Ternary Complexes in Solution. X. The Influence of the Size of the Chelate Rings on the Stability of Mixed-Ligand Copper(II) Complexes

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The influence of the ring size on the stability of ternary Cu complexes was investigated (25°; I = 0.1). The differences Δ log if = log. K CuAB — log K cuB and the constants log X due to the equilibrium Cu(2,2'-bipyridyl) + CuL ⇌ 2Cu-(2,2'-bipyridyl)L were compared for ligands forming five- and six-membered chelates: L = oxalate (log X = ~4.9), glycinate (3.05), and ethylenediamine (1.10) or malonate (5.49), β-alaninate (2.68), and 1,3-propylenediamine (0.06). The same was done for the equilibrium Cu(pyrocatecholate)2 + CuL ⇌ 2Cu(pyrocatecholate)L, with L = α-picolylamine (log X = 4.64), 4-aminomethylimidazole (3.46), and ethylenediamine (2.65) or α-aminoethylpyridine (4.52), histamine (4.86), and 1,3-propylenediamine (4.11). It is concluded that the differences in stability between comparable pairs of ternary complexes are, at least, partly due to the different magnitudes of log K cuL — log K CuL of the binary complexes with five- or six-membered chelates, the difference usually being greater for the latter one. This especially favors the formation of ternary complexes containing a five- and a six-membered chelate ring. Besides this, however, it seems that mixed-ligand complexes containing two five-membered chelates are slightly favored compared to those with a five- and a six-membered ring. © 1971, American Chemical Society. All rights reserved. 2+ CuA Cu Cu CuL 2 2 2 2

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Inorganic Chemistry

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