M.B. Ćelap

Effect of the geometrical configuration of facial and meridional isomers of cobalt(III) and chromium(III) complexes on their RF values

Abstract The RF values of eight pairs of facial and meridional of cobalt(III) and chromium(III) complexes have been determined by thin-layer chromatography with 21 solvent systems and by paper chromatography with 26 solvent systems. In the thin-layer chromatographic separations on silica gel the meridional isomers always exhibited higher RF values when single component solvent systems were used. On the contrary, with multicomponent solvent systems, in 30% of cases the elution order of the isomers was reversed. In paper chromatographic separations with solvent systems not containing strong acids the meridional isomers were found to exhibit higher RF values. However, when solvent systems containing strong acids were used in some cases the elution order was reversed. Possible mechanisms for the separation of those isomers are considered.

New synthesis and thin-layer chromatography of tris-(alkylxanthato)cobalt(III) complexes

A new method for the preparation of tris(alkylxanthato)cobalt(III) complexes, [Co(S2COR)3], involving the reaction of sodium hexanitrocobaltate(III) with alkylxanthates, is described. It has been established that the RF values of these complexes, obtained by silica gel thin-layer chromatography, increase with increasing carbon number of the alkyl group, and that there is a linear relationship between this number of carbon atoms in an homologous series and the RM values of the complexes. The mechanism of the chromatographic separations is thought to be based on adsorption.

Synthesis and structure of aminocarboxylato(1,4-diaminobutane)dinitrocobalt(III) complexes. Crystal and molecular structure of the δ-(—)589- CIS(NO2)trans(NH2)-(R-alaninato)(1,4-diaminobutane)dinitrocobalt(III) diastereomer.

Abstract The preparation of the trans(NO2) and cis (NO2),trans(NH2) isomers of the (1,4- diaminobutane)(glycinato)dinitrocobalt(III) complex, as well as trans(NO2), δ-(—)589-cis (NO2),trans(NH2) and λ-(+)589-cis (NO2),trans(NH2) isomers of the (R-alaninato) (1,4-di- aminobutane)dinitrocobalt(III) complex, is described. These substances were obtained in the reaction of sodium hexanitrocobaltate(III) with 1,4-diaminobutane and the corresponding amino acid. The separation of the isomers was performed on an anionic Dowex 1-X4 column. The composition of the isolated substances was determined by elemental microanalysis. The position of the nitro groups in the substances obtained was determined from their electronic absorption spectra in the UV region and by paper chromatography. The X-ray crystal structure of the δ-(—)589-cis(NO2),trans(NH2)-(R-alaninato)(1,4-diamino-butane) dinitrocobalt(III) diastereomer was established and the arrangement of ligands in other isomers prepared was established by comparing their electronic absorption spectra with the alaninato diastereomer.

Thin-layer chromatography on silica gel of a homologous series of bis(alkylxanthato)nickel(II) complexes

Abstract The direct synthesis of bis(alkylxanthato)nickel(II) complexes is described, consisting in the addition of nickel(II) chloride to a previously heated solution of sodium hydroxide and carbon disulphide in the corresponding alcohol. It was established that the RF values of complexes obtained with non-aqueous solvent systems increased with increasing number of carbon atoms in the xanthato ligands used, but the opposite order was found using an aqueous solvent system. There is a linear dependence between the RM values of complexes and the number of carbon atoms in the hydrophobic part of the ligands. In the former instance the proposed separation mechanism involves the formation of hydrogen bonds between strongly electronegative atoms of the ligands (O and S) and silanol groups of silica gel and in the latter a hydrophobic interaction mechanism was assumed. In this manner it was established that the previously found regularities for octahedral alkylxanthatocobalt(III) complexes can be applied also to the investigated square-planar alkylxanthatonickel(II) complexes.

Investigation of conformational and vicinal optical activity of mono(s-aminocarboxylato) cobalt(III) complexes

Abstract Conformational and vicinal optical activities of four classes of mixed mono( S -aminocarboxylato)cobalt(III) complexes, lacking configurational dissymetry, were studied. The efrect of conformational changes of aminocarboxylato chelate ligands containing various ring side alkyl groups, caused by the replacement of a relatively small ammine by a bulky nitro ligand, were followed. Solution CD spectra supported the earlier conclusion that the more axially oriented chelate ring side group of an S -aminocarboxylato ligand produces a larger negative dichroism in the region of the first, spin allowed, d-d electronic transition. Solid-state CD spectra enabled the application of the theoretically based sector rules for optical activity in cobalt(III) complexes to the four investigated complexes, for which crystal structures have been determined. It emerged that an axial orientation of the aminocarboxylato chelate ring side alkyl is very effective, not only due to the displacement of this alkyl from an equatorial position, nodal plane of the Sector rule, but also due to the proximity of the oriented group to the metal centre.

Simultaneous effect of geometrical isomerism and chelate ring size of tris(aminocarboxylato)cobalt(III) complexes on their behaviour in thin-layer chromatography on silica gel and alumina

Abstract The simultaneous effects of geometrical isomerism and chelate ring enlargement of octahedral complexes on their RF values obtained by chromatographing twelve isomers of complexes of the general formula [Co(gly)3-α (β-ala)n] (n = 0–3; glyH = glycine; β-alaH = β-alanine) on thin layers of silica gel and alumina by using five monocomponent solvent systems was investigated. It was found that in most instances the effect of the ring size was predominant. Within the series of facial or meridional isomers it was established that with the enlargement of the chelate ring the RF values of the complexes decreased on silica gel but increased on alumina, which was ascribed to the formation of hydrogen bonds of different strengths between the complexes and the adsorbent.

The influence of trans-ligands on the Co–NO2 bond length in mixed cobalt(III) complexes

An X-ray crystal structure investigation of 12 cobalt(III) complexes has shown that the Co–NO2 bond length is shorter by an average of 0·05 A when trans-to carboxylato-oxygen than when trans- to a nitrogen ligator.