New light on an old debate: does the RCN-PtCl2 bond include any back-donation? RCN←PtCl2 backbonding vs. the IR νCN blue-shift dichotomy in organonitriles-platinum(ii) complexes. A thorough density functional theory - energy decomposition analysis study

For a series of organonitrile [RCN (R = Me, CF3, Ph, CH3Ph, CF3Ph)] ligands, the nature of the N-Pt bond in the related cis-/trans-(RCN)(2)PtCl2 complexes has been computationally investigated by Density Functional Theory. A fragment based bond analysis has been performed in the canonical Kohn-Sham molecular orbitals framework, and it has been ultimately assessed that this bond is characterized both by N -> Pt sigma and by N <- Pt pi contributions. Voronoi Deformation Density charges further confirms the occurrence of N <- Pt pi interactions. Moreover, the Energy Decomposition Analysis-Natural Orbital for Chemical Valence (EDA-NOCV) method shows that the strength of the N <- Pt pi interaction is not negligible by contributing to about 30-40% of the total orbital interaction. Finally, the well-known nu(CN) blue-shift occurring upon coordination to Pt-II, has been thoroughly investigated by exploiting the EDA-NOCV and by evaluating nu(CN) and force constants. The origin of the nu(CN) blue-shift in these systems has been discussed on the basis of the CN bond polarization. N <- Pt pi backbonding causes only a systematic decrease of the observed nu(CN) blue-shift when compared to the one calculated for RCN-X (X = H+, alkaline, Lewis acids) herein reported (X = purely sigma acceptors).