Abstract
The mechanism of substitution water exchange reactions in square planar trans‐Pt[(NH~3~)~2~T(H~2~O)]^n+^ complexes is studied (TH~2~O, NH~3~, OH^−^, F^−^, Cl^−^, Br^−^, H~2~S, CH~3~S^−^, SCN^−^, CN^−^, PH~3~, CO, CH~3~^−^, H^−^, C~2~H~4~). The trans effect is explained in terms of σ‐donation and π‐back‐donation whose relative strengths are quantified by the changes of electron occupations of 5d platinum atomic orbitals. The σ‐donation strength is linearly correlated with the PtH~2~O (leaving ligand) bond length (trans influence). The kinetic trans effect strength correlates proportionally with the σ‐donation ability of the trans‐ligand except the ligands with strong π‐back‐donation ability that stabilizes transition state structure. The σ‐donation ability of the ligand is dependent on the σ‐donation strength of the ligand in the trans position. Therefore the trans effect caused by σ‐donation can be understood as a competition between the trans‐ligands for the opportunity to donate electron density to the central Pt(II) atom. The influence of the trans effect on the reaction mechanism is also shown. For ligands with a very strong σ‐donation (e.g. CH~3~^−^ and H^−^), the substitution proceeds by a dissociative interchange (I~d~) mechanism. Ligands with strong π‐back donation ability (e.g. C~2~H~4~) stabilize the pentacoordinated intermediate and the substitution proceeds by a two step associative mechanism. For ligands with weak σ‐donation and π‐back‐donation abilities, the highest activation barriers have to be overcome and substitutions can be described by an associative interchange (I~a~) mechanism. The results are supported by the energy decomposition and the natural orbital analysis. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008