Metal complexes of amino acids. 3. The circular dichroism of cobalt(3) ammine and ethylenediamine complexes with L-amino acids.

Abstract

Abstract In order to study the vicinal effect of the l-amino acid coordinated in a complex on the central metal ion, a number of cobalt(III) l-amino acid complexes of three types were prepared, i. e., [Co(NH3)5(l-amH)]X3, [Co(l-am)(NH3)4]SO4, and trans-[Co en2(l-amH)2]X3, where l-amH denotes an l-amino acid, and where X is Cl, Br, or I. The circular dichroism (CD) and the rotatory dispersion (RD) spectra of their aqueous solutions were measured in the wavelength range from 650 to 300 mμ. In the region of the first absorption band, both the [Co(NH3)5(l-amH)]3+ and the trans-[Co en2(l-amH)2]3+ ions show one or two CD bands to be expected in complexes of the C4v symmetry with a structure of the [Co N5O]-type and of the D4h symmetry with a structure of the trans-[Co N4O2]-type respectively. The positions of the CD bands observed in the trans complexes coincide well with the maxima of the split components, Ia and Ib, of the first absorption band. The CD spectra of different [Co(l-am)(NH3)4]2+ ions, in which l-am is a bidentate ligand, show a variety of behavior; they are discussed from a stereochemical point of view. An intense CD band of the [Co(l-prol)(NH3)4]2+ ion at about 510 mμ can be attributed to the vicinal effect of an asymmetric nitrogen atom of the coordinated l-prolinate ion. For the trans-[Co en2(l-amH)2]X3 complexes, the rotatory parameters and rotatory strengths were calculated by means of analyses of the experimental CD and RD curves.