Thermodynamics of formation of binary and ternary complexes of zinc(II) with halide and thiocyanate ions and 2,2′-bipyridine in dimethylformamide

Abstract

The formation of binary and ternary complexes of zinc(II) with bromide, iodide, and thiocyanate ions and 2,2′-bipyridine (bipy) has been studied by calorimetry in dimethylformamide (dmf) at 25 °C. The formation of the ternary complexes [ZnX_m(bipy)_n]^{(2 –m)+}(denoted as [1mn]), such as [111], [112], and [121] for X = Br and I and [111], [112], [121], and [122] for X = SCN is revealed, along with the binary complexes [ZnX_n]^(2–n)+(n= 1–3 for X = Br, n= 2 or 3 for X = I, and n= 1, 3, or 4 for X = SCN) and [Zn(bipy)_n]²⁺(n= 1–3), and their formation constants, reaction enthalpies, and entropies were obtained. The thermodynamic parameters are discussed together with those for the (2,2′-bipyridine)chlorozinc(II) complexes previously determined. A four-co-ordinate structure is suggested for [ZnX₂(dmf)₂](X = Br or I), [ZnX₃(dmf)]^–(X = Br, I, or SCN), and [Zn(NCS)₄]^2–, and thus the six-co-ordinate, octahedral structure of [Zn(dmf)₆]²⁺ changes upon complexation. The complexes [Zn(NCS)(dmf)₅]⁺ and [Zn(bipy)_n]²⁺(n= 1–3) remain octahedral. The extent of formation of the [111] complex is weak in all halide systems. The [121 ] complex is dominant in the chloride system, significant in the bromide, but very weak in the iodide system, and the reaction enthalpies and entropies suggest that an octahedral–tetrahedral equilibrium, [ZnX₂(bipy)(dmf)₂](O_h⇌[ZnX₂(bipy)](T_d)+ 2dmf (X = Cl or Br), is established in solution. On the other hand, the [112] complex, which is not found in the chloride system, becomes dominant in the bromide and especially so in the iodide system. The formation of the [ZnI (bipy)₂]⁺ complex, [Zn(bipy)₂]²⁺+ I^–→[Znl(bipy)₂]⁺, is accompanied by unusual ΔH^⊖ and ΔS^⊖ values, implying that a specific interaction operates between iodide ion and 2,2′-bipyridine molecules in the vicinity of the zinc(II) ion.