Formation and structure of thiocyanatochromium(III)-complexes

The formation of [Cr(NCS)(n)X6-n](3-), [Cr(NCS)(6)](3-), [Cr(NCS)(4)L-2](-) and [Cr(NCS)(4)LL'](-) type complexes (X - halides or pseudohalides, L,LL' - monodentate or bidentate amines or phosphines) were discussed on the bas is of the authors' studies (over 30 new [Cr(NCS)(4)L-2](-) and [Cr(NCS)(4)L L'](-) derivatives). The isolation of a great number of salts by double dec omposition reactions with ammonium-, phosphonium-, metal- and metal(III)ami ne cations ([M(amine)(6)](3+), [Ni(amine)(5)X](2+), [M(amine)(4)X-2](+); M- Co, Cr) proves the composition of the these complexes. The problems of the geometric and bonding isomerism were also discussed. The co-ordination of the NCS ligand through the N and S atom was studied by performing HMO calculations concerning the d-bond order of the NC and CS b onds and by comparing them with IR spectral data and force constants calcul ated from the <(nu)over tilde>(CN) and <(nu)over tilde>(CS) frequencies. In the case of M[Cr(NCS)(6)] type complexes the influence of the co-ordinated L amine was investigated. By recording the electric spectra of [Cr(NCS)(4)L-2](-) type complexes the ligand field transition bands were used to calculate ligand field parameter s, nephelauxetic ratios and PC values for the Cr-N bond. The correlation be tween the covalent character (PC) and base strength of the ligands was disc ussed.