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

Abstract

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)₂PtCl₂ 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 σ and by N←Pt π contributions. Voronoi Deformation Density charges further confirms the occurrence of N←Pt π interactions. Moreover, the Energy Decomposition Analysis-Natural Orbital for Chemical Valence (EDA-NOCV) method shows that the strength of the N←Pt π interaction is not negligible by contributing to about 30–40% of the total orbital interaction. Finally, the well-known ν_CN blue-shift occurring upon coordination to Pt^II, has been thoroughly investigated by exploiting the EDA-NOCV and by evaluating ν_CN and force constants. The origin of the ν_CN blue-shift in these systems has been discussed on the basis of the CN bond polarization. N←Pt π backbonding causes only a systematic decrease of the observed ν_CN blue-shift when compared to the one calculated for RCN–X (X = H⁺, alkaline, Lewis acids) herein reported (X = purely σ acceptors).