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(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.
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.
TG, DTA and other analytical methods were applied to investigate the thermal behaviour and structures of the compounds Mg(ClAc)2(Py)22H2O (I), Mg(Cl2Ac)2(Py)H2O (II), Mg(Cl3Ac)2(Py)6H2O (III) and Mg(SCN)2(Py)32H2O (IV), where ClAc=ClCH2COO−, Cl2Ac=Cl2CHCOO−, Cl3Ac=Cl3CCOO− and Py=Pyridine. The compositions of the complexes and the solid-state intermediates and products of thermolysis were identified
by means of elemental analysis. Possible schemes of destruction of the complexes are suggested. The final products of the
thermal decompositions were MgO (I–III) and MgS (IV). The IR data suggest unidentate coordination of the carboxylate ions
to Mg(II) in complexes I–III. Py is coordinated to the Mg(II) through the nitrogen atom of its heterocyclic ring.
The stoichiometry of thermal decomposition was studied for the following Mg(II) nicotinamide (NA) complexes: Mg(Ac)2(NA)52H2O (I), Mg(CIAc)2(NA)66H2O (II), Mg(Cl2Ac)2(NA)65H2O (III) and Mg(Cl3Ac)2(NA)62H2O (IV), where Ac=CH3COO−, ClAc=ClCH2COO−, Cl2Ac=Cl2CHCOO− and Cl3Ac=Cl3CCOO−. Heating the compounds results first in the release of water molecules. The NA molecules are released in one step (complexes
II and III) or in two steps (complexes I and IV). The compositions of the complexes, the solid-state intermediates and the
products of thermolysis were identified by means of elemental analysis and complexometric titration. The results reveal that
MgO is left as residue at the end of thermal degradation of compounds I–IV, NA is coordinated to Mg(II) through the nitrogen
atom of the heterocyclic ring. The IR data indicate unidentate coordination of the carboxylate ions to the Mg(II) in complexes
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.