Electrochemical And Structural Studies Of Nickel(Ii) Complexes With N2O2 Schiff-Base Ligands .2. Crystal And Molecular-Structure Of N,N'-1,2-Ethane-1,2-Diyl-Bis(2-Hydroxyacetophenonylideneiminate)Nickel(Ii), N,N'-1,2-Cis-Cyclohexane-1,2-Diyl-Bis(2-Hydroxyacetophenonylideneiminate)-Nickel(Ii) And N,N'-1,2-Benzene-1,2-Diyl-Bis(3,5-Dichlorosalicylideneiminate)Nickel(Ii)