The Co(III)N(,3)O(,3) complex, Co(PLASP) (L-Phe) (.) 3H(,2)O, was prepared by the oxidation of Co(II) to Co(III) in the presence of N-(2-pyridylmethyl)-L-aspartate, PLASP('2-), and L-phenyl-alaninate, L-Phe('-). An X-ray structure of the complex established that of the four possible isomers, the only one isolated was the facial isomer in which the (beta)-CO(,2)('-) and pyridyl groups of PLASP('2-) are mutually trans. The bidentate, L-Phe('-), is coordinated with its amino group trans to the (alpha)-CO(,2)('-) of PLASP('2-) and its (alpha)-CO(,2)('-) group trans to the secondary amino group of PLASP('2-). This same isomer was the only one isolated for a series of Co(PLASP) (AA) complexes prepared from Co(II) and Co(III) reactants where AA('-) is one of the following amino acidates: glycinate (Gly('-)), L-alaninate (Ala('-)), (alpha)-aminoiso-butyrate ((alpha)-AIBA('-)), L-threoninate (Thr('-)), L-prolinate (Pro('-)), D or L-Phe('-), D or L or D,L-valinate (Val('-)), and D or L-asparaginate (AsN('-)). Two isomers of the Co(N-Cm-L-Pyala)-(D-Thr) complex were prepared by oxidation of Co(II) to Co(III) in the presence of N-carboxymethyl-L-(beta)-(2-pyridyl)-(alpha)-alaninate, N-Cm-L-Pyala('2-), and D-Thr('-). The two isomers, one facial and the other meridional, were separated by chromatography. Based upon its visible spectrum the meridional isomer was assigned a structure in which the amino nitrogen of D-Thr('-) is trans to the pyridyl group of N-Cm-L-Pyala('2-). This was confirmed by an X-ray structure determination of mer-Co(N-Cm-L-Pyala) (D-Thr) (.) 1/2H(,2)O. It was concluded that the presence of the meridional isomer was due to the flexibility of the N-carboxymethyl chelate ring. A series of Co(N-Cm-L-Pyala) (AA) complexes were prepared by oxidation of Co(II) to Co(III) where AA('-) is one of the following: Gly('-), (alpha)-AIBA('-), D,L-Ala('-), D or L or D,L-Val('-), D-Thr('-) and D-AsN('-). From the distribution of isomers it appeared that if steric factors (interaction of bulky groups) are absent, the facial isomer is structurally preferred, while if steric factors are present, the meridional isomer is preferred. Several mixed ligand complexes of the form Co(N-Cm-L-Hist) (AA), where N-Cm-L-Hist('2-) is N-carboxymethyl-L-histidinate and AA('-) is either (alpha)-AIBA('-), D-Thr('-), D-AsN('-) or D or L-Val('-), were prepared by the K(,2)S(,2)O(,8) oxidation of Co(II) to Co(III) in the presence of N-Cm-L-Hist('2-) and AA('-). Of the four possible isomers, the only one isolated was the facial isomer in which the N-carboxymethyl CO(,2)('-) group of N-Cm-L-Hist('2-) is trans to the imidazole. Structural assignments for the above complexes were based on visible, CD and nmr spectra and/or X-ray structure analyses. CD spectra of the complexes were resolved into a contribution from the AA('-) chelate ring (Y) and one from the rest of the molecule (X). For the complexes above, it was found that the sign of the low energy peak of the Y term is positive for a D-AA('-) and negative for an L-AA('-). The stability of each complex isolated was related to a combination of electronic, structural and steric factors.