A structural and isotopic NO2– exchange rate comparison of trans-[Co(ao–H–ao)(NO2)2] and cis-[Co(Hao)2(NO2)2]NO3

Abstract

The X-ray crystal structures of two non-isomeric (α-amineoxime)dinitrocobalt(III) complexes, trans-[Co(ao–H–ao)(NO₂)₂] and cis-[Co(Hao)₂(NO₂)₂]NO₃(Hao = 3-amino-3-methyl-2-butanone oxime), have been refined to conventional R values of 0.034 (trans) and 0.032 (cis). The neutral trans complex crystallizes in the orthorhombic space group P2₁2₁2₁ with a= 6.786(3), b= 11.553(1), and c= 20.908(2)Å. The cis nitrate salt crystallizes in the monoclinic space group P2₁/n with a= 8.1 60(3), b= 17.250(3), c= 13.745(2)Å, and β= 104.77(2)°. Only the former species has an intramolecular hydrogen bond, and this is, to our knowledge, the first comparison of non-isomeric (due to charge)cis–trans complexes. The differences in the organic ligand and nitro bond distances and angles are discussed. The structural, u.v.-visible, and n.m.r. spectroscopic parameters are presented. The bond distances to the nitro groups are compared with their isotopic exchange lability in aqueous solution. It is concluded that the kinetic lability of the vitro ligands, as measured by ¹⁵N isotopic exchange rates is not directly related to the metal–nitrogen bond distance in the solid state.