Is the charge on the nitrile carbon atom a driving force of the nucleophilic addition to coordinated nitriles?

Abstract

The availability of the electrostatic model for the interpretation of nucleophilic addition to nitriles—free and coordinated to platinum—has been discussed. The analysis of effective atomic charges calculated at the HF/6-31G*, B3LYP/6-31G* and MP2/6-31G* levels of theory using both Mulliken and Natural Bond Orbital (NBO) schemes was performed for the neutral and cationic dinitrile complexes of Pt II and Pt IV trans-[Pt IIL 2(NCCH 3) 2] 2+ and trans-[Pt IVCl 2L 2(NCCH 3) 2] 2+ (L=Cl −, NH 3, PH 3, NCCH 3). The oxidation state of the metal, overall charge of the complex moiety and nature of the ligand L only weakly affect the atomic charge on the C atom of the CN group of the bound nitrile (especially the Mulliken one). Meanwhile, the significant enhancement of the latter charge from the free to the coordinated ligand was found for the NBO method. The activation of the nitriles in the complexes compared to the free ligand towards the nucleophilic addition to the CN bond can be interpreted in terms of charge control, but the enhancement of the reactivity from the complexes of Pt II to those of Pt IV should be explained by other arguments (orbital control and the effect of the transition state). The nature of the chemical bonds was investigated using the AIM and NBO methods and the charge decomposition analysis (CDA). The nitrile ligands do not exhibit noticeable π-acceptor properties in the complexes, and the electrostatic interaction plays an important role in the formation of the coordination bond.