TERNARY COMPLEXES IN SOLUTION .31. EFFECT OF THE VARYING PI-ACCEPTING PROPERTIES OF SEVERAL BIPYRIDYL-LIKE LIGANDS ON THE STABILITY OF MIXED-LIGAND COMPLEXES ALSO CONTAINING PYROCATECHOLATE AND COBALT(II), NICKEL(II), COPPER(II), OR ZINC(II)

The stability constants of the ternary Co2+, Ni2+, Cu2+, and Zn2+ complexes containing pyrocatecholate (pyr) and, as a second ligand, a heteroaromatic N base, namely, bis(2-pyridyl)amine (2, 2'-dipyridylamine, dpa), bis(2-pyridyl)methane (2, 2'-dipyridylmethane, dpm), or bis(2-pyridyl) ketone (2, 2'-dipyridyl ketone, dpk), were determined in aqueous solution by potentiometric titration (I=0.1 M, NaC104; 25°C). For the equilibrium M(pyr)2+M(dpx)2⇌2M(dpx)(pyr), where dpx=dpa, dpm, or dpk, the following constants, log X, were determined: Co(dpa)(pyr) 3.79 (0.69), Co(dpm)(pyr) 4.79 (1.13); Ni(dpa)(pyr) 3.95 (0.37), Ni(dpm)(pyr) 4.47 (0.43); Cu(dpa)(pyr) 6.17 (0.37), Cu(dpm)(pyr) 5.8 (0.66), Cu-(dpk-H2O)(pyr) 6.6 (0.82) [dpk-H2O=geminal diol of dpk]; Zn(dpa)(pyr) 3.92 (0.57), Zn(dpm)(pyr) 4.31 (0.83). The constants given in parentheses are due to [FORMULA OMMITED] and refer to the equilibrium M(dpx)2++M(pyr)⇌M(dpx)(pyr)+M2+. All of these mixed-ligand complexes are considerably more stable than one would expect from purely statistical reasons. In addition, all metal ions form the more stable ternary complex, as is especially evident from the Δ log KM values, with this heteroaromatic N base which is the better π acceptor; i.e., the tendency for the formation of the M(dpx)(pyr) complexes decreases within the series dpk>dpm>dpa. This demonstrates that for the enhanced stability of ternary complexes (with Co2+, Ni2+, Cu2+, or Zn2+) containing an O donor and a heteroaromatic N base the π-acceptor properties of the latter are crucial, but it should be noted that the stability enhancement with Ni2+ is less pronounced than with its neighboring elements. © 1979, American Chemical Society. All rights reserved.