U ovoj doktorskoj disertaciji proučavano je građenje i termohromno ponašanje hloro kompleksa kobalt(II) jona u tri različita medijuma: amonijum-nitrat + zformamid (z= 2, 3, 4, 5, 6 i 20), amonijum-nitrat + zN-metilformamid (z = 3, 4, 5, 6 i 20) i amonijum-nitrat +  zN,N-dimetilformamid (z = 3) u temperaturnom intervalu od 308,15 do 348,15 K. Cilj ovog istraživanja je bio određivanje stabilnosti termohromnih kompleksa kobalta(II) koji se grade sahloridnim i nitratnim jonima i molekulima rastvarača, pronalaženje kvalitativne i kvantitativne zavisnosti konstanti stabilnosti kompleksa kobalta(II) od temperature i molskog odnosa komponenti ispitivanih sistema. Termodinamički parametri, koji karakterišu reakcije asocijacije u ispitivanim sistemima izračunati su na osnovu dobijenih konstanti stabilnosti na različitim temperaturama. Drugi deo ovog rada obuhvata fizičko-hemijsku karakterizaciju ispitivanih sistema, i  to: merenje gustine, provodljivosti i njihove viskoznosti. Na osnovu dobijeni In this thesis absorption spectra of cobalt(II) chloride in three ammonium nitrate binary mixtures with organic solvents (formamide, N-methylformamide and  N,N-dimethylformamide) at different temperatures (from 308.15 to 248.15 K) and compositions have been investigated in the wavelength range 400-800 nm. Influence of the temperature and composition of the mixture on complex formation between cobalt(II) and nitrate ions, chloride ions  and/or solvent molecules have been studied. Thermodynamic parameters for cobalt(II)-ligand association in different solventswere also determined. The second part of this work involves physical-chemical characterization of the studied systems, namely: density, electrical conductivity and viscosity measurements. On the basis of the obtained data, the types and nature of the solute-solvent interactions that occur, as well as their impact on the cobalt(II) complex formation were discussed. In all investigated systems, complexes of cobalt(II) change the geometry