X-RAY STRUCTURE OF (DIBROMO(PCO-O,O')(PCO-O)COPPER(II)) (PCO = 2-PYRIDINECARBOXAMIDE 1-OXIDE), A SIMPLE STRUCTURE WHICH CONTAINS A UNIDENTATE AND BIDENTATE FORM OF THE SAME LIGAND

The X-ray structure is reported for the complex Cu(pco)2Br2 of 2-pyridinecarboxamide 1-oxide (pco). The complex is triclinic, P1BAR, with a = 7.431(7), b = 7.869(10), c = 13.739(13) angstrom, alpha = 91.04(7), beta = 90.83(7), gamma = 93.56(6)degrees and Z = 2. The complex is five-coordinate. Both bromide ions are coordinated. One pco molecule is bidentate, coordinating via the N-oxide and amide oxygen atoms. The other pco is unidentate, coordinating via the N-oxide oxygen, and with the amide group uncoordinated. In the unidentate pco molecule the uncoordinated amide group has its oxygen trans to the N-oxide oxygen, and there is strong hydrogen bonding between one of the amide hydrogens and the N-oxide oxygen. This hydrogen bonding may stabilise the unidentate behaviour of pco. Such hydrogen bonding is impossible with the analogous pyridine ligand 2-pyridinecarboxamide (pc) and this may explain why pc does not apparently act as a unidentate to simple metal salts. The IR spectrum of Cu(pco)2Br2 is compared with the spectra of other pco complexes which mostly appear to contain only bidentate pco.