Thermal investigation and stereochemical studies of some cyclic diamine complexes of nickel(II), zinc(II), and cadmium(II) in the solid state

Abstract

Nickel(II), zinc(II), and cadmium(II) complexes of piperazine (pipz), N-methylpiperazine (mpipz), and 1,4-diazacycloheptane (dach) with the compositions [NiL₂(NCS)₂], [Ni(dach)₂][SCN]₂, [ZnL(NCS)₂], [Zn(dach)₂][SCN]₂, [CdL(NCS)₂], and [Cd(dach)(NCS)₂](L = pipz or mpipz) have been synthesised. Attempts to prepare N,N′-dimethylpiperazine complexes failed. Some intermediate complexes were isolated by pyrolysis. Configurational and conformational changes have been studied by elemental analyses, i.r. spectra, magnetic moment measurements, and thermal analysis. All the complexes of pipz and mpipz appear to be octahedral and those of dach to be square planar. Activation energies (E_a), enthalpy (ΔH) and entropy changes (ΔS) for the dehydration and decomposition reactions show that the order of stability of the complexes (with respect to E_a) follows the trend pipz > mpipz > dach. A linear correlation has been found between E_a and ΔS for the decomposition of the nickel complexes.