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)]·4H2O (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 ADA2− as tripodal–tetradentate ligand. This finding is explained on the basis of a structural comparison with the compounds of [Ni(ADA)(H2O)2] (1) and [Ni(ADA)(Him)(H2O)]·1.5H2O (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.