TERNARY COMPLEX-FORMATION BY DIAQUANITRILOTRIACETATOCHROMIUM(III) WITH 2-2'-BIPYRIDINE AND 1,10-PHENANTHROLINE IN AQUEOUS-MEDIA - A KINETICINVESTIGATION

In solution the complexes [Cr(nta)(H2O)(2)] (1) and [Cr(Hnta)(H2O)(2)] (+) (1H) co-exist in a protonic equilibrium. 1 and 1H react with I,10- phenanthroline to form [Cr(nta)(phen)] (Ip) and [Cr(Hnta)(H2O)(phen)]( +) (1HP) respectively. 2,2'-bipyridine (bipy) reacts similarly and for ms [Cr(nta)(bipy)] (Ib) and [Cr(Hnta) (H2O)(bipy)](+) (1Hb). The relat ive proportions of Ip End Ib in solution increases with increased pH. 1Hp and 1Hb have been isolated and characterized in solution. Rate con stants for the formation of 1)Ip and 1Hb are more than 10(3) times lar ger than the corresponding rate constants for the formation of Ip and Ib. This observation is probably the first direct demonstration of an anchimeric effect exhibited by the flanging carboxylate arm of a tride ntate nta. Aquation of Ip, :Lb, 1Hp and 1Hb are acid independent in th e pH range 3.85-5.4. (C) 1997 Elsevier Science Ltd.