Synthesis and characterization of the trans(O6) isomer Ni(II) complex containing symmetrical edta-type ligand with mixed carboxylate and diamine rings: Quantum-mechanical evaluation of different isomers

Abstract The trans (O 6 ) isomer ( trans (O 6 ) refer to two six-membered rings in trans position) of the [Ba(H 2 O) 4 ][Ni(1,3-pddadp)]·4H 2 O (1,3-pddadp = 1,3-propanediamine- N,N′ -diacetate- N,N′ -di-3-propionate) was synthesized and spectroscopically characterized (IR, UV–Vis). The trans (O 6 ) geometry was confirmed by X-ray diffraction analysis. Single crystal X-ray diffraction of the complex revealed an octahedral geometry around the Ni(II) centre. Extensive strain analysis of [M(edta-type)] 2− complexes (M = Ni, Co, Cu) has been performed and discussed in detail. DFT/NBO analysis (DFT = Density Functional Theory, NBO = Natural Bond Orbital) was performed for all isomers of [Ni(1,3-pddadp)] 2− complex. DFT theory shows that trans (O 6 ) isomer is 0.53 kcal mol −1 more stable than trans (O 5 O 6 ) and 1.95 kcal mol −1 more stable than trans (O 5 ). NBO analysis for all three isomers predicts the existence of a 3-center 2-electron (β system) A: -Ni-: B hyperbonds along with O:-C-:O carboxylate triads. The Second-Order Perturbation Theory Analysis predicts molecular stabilization that comes from delocalization caused by n A  →  σ NiB* or n B  →  σ NiA* charge transfer.