The Cu(II) ion-catalysed kinetics of oxidation of H ₂O ₂ by [Ni^IIIL] [where L = L₁ (cyclam) and L ₂ (1,8-bis(2-hydroxyethyl)-1,3,6,8,10,13-hexaazacyclotetradecane)] was studied in the pH range of 3.6–5.6 in acetic acid-acetate buffer medium at 25◦C in the presence of sulphate ion. The ionic strength (I) was maintained at 0.5 M (NaClO₄). The rate constants showed an inverse acid dependence and [Ni^IIIL ₂] was observed to be more stable than [Ni^IIIL₁]. The rate of the reaction of both complexes with hydrogen peroxide shows contrastingbehaviour at pH > 2.5 when compared to the same reaction in perchloric acid medium. DFT calculations performed on the complexes [Ni^IIIL₁ (SO₄)(OAc)] and [Ni^IIIL ₂ (SO₄)(OAc)] reveal that both the acetate and sulphate ligands are axially coordinated to the metal centre. In addition, there is strong hydrogen bonding between the axial ligand and NH hydrogen of the macrocyclic ligand. The computed covalent bond ordersin the aqueous medium predict that the acetate forms stronger coordinate bond with Ni ion than the sulphate ligand. The hydroxyl group present in one of the pendant groups of L ₂ forms a strong hydrogen bond with thesulphate ligand which leads to additional stability in [Ni^IIIL ₂ (SO₄)(OAc)].

Density functional theory calculations at M052X/6-311++ G^** level were performed to under-stand the structure and stability of Ni(II) tetraaza macrocyclicdicarbinolamine complex 1. The preferentialstability of 1 over the hitherto unknown Ni(II) complex having fully conjugated macrocyclic ligand 2,is examined by analyzing geometric and electronic structures and energy considerations. The present calcula-tions predict that in the trans (C₂) structure, 1 is 102 kcal/mol more stable than its components 2 and 2(OH) at M062X-D3/def2-QZVP//M052X/6-311++ G^** level. This significant stabilization explains the formation of 1 as experimentally observed. The calculations support a distorted square planar environment for Ni in 1,inagreement with the observed spectral and magnetic properties. In order to understand the stability of 1 ,weexamined the second-order stabilizing interactions in natural bond orbital (NBO) basis, the role of the noncovalent dispersion energy, macrocyclic cavity size, Ni-ligand covalent bond strength, natural electronic population on the atomic centers and the nature of the frontier molecular orbitals in the complexes. The present study reveals that the higher stability of 1 over 2 is primarily due to the stronger covalent bonds between the Ni(II) centre, and two of the coordinating nitrogen atoms in 1 than in 2 and significant second-order stabilizing interactions originating from the NBOs involving the oxygen atoms