PREPARATION, CHARACTERIZATION AND REACTIVITIES OF CHROMIUM(III) COMPLEXES OF A HOMOLOGOUS SERIES OF SCHIFF-BASE LIGANDS

Complexes of general formulation [Cr(Schiff-base)(H2O)(2)]X, where Sch iff-base = 1, 2-bis(salicylideneamino)ethane (salen), 1, 3-bis(salicyl ideneamino) propane (salprn) and 1, 4-bis(salicylideneamino)butane (sa lbuen) and X = ClO4 or Cl have been prepared and characterised. Single -crystal structure of the complex trans[Cr(salprn)(H2O)(2)]Cl has been resolved. Evidence for less distorted coordination geometry around ch romium in the salprn analogue as compared to that in trans-[Cr(salen)( H2O)(2)](+) has been presented. The kinetics of the aqua ligand substi tution in the complexes Cr(Schiff-base)(H2O)(2)(+) by thiocyanate have been investigated under pseudo-first order conditions with [NCS-] >> [Cr(III)], T = 10-50 degrees C, [H+] = 10(-8) - 10(-1) M, I = 1.0 M (L iClO4). An attempt has been made to discuss the anomalous lability of trans-[Cr(salen)(H2O)2](+) reported earlier in comparison to the react ivities of salprn and salbuen analogues. Bimolecular rate constants fo r the aqua ligand substitution by NCS- in [Cr(Schiff-base)(H2O)(2)](+) at 25 degrees C vary by more than two orders of magnitude depending o n the nature of the Schiff-base ligand. The reactivity order is discus sed in terms of the possible steric strain imposed by the equatorially coordinated quadridentate ligand.