The paper deals with the chemical and physical factors influencing the thermal octahedral ↔ square planar changes of nickel(II) complexes in the solid phase. The relationship between these transformations and the tetragonal distortion of the octahedral ligand field is discussed. Depending on the coordination of the ligands, these transformations can be divided into two groups: octahedral monomer ↔ square planar monomer, and octahedral polymer ↔ square planar monomer changes. Attention is directed only to octahedral and square planat Ni(II) complexes (square planar complexes with chromophores [NiN4], [NiN2O2] and [NiO4]), which can be isolated in the solid state before and after heating. The possibility of such a configurational change seems to be dependent upon the thermal stabilities of the initial and final complexes, the electronic and steric properties of the ligands, the complexing ability of the central atom, and particularly the equatorial-axial interactions of the ligands via the central atom.
The Stoichiometry of thermal decomposition was studied for the following compounds: Ni(NCS)2(2-Mepy)2 (I), (Me=methyl, py=pyridine), Ni(NCS)2(2-Etpy)2 (II) (Et=ethyl), Ni(NCS)2(2-Clpy)2 (III), Ni(NCS)2(2-Brpy)2 (IV), Ni(NCS)2(2-NH2py)2 (V), Ni((NCS)2(2-NH2py)2·3/4 (C2H5)2O (VI). The release of volatile ligands 2-Rpy is a one-step process for complexes I, II, III and IV, while for V and VI it is a two-step process, Ni(NCS)2(2-NH2py)1 (VII) being formed as an intermediate complex. It was found that complexes I and II are square-planar; the others exhibited pseudo-octahedral geometry. The differences in stereochemistry of the above complexes are explained by the different electronic properties of 2-Rpy.
The Stoichiometry of thermal decomposition was studied for the following compounds: Ni(NCS)2(pip)4 (I), (pip=piperidine), Ni(NCS)2(pip)2py·H2O (II), (py=piridine), Ni(NCS)2(4-Mepip)3 (III), Ni(NCS)2(3-Mepip)3 (IV) and Ni(NCS)2(3.5-Me2pip)3 (V). In complexes I, II, III and IV the loss of the volatile ligands (on the TG curve to 300 °C) occurs in three steps and in complex V in two steps. The loss of the last molecules of volatile ligands is accompanied by the decomposition of NCS groups. Spectral data and magnetic moment values for the initial complexes I and II (together with the defined intermediates) indicated pseudooctahedral configuration while pentacoordination for complexes III, IV and V. Structural changes of the complexes studied in thermal decomposition reactions are discussed.
Thermogravimetry (TG), differential thermal analysis (DTA) and other analytical methods have been applied to the investigation
of the thermal behaviour and structure of the compounds Mg(Ac)2 2H2 O(I), Mg(ClAc)2 2H2 O(II) and Mg(Cl2 Ac)2 H2 O(III) (Ac =CH3 COO− , ClAc =ClCH2COO− , Cl2Ac =Cl2 CHCOO− ). The solid phased intermediate and resultant products of thermolysis had been identified. The possible scheme of destruction
of the complexes is suggested. The halogenacetato magnesium complexes (II–III) are thermally more stable than the acetatomagnesium
complex I. The final products of the decomposition of compounds were MgO. Infrared (IR) data suggest to a unidentate coordination
of carboxylate ions to magnesium ions in complexes I–III.
The thermal decomposition of the complexes Mg(Clac)2 (ron)2 3H2 O(I), Mg(Cl2 ac)2 (ron)2 3H2 O(II) and Mg(Cl3 ac)2 (ron)2 3H2 O(III), where Clac =ClCH2 COO- , Cl2ac =Cl2 CHCOO- , Cl3ac =Cl3 CCOO- and ron =3-pyridylcarbinol (ronicol) had been investigated in air atmosphere in temperature range 20–1000C by means of TG and DTA.
The composition of the complexes and the solid state intermediate and resultant products of thermolysis had been identified
by means of elemental analysis and complexometric titration. The possible scheme of destruction of the complexes is suggested.
The final product of the thermal decomposition was MgO. The thermal stability of the complexes can be ordered in the sequence:
I<III<II. IR data suggest that ronicol was coordinated to Mg(II) through the nitrogen atom of its heterocyclic ring.
The influence of factors caused by different experimental conditions on the stoichiometry of thermal decomposition of the complex Ni(NCS)2(β-picoline)4 was studied. By means of TG it was found that the release ofβ-picoline was shown by the derivatograph (air atmosphere, sample of 50–500 mg, rate of temperature increase 1–6°/min, crucible with a hole in the bottom) in three steps (−2, −1, −1), by the micro-balance (sample of 2.59 mg) in two steps (−2, −2), and by the vacuum thermobalance in one step (−4). Under quasi-isothermal and quasi isobaric conditions in a special crucibleβ-picoline was released in three steps (−1, −2, −1). An analogous result was obtained with the derivatograph using a crucible with a lid. The reasons for these differences are discussed.