It has been established, using spectroscopic methods, that 4-imidazoline-2-thione and its biologically active derivatives (1-methyl-4-imidazoline-2-thione and ethyl 3-methyl-2-sulfanylidene-imidazole-1-carboxylate), as well as thiourea, in a reaction with aquapentacyanoferrate(II/III) give the corresponding mononuclear pentacyano(ligand)ferrate(II/III) complex. The formation of pentacyano(imidazoline-2-thione)ferrate(II) is a simple ligand substitution reaction as opposed to the formation of pentacyano(imidazoline-2-thione)ferrate(III), whose more complex mechanism has been confirmed to include aquapentacyanoferrate(II) as a catalyst. Equilibrium constant and kinetic parameters for the formation of both pentacyano(imidazoline-2-thione)ferrate(II) and pentacyano(imidazoline-2-thione)ferrate(III) complexes have been determined. Kinetic parameters for the dissociation of pentacyano(imidazoline-2-thione)ferrate(II) complexes have also been determined. Pentacyano(imidazoline-2-thione)ferrate(III) complexes have been shown to be significantly more stable and inert than analogous complexes of iron(II). On the basis of spectroscopic and kinetic parameters it has been concluded that selected imidazoline-2-thiones coordinate pentacyanoferrate(II/III) through a sulfur atom. Imidazoline-2-thiones can be considered weak -acceptor and strong -donor ligands. Crystal and molecular structure of pentacyano(thiourea)ferrate(III) salt has been determined. It is the first reported structure of a pentacyano(ligand)ferrate(III) complex in which the ligand is not bound through the nitrogen atom.