VOLUMES OF ACTIVATION FOR RACEMIZATION OF MIXED 1,10-PHENANTHROLINE AND 2,2'-BIPYRIDYL COMPLEXES OF NICKEL(II) FROM HIGH-PRESSURE SOLUTION KINETICS

The effect of pressure on the rates of racemization of Ni (phen)2(bpy)2+ and Ni (phen) (bpy)22+ in solution has been evaluated. Reactions of Ni (phen)2(bpy)2+ exhibit only a minor pressure dependence of the rate, with a volume of activation (ΔV⧧) of+0.6 cm3 mol-1 and-0.1 cm3 mol-1 for racemization in 0.01 and 1.0 M aqueous HC1, respectively. Aquation of Ni (phen)2(bpy)2+ in 1.0 M aqueous HC1 exhibits no measurable pressure dependence, with ΔV⧧ = 0.0 cm3 mol-1. Racemization of Ni (phen)(bpy)22+ proceeds with a minor acceleration upon the application of pressure, with ΔV⧧=-1.9 cm3 mol-1 and ΔV⧧=-5.2 cm3 mol-1 in 0.01 and 1.0 M aqueous HC1, respectively. Variation of ΔV⧧ with acidity observed suggests an acid-dependent pathway is the dominant mechanism at high acidity but is not important at low acidity. A common mechanistic pathway for racemization and aquation of these complexes is favored since reported rates, enthalpies, entropies and volumes of activation are very similar. A one-ended dissociative mechanism via a half-bonded bpy ligand is consistent with experimental ΔV⧧ data. © 1978, American Chemical Society. All rights reserved.