Pnicogen bonds in complexes with CO and CS: differentiating properties
Ab initio MP2/aug’-cc-pVTZ calculations have been performed on pnicogen-bonded complexes with CO and CS as electron-pair donors to PH2X, for X = F, NC, OH, CN, CCH, and H. CO:PH2X and OC:PH2X complexes are stabilised by traditional pnicogen bonds. CS is an electron-pair donor through its in-plane π system to four PH2X molecules. It forms C··· P phosphorus-shared bonds with some ion-pair character with PH2F, PH2(OH-Z), and PH2(OH-E), and traditional pnicogen bonds with all PH2X except PH2F. C-O and C-S stretching frequencies are blue-shifted for C···P pnicogen bonds, and red-shifted for O···P and S···P bonds. EOM-CCSD spin-spin coupling constants 1pJ(P-C) for OC:PH2X and 1pJ(P-O) for CO:PH2X are characteristic of complexes stabilised by traditional pnicogen bonds. Coupling constants 1pJ(P-C) as a function of the P-C distance for SC:PH2X illustrate the evolution of the C···P pnicogen bond. They increase as the P-C distance decreases in complexes with traditional bonds, reach a maximum for SC:PH2OH transition structures as the P-C distance further decreases and the bonds gain phosphorus-shared character, and then change sign and continue to decrease as the P-C distance further decreases and the phosphorus-shared pnicogen bonds gain ion-pair character. They approach the values of 1J(P-C) for the cation (H2PCS)+.
