A detailed theoretical study of the interaction of thiourea with cis-diaqua(ethylenediamine) platinum(II)

Density functional theory (DFT), Hartree-Fock (HF) and Möller-Plesset second order perturbation (MP2) calculations were applied to study the mechanism of the two-step substitution reaction of cis-[Pt(en)(H2O)2]2+ (en = ethylenediamine) and thiourea. Pentacoordinated platinum transition state (TS) geometries for both steps of the title SN2 reaction were fully optimized at HF, B3LYP and MPW1PW91 levels of calculations. Distorted trigonal-bipyramidal TS's with rather small entering ligand to metal to leaving ligand angles (≈800) and with prolonged bonds to the leaving and entering ligands (RPtO ≈ 2.4 Å and RPtS ≈ 2.85 Å) were investigated in this paper. Natural bond orbital (NBO) analysis method was performed for the investigation of major stabilizing orbital interactions. The self-consistent reaction field (SCRF) methods were used to study the reaction in liquid phase. Both steps of the reaction are exothermic and the computed activation enthalpy (ΔH‡) and entropy (ΔS‡) in the rate-determining steps (RI → TS → PI) are in good agreement with the experimental results. Thorough study of the reaction for improved values of energies and activation parameters was carried out at PCM/MP2 and PCM/B3LYP levels using 6-311+(d,p) and 6-311++(2d,2p) basis sets. The structural and activation parameters of the reaction suggest an associative interchange (Ia) substitution mechanism.