The complexes Delta-fac-Cr(S-trp)(3) and Delta-fac-Cr(S-trp)(3) (where S-trp is the unsymmetric bidentate ligand (S)-tryptophanato) have been prepared by heating [Cr(en)(3)]Cl-3 with excess S-trpH in water or dimethyl sulfoxide (DMSO) solution. The two diastereoisomeric species are separated by extracting the initial solid product with acetone, in which only Delta-fac-Cr(S-trp)(3) is soluble. Geometric isomer identifications are made from an analysis of uv-visible spectral data and indicate isolation of just the facial isomer. Optical isomer assignments are based on circular dichroism (CD) measurements. photochemical studies in DMSO solution reveal that on ligand-field (d-d) excitation at 488 nm the Delta-fac-Cr(S-trp)(3) isomer is essentially photoinert, while the Delta-fac-Cr(S-trp)(3) isomer undergoes complete conversion to Delta-fac-Cr(S-trp)(3) with a quantum yield of 0.03. Complete photoinversion demonstrates a key stereochemical role for the asymmetric C of (S)-tryptophan. The observation of the same inversion process thermally (at elevated temperatures) indicates that the Delta-fac-Cr(S-trp)(3) diastereoisomer has a considerably higher stability than that of the ii species. This conclusion is in accord with molecular mechanics calculations, which estimate the Delta diastereoisomer to be more stable by 7 kcal mol(-1). The observation of optical inversion without fac --> mer geometric isomerism is suggestive of a Bailar trigonal twist mechanism and may constitute the first example of a photoinduced Bailar twist. Using X-ray crystallographic data available for the glycine analog, a normalized chelate bite value of 1.31 is estimated for the fac-Cr(S-trp)(3) system. Consistent with our experimental data, recent theoretical studies by Rodger and Johnson conclude that a Bailar twist will be energetically preferred over a Ray and Dutt twist for bite values less than 1.4.