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The correlation was investigated between the observed heat of ligation and calculated quantum chemical quantities for octahedral [M(H2O)6-x(NH3)x]2+ (M=Fe(II), Ni(II)) complexes by EHMO(Extended Huckel Molecular Orbital) and ZINDO/1(Zemer's Intermediate Neglected of Differential Overlap)method. The net charge of Fe2+ and Ni2+ ion of octahedral [M(H2O)6-x(NH3)x]2+(M=Fe(II), Ni(II)) complexes(x=O, 1, …, 6) decreased with substituting NH3 for H2O molecules. It has found that a good correlation exists between the observed heat of ligation and the calculated quantum chemical quantities such as net charge of central atom, enthalpy of formation, and total dissociation energy. From this finding, we have obtained the following semiempirical linear equation ΔHobs=-0.2858qFe+0.8813(r=0.97), ΔHobs=-0.8981qNi+1.7929(r=0.95), ΔHobs=-0.0031Hf(Fe)+0.5725(r=0.97), ΔHobs=-0.0095Hf(Ni)+0.9193(r=0.97), ΔHobs=0.0476Ediss(Fe)+0.6434(r=0.94), ΔHobs=0.1401Ediss(Ni)+1.1393(r=0.93)
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