The unexpected tridentate role of the tripodal ligand N-(carbamoylmethyl)iminodiacetato(2-) (ADA) in a new mixed-ligand nickel(II) complex with 2,2′-bipyridine (bipy) as secondary ligand:: structure of [Ni(ADA)(bipy)(H2O)]-4H2O

The crystal structure of a new mixed-ligand nickel(II) complex of N-(carbamoylmethyl)iminodiacetato(2-) (ADA) and 2,2'-bipyridine (bipy), with formula [Ni(ADA)(bipy)(H2O)] . 4H(2)O (compound 3), reveals an unexpected tridentate ADA ligand, with a free N-(2-amidomethyl) arm, in contrast with all other known structures of metal-ADA chelates which have ADA(2-) as tripodal-tetradentate ligand. This finding is explained on the basis of a structural comparison with the compounds of [Ni(ADA)(H,0)21 (1) and [Ni(ADA)(Him)(H2O)].1.51-H2O (2, Him=imidazole) which reveals significant changes in Ni-N(amino tertiary) and Ni-O(amide) bond lengths as well as in the ADA denticity and ligand conformation concerning the iminodiacetato(2-) moiety (IDA) in these Ni(II) chelates. The ADA-ligand conformation is mer-IDA + O(amide), fac-IDA + O(amide) or only fac-IDA-tridentate in 1, 2 or 3 respectively. (C) 2002 Elsevier Science B.V. All rights reserved.