Metal complexes of sulphur–nitrogen chelating agents. Part 11. Synthesis, characterisation, and thermodynamics of nucleophilic substitution reactions of monohalogeno nickel(II) complexes of tridentate ligands of the type SNN in solution and the solid state

Abstract

Square-planar nickel(II) complexes [NiLX](X = Cl, Br, I, or SCN) have been obtained through deprotonation of the ligands methyl 2-(β-aminoethylamino)cyclopent-1-enedithiocarboxylate (HL¹), methyl 2-(β-aminoisopropylamino)cyclopent-1-enedithiocarboxylate (HL²), and methyl 2-(β-diethylaminoethylamino)cyclopent-1-enedithiocarboxylate (HL³). Diamagnetic complexes of composition [NiL(MeCN)][ClO₄](L = L¹–L³) and [Ni₂L₂(HL)][ClO₄]₂(L = L¹ or L²) have also been synthesised. The equilibrium constants for the nucleophilic substitution reaction [NiL²X]+ B [NiL²(B)]⁺+ X^–(X = Cl, Br, or I; B = pyridine and its methyl-substituted derivatives) have been determined spectrophotometrically in the temperature range 25–50 °C. A linear correlation has been obtained between the ΔH^⊖ and ΔS^⊖ values. The enthalpy changes (ΔH₂) and activation energies (E_a*) for the thermal dissociation [NiL²(B)]X(s)→[NiL²X](s)+ B(g) have been determined. A linear correlation has been found between the (ΔS₂) and E_a* values.