In this thesis, thermochemical data for two different groups of transition-metal complex are reported. The first group of complexes is of the type MX2Ln, where M is a first-row transition-metal or Cd, X is a halogen and L is a heterocyclic ligand. The heterocyclic ligands are as follows: thiazole, benzothiazole and their methyl-derivatives} benzoxazole and substituted benzoxazoles; 3-methylpyrazole and the 2- and 3-halogenopyridines. These MXgLn complexes decompose, as indicated below, on raising the temperature.