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)(-), H-2(CH3)P:F-, H-2(CCH)P:F-, H2FP:NC-, H2FP:Cl-, H2FP:CN-, and H-2(NC)P:Cl-. Charge transfer from A to the P-A' sigma* 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 (1p)j(P-A) differentiate between shorter ion-molecule pnicogen bonds with partial covalent character and longer P center dot center dot center dot A ion-molecule pnicogen bonds. Similarly, coupling constants (1)J(P-A') differentiate between longer covalent P-A' bonds with partial ion-molecule character and shorter P-A' covalent bonds.