Kinetic studies on the stability enhancement of mixed ligand copper(II) complexs with 2,2'-bipyridyl and acetylacetonate.

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The rate constants for the formation and dissociation of the binary Cu2+-acetylacetonate(acac−) complex and the ternary 2,2′-bipyridyl(bpy)-Cu2+-acac− complex have been determined by the temperature-jump method. For the reactions Cu2++acac−\oversetk1b\undersetk-1b\rightleftharpoonsCu(acac)+, Cu(acac)++acac−\oversetk2b\undersetk-2b\rightleftharpoonsCu(acac)2, and Cu(bpy)2++acac−\oversetk1t\undersetk-1t\rightleftharpoonsCu-(bpy)(acac)+, the rate constants are k1b=9.0×108 M−1 s−1, k-1b=3.2 s−1, k2b=7.0×107 M−1 s−1, k-2b=13 s−1, k1t=1.1×109 M−1 s−1 and k-1t=1.9 s−1. A comparison of the reactions shows that the forward rate for the Cu(bpy)-(acac)+ formation is larger than that for Cu(acac)+ and, subsequently, the rate of the complex formation between Cu(bpy)2+ and acac− is enhanced by the coordinating bpy. On the other hand, the reverse rate for the ternary complexation reaction is smaller than for the binary. This illustrates that the dissociation of acac− of Cu(bpy)-(acac)+ becomes more difficult owing to the coordination of the bpy. The above two effects of the bpy may be driving forces producing high stability, i.e., a positive value of ΔlogK\left(K=\frac[Cu(bpy)[L][Cu(bpy)][L]\right) for the ternary copper(II) complexes with bpy and other ligands containing O as donor atoms.
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