Pnicogen-Bonded Anionic Complexes

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the pnicogen-bonded complexes H<sub>2</sub>YP:X<sup>–</sup>, for X,Y = Cl, NC, F, CCH, and CH<sub>3</sub>. Of the 36 possible complexes, only 21 are unique equilibrium structures. All substituents form (H<sub>2</sub>XPX)<sup>−</sup> 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 (H<sub>2</sub>XPX)<sup>−</sup>, H<sub>2</sub>(CH<sub>3</sub>)­P:F<sup>–</sup>, H<sub>2</sub>(CCH)­P:F<sup>–</sup>, H<sub>2</sub>FP:NC<sup>–</sup>, H<sub>2</sub>FP:Cl<sup>–</sup>, H<sub>2</sub>FP:CN<sup>–</sup>, and H<sub>2</sub>(NC)­P:Cl<sup>–</sup>. Charge transfer from A to the P–A′ σ* orbital stabilizes H<sub>2</sub>YP:X<sup>–</sup> 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 CH<sub>3</sub> > 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 <sup>1p</sup><i>J</i>(P–A) differentiate between shorter ion–molecule pnicogen bonds with partial covalent character and longer P···A ion–molecule pnicogen bonds. Similarly, coupling constants <sup>1</sup><i>J</i>(P–A′) differentiate between longer covalent P–A′ bonds with partial ion–molecule character and shorter P–A′ covalent bonds.