THE RATES OF EXCHANGE OF WATER MOLECULES FROM AL(III)-METHYLMALONATE COMPLEXES: THE EFFECT OF CHELATE RING SIZE - ACIDITY CONSTANTS OF SILICIC ACID AND THE IONIC PRODUCT OF WATER IN THE MEDIUM RANGE 0.5-2.0 M NA(CL) AT 25 C

Abstract

Rate coefficients are reported for exchange of hydration waters in the inner-coordination-sphere of Al(III)-methylmalonate complexes with bulk solution as determined via 17O-NMR. Surprisingly, water molecules in the thermodynamically less-stable complexes containing six-membered chelates are much more labile than those in five-membered oxalate-Al(III) complexes. The rate parameters for the Al(mMal)+ complex are: kex298=6.6.102 s-1,ΔH#=66(+/-1) kJ mol-1;ΔS#=30.6(+/-2) J mol-1 K-1 and for the Al(mMal)2- complex are kex298=6.9.103 s-1,ΔH#=55(+/-3) kJ mol-1;ΔS#=12.8(+/-11) J mol-1 K-1 The surprising trend in reactivity is attributable either to differences in the Lewis basicities of oxygens in bidentate oxalate and methylmalonate ligands, or to rapid dissociation/reassociation of one of the acetate groups to the metal center. These results identify a useful case where trends in the apparent labilities of dissolved and presumed surface complexes deviate sharply. This deviation could be usefully exploited to probe surfaces if ligand-promoted dissolution rates could be compared at conditions where inner-sphere and outer-sphere chelate complexes could be distinguished spectroscopically. We expect inner-sphere oxalate to have a smaller labilizing effect than malonate or methylmalonate. A contrary result would indicate structural dissimilarity between complexes on the surface and in solution, or perhaps steric hindrance.