Stereoselectivity in the complex formation of penicillamine with nickel(II)

Stability constants of nickel(II) and zinc complexes of optically active and racemic cysteine and penicillamine (dianion: penam) in aqueous solution have been determined. For the complex Ni(penam)22− a significant stereoselective effect in favour of the racemic mixture (MLL + MDD) was observed, the meso complex MDL having an excess Gibbs energy of 0.5‐0.6 kcal/mole (2.1‐2.5 kJ/mole) relative to the racemic mixture. For Zn(penam)22− a small effect, on the edge of significance, in favour of the meso complex MDL was found; no stereoselectivity was observed for the cysteine complexes.