Optically active cis-dianionobis(ethylenediamine)chromium(III) complexes—II absolute configurations

Abstract

Racemic cis[ M(N 3) 2(en) 2]NO 3 ( M = Cr, Co) have been resolved using potassium antimony-(+)-tartrate to give the (+)− M(N 3) 2(en) 2 + ions as the isomorphous less soluble diastereoisomerides with the Γ absolute configuration. Using Γ-(+)- M(N 3)(en) 2 + and Γ-(+)-Cr(NCS) 2(en) 2 +, the following cations have been isolated in solution: Γ-(+)-Cr(N 3)(en) 2(OH 2) 2+, Γ-(+)-Cr(NCS)(en) 2(OH 2) 2, Γ-(+)-Cr(en) 2(OH 2) 2 3+, Γ-(+)-Cr(OH) 2(en) 2 +, Γ-(+)-CrCl 2(en) 2 +, Γ-(+)-CrCl(en) 2(OH 2) 2+, Γ-(+)-Co(N 3)(en) 2(OH 2) 2+ and Γ-(+)-Co(en) 2(OH 2) 2 3+. Ion-exchange techniques have been used extensively in the isolation of these chiral species and ORD and CD spectral parameters have been determined for these cations. These data give rise to an empirical rule for the assignment of absolute configuration for Cr(III) complexes of this type, where the aniono ligand is not a halogen, namely, that for the Γ configuration, the Cotton effect of the ORD curve under the 4A 2g → 4T 2g transition is positive.