Thermal and structural studies of amide complexes of transition metal(II) chlorides. II: Kinetics

Abstract

The possibility of correlating the kinetic parameters for the thermal decompositions of a series of NiLCl 2 complexes, where L= N-methylformamide ( nmf), N-methylacetamide ( nma) or acetamide ( aa), with the nature (steric and electronic effects) of the coordinated ligands and the strengths of the metal–ligand bonds has been explored. The rates of removal of L in single endothermic processes were measured using isothermal TG in nitrogen. Plots of α against time are deceleratory and are best described by either the R2 or R3 expressions. An empirical (B2) expression: v r=1−( kt) b , was introduced to give the best description of the results for the Ni( nma)Cl 2 complex. Comparable E a values were obtained using various isothermal and non-isothermal methods of analysis. E a values for the NiLCl 2 system (calculated using the R3 model) generally increased with an increase in basicity of the amide ligand: N- methylformamide∼acetamide<N- methylacetamide E a values were found to be lower than the corresponding decomposition enthalpies (Δ H L), indicating that cleavage of the nickel–amide bond could be the rate-controlling event. The order of E a values was found not to coincide with that of the Δ H L values. Extrapolated onset temperatures ( T e) and temperatures at maximum decomposition rate ( T max) were determined from TG and DSC curves. No simple correlation was found between E a, Δ H L, T e or T max and the spectral properties of the complexes.