CAPPED TRIGONAL PRISM COORDINATION GEOMETRY IN A COBALT(II) CHELATE WITH A 24-MEMBERED MACROCYCLIC LIGAND IN WHICH 2 ETHYLENEDIAMINETETRAACETATO AND 2 ETHYLENEDIAMINE GROUPS ARE LINKED BY AMIDE BONDS

An X-ray crystal analysis was carried out on the cobalt(II) compound o f a 24-membered macrocyclic ligand with four pendant acetato groups, C o2(C24H36N8O12) . 8H2O: the ligand molecule was prepared by a condensa tion reaction between ethylenediaminetetraacetic dianhydride and ethyl enediamine. The cobalt(II) compound crystallized in the hexagonal spac e group P6(2) (or P6(4)) with a=17.957(1), c=10.274(1) angstrom and Z= 3. Each ligand molecule is coordinated to two cobalt ions, which are c rystallographically equivalent to each other. Each metal ion has six w ell-defined coordination bonds with two oxygen atoms from carboxylato groups (Co-O=2.062 and 2.177 angstrom an oxygen atom from an amide gro up (Co-O=2.101 angstrom), two amine nitrogen atoms (Co-N=2.229 and 2.3 00 angstrom) and an oxygen atom from a water molecule (Co-O=2.058 angs trom). These six Co-X bonds form a quasi-trigonal prism around the Co atom. When an additional amide oxygen atom located at a distance of 2. 720 angstrom from the Co atom is included in the coordination sphere, the coordination geometry is described as a capped trigonal prism. Thi s less common coordination geometry is due to the inflexibility of the ligand molecule. A weak d-d band was observed at 515 nm in aqueous so lution.