Linear and Bent Noncovalent M•••S=C=N-M' Interactions: The Case of Palladium(II) and Platinum(II) Thiocyanate Species

Analysis of X-ray crystal structures of newly prepared thiocyanate Pd-II and Pt-II complexes revealed the bifurcated ((M) double under bar-(N) double under bar)center dot center dot center dot S contacts in which angles.M center dot center dot center dot S=C are close to 180 degrees. All M center dot center dot center dot S=C=N-M' noncovalent interactions-identified in this study and also those found on the Cambridge Structural Database search-were systematized as linear (approximately 180 degrees) and bent (angle M center dot center dot center dot S=C 80-120 degrees). Results of appropriate theoretical study indicate that, although the linear configuration of the Pd-II and Pt-II structures is primarily determined by crystal packing effects, the attractive M center dot center dot center dot S forces are far from negligible and the linear structures correspond to the energy minimum. Furthermore, the linear arrangement is mostly determined by orbital interactions, while the bent configuration by electrostatic forces. Molecular electrostatic potential (MEP) surfaces along with the quantum theory of "atoms-in-molecules" (QTAIM) and the noncovalent interaction plot index (NCIPlot) computational tools were used to characterize the bifurcated ((M) under bar-(N) under bar)center dot center dot center dot S interactions. The nature of the noncovalent M center dot center dot center dot S=C=N-M' forces in the Pd-II and Pt-II structures was established by NBO analysis: in both cases, the predominant donor-acceptor orbital interactions include the charge transfer from lone pair orbitals located at the M atom to the sigma*-orbital of the S-C bond of a thiocyanate ligand from a neighboring complex.