DFT study and dynamic NMR evidence for cis-trans conformational isomerism in square planar Ni(II) thioselenophosphinate, Ni(SeSPPh2)2

Theoretical (DFT) and experimental (dynamic NMR) study of cis-trans conformational isomerism in Ni(II) square planar thioselenophosphinate, Ni(SeSPPh2)2, have been carried out. The DFT investigation [B3LYP/6-311++G(d,p), gas] of this complex reveals that its cis-to-trans isomerization occurs through two minimal energy crossing points (MECPs) located at the intersection of the lowest singlet and triplet PESs (potential energy surfaces). The calculated relative energies of the MECPs, 5.6 and 3.6 kcal/mol, correspond to low-energy barriers of “cis-Ni(SeSPPh2)2 [S] ⇌ pseudoterahedral-Ni(SeSPPh2)2 [T]” and “trans-Ni(SeSPPh2)2 [S] ⇌ pseudoterahedral-Ni(SeSPPh2)2 [T]” spin crossover rearrangements. The dynamic 31P NMR study of Ni(SeSPPh2)2 is fully confirms these computations: below 208 K, the cis- and trans-isomers are in a dynamic equilibrium. The free energy of activation for this fluxional process calculated from the coalescence temperature is ΔG≠ = 11.5 kcal/mol.