Structure-directing sulfur • • •metal noncovalent semicoordination bonding

The abundance and geometric features of nonbonding contacts between metal centers and `soft' sulfur atoms bound to a non-metal substituent R were analyzed by processing data from the Cambridge Structural Database. The angular arrangement of M, S and R atoms with angle(R-S center dot center dot center dot M) down to 150 degrees was a common feature of the late transition metal complexes exhibiting shortened R-S center dot center dot center dot M contacts. Several model nickel(II), palladium(II), platinum(II) and gold(I) complexes were chosen for a theoretical analysis of R-S center dot center dot center dot M interactions using the DFT method applied to (equilibrium) isolated systems. A combination of the real-space approaches, such as Quantum Theory of Atoms in Molecules (QTAIM), noncovalent interaction index (NCI), electron localization function (ELF) and Interacting Quantum Atoms (IQA), and orbital (Natural Bond Orbitals, NBO) methods was used to provide insights into the nature and energetics of R-S center dot center dot center dot M interactions with respect to the metal atom identity and its coordination environment. The explored features of the R-S center dot center dot center dot M interactions support the trends observed by inspecting the CSD statistics, and indicate a predominant contribution of semicoordination bonds between nucleophilic sites of the sulfur atom and electrophilic sites of the metal. A contribution of chalcogen bonding (that is formally opposite to semicoordination) was also recognized, although it was significantly smaller in magnitude. The analysis of R-S center dot center dot center dot M interaction strengths was performed and the structure-directing role of the intramolecular R-S center dot center dot center dot M interactions in stabilizing certain conformations of metal complexes was revealed.