Entropy-controlled formation of metal complexes with hindered ligands. The system zinc(II)1,3,5-cis,cis-triaminocyclohexane

The enthalpy and entropy of the reaction of H+ and Zn [7440-66-6] with 1,3,5-cis,cis-triaminocyclohexane [26150-46-9] (tach) are given at 25° in 0.1M KCl. Tach has 3 primary amine groups at the corners of an equilateral triangle. The complex Zn(tach)2+ is less stable than the corresponding complexes with linear triamines. The stereochem. of the Zn complexes in aq. soln. is somewhat uncertain. The rigidity of the ligand explains the large entropy contribution which alone accounts for the stability of Zn(tach)2+. The reaction of Zn(tach)2+ with OH- is exothermic and is favored by the entropy change. The exothermic nature reflects the formation of a stronger metal-O bond following removal of a proton