­P:Cl. Charge transfer from A to the P–A′ σ* orbital stabilizes H2YP:X complexes and leads to a reduction of the negative charge on X. For fixed X, the smallest negative charge occurs in the symmetric complex. Then, for a given X, the order of decreasing negative charge with respect to Y is CH3 > CCH > CN (bonded through C) > F > NC (bonded through N) > Cl, which is also the order of decreasing P–A distance. EOM-CCSD spin–spin coupling constants 1pJ(P–A) differentiate between shorter ion–molecule pnicogen bonds with partial covalent character and longer P···A ion–molecule pnicogen bonds. Similarly, coupling constants 1J(P–A′) differentiate between longer covalent P–A′ bonds with partial ion–molecule character and shorter P–A′ covalent bonds.