Stability constants of inner- and outer-sphere complexes of hydrated tris(1-(2-thienyl)-4,4,4-trifluoro-1,3-butanedionato) lanthanide(III) with tris(2,4-pentanedionato) cobalt(III)

Abstract

Binuclear complexation between hydrated tris(1-(2-thienyl)-4,4,4-trifluoro-1,3-butanedionato)lanthanide(III) (Ln(tta) 3) and tris(2,4-pentanedionato)cobalt(III) (Co(acac) 3) in benzene has been studied by a liquid–liquid distribution method and spectroscopic methods such as electronic absorption for d–d transition, IR for OH vibration of coordinated water molecules, and 59Co NMR. The stability constants of a binuclear complex ( β s,1) across the lanthanide series were determined by both distribution and spectrophotometric methods. The β s,1 values for La to Nd were close to each other and much higher than those for Er to Lu, which were almost constant. As a result, there was the large difference in β s,1 between the light and the heavy Ln. The spectrochemical studies by IR and 59Co NMR suggested that the binuclear complex has two different structures with inner- and outer-sphere coordination. In the former Co(acac) 3 displaces the coordinated water molecules in the Ln chelate and directly coordinates to the central Ln ion through the coordinating oxygen atoms of the acac ligands. In the latter hydrogen-bonding is formed between the coordinating oxygen atoms in Co(acac) 3 and hydrogen atoms of coordinated water molecules in the Ln chelate. Both binuclear complexes coexist in the solution of light Ln, while the outer sphere type mostly exist in heavy Ln.