Crystalline‐State Reaction of Cobaloxime Complexes by X‐ray Exposure. XII. A New Type of Racemization with a Negligibly Small Change of the Cell Dimensions [1

Abstract

AbstractCrystals of [(R)‐1‐cyanoethyl] bis(dimethylglyoximato)(diphenylmethyl‐phosphine)cobalt(III), [Co(C3H4N)(C4H7N2O2)2(C13H13P)], on exposure to X‐rays undergo crystalline‐state racemization with a negligibly small change of the cell dimensions. At the initial stage the two crystallographically independent molecules are related by pseudo inversion symmetry in the A2 cell. On X‐ray irradiation to the crystal, the pseudo inversion center becomes a true crystallographic one and the space group becomes A2/a. Both the chiral cyanoethyl groups are converted to the disordered racemates. The rate of the racemization was measured by the intensity variation of the reflections that should disappear after the racemization. The rate constants at 293 and 343 K are calculated to be 4.81 and 5.07 × 10−6 s−1, respectively, assuming first‐order kinetics. The cavities for the two cyanoethyl groups in the initial structure have the largest volumes known in this and related crystals. The positive correlation between the reaction rate and the volume of the cavity for the reactive group holds good for the present crystal.