Potentiometric and Spectroscopic Studies with Chromium(III) Complexes of Hydroxysalicylic Acid Derivatives in Aqueous Solution

Chromium is a first row transition element in the periodic table. It may resembles the group 16 elements of the oxygen group but only in the acidity of the trioxide and in its covalent nature. Cr(III) with its d configuration forms thousands of inert complexes; an important characteristic of these complexes, in aqueous solutions, is their relative kinetic inertness. The radius of Cr(III) is 0.76 Å for coordination number 6 and it acts as a hard acid in aqueous solution. Its complex-formation equilibria are rather complicated, due to the strong affinity of Cr(III) ions to bind hydroxo groups. The hexaaqua ion [Cr(H2O)6] in aqueous solution is acidic (pKa= 4) and gives predominantly hydroxo ions Cr(OH), Cr(OH)2 and dimeric hydroxo bridged species by hydrolytic reactions; further polymerization proceeds stepwise via OH-bridges to give trimers [Cr3(μ -OH)4 (OH)(n+1)] (n = 0, 1, 2), tetramers, hexamers, etc.1−2. Phenolate oxygens have been shown to be very effective donors, especially for metal ions with a strong tendency to form hydroxo species in aqueous solutions3−4. Salicylic acid (SA) is the simplest representative of the phenolic ligands and it contains 2 potential binding sites; it also provides an anchoring carboxylate group. Thus it can act as a monoor bidentate ligand; the prevelence of 1 of the 2 possibilities depends on ∗Corresponding author