Effect of methyl substitution in 1,2-ethanediaminenickel(II) complexes upon their thermal stabilities.

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The enthalpy changes (ΔH) in the reaction of three types of the nickel(II) complexes, [Ni(AA)2(H2O)2]X2·nH2O, [NiX2(AA)2], and [Ni(AA)2]X2, with aqueous potassium cyanide solution were calorimetrically measured at 25 °C, where AA denotes a diamine selected from 1,2-ethanediamine (en), N-methyl-1,2-ethanediamine (N-men), N,N-dimethyl-1,2-ethanediamine (NN-dmen), N,N′-dimethyl-1,2-ethanediamine (NN′-dmen), and N,N,N′-trimethyl-1,2-ethanediamine (NNN′-tmen); X is Cl, Br, I, NO3, or ClO4; n=0, 2, or 3. The thermal stabilities of the complexes in the solid state defined by the enthalpy changes (not by the free energy changes) were compared with one another, so that the thermal stabilities of the complexes are highly dependent on numbers and/or positions of N-methyl groups on diamines. In the series of [NiX2(AA)2] and [Ni(AA)2]X2 type complexes, the NN′-dmen complexes are the most stable. Except for the NN′-dmen complexes, as the numbers of N-methyl group(s) contained in the diamines increase, the complexes have the tendency to become less stable. The relative thermal stabilities of the present complexes were discussed in terms of the basicities of diamines and steric effect of N-methyl groups.
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