Kinetics and Mechanism of Chelation of Nickel(II) by a Tridentate a-[(2-Hydroxyphenyl)azo]-a-acetoacetonitrile and an a-(8-Quinolylazo)-a-acetoacetonitrile Dye
Absorption curves of solutions containing a dye ligand and its 1:1 and 1:2 nickel(II) complexes have been analyzed by factor analysis to extract the (often) inaccessible absorption curve of the 1:2 complex and its formation constant, K2. The method was applied to an , '-dihydroxy phenylazo dye, Solochrome Violet (SV), and to two a-[(2-hydroxyphenyl)azo]-a-acetoacetonitrile dyes. Required inputs are a family of absorption curves containing various proportions of the dye and two complexes, reference curves for the dye and the 1:1 complex, the pKa's of the ligand'groups; and an independently determined value of the formation constant of the 1:1 complex, K¡. The derived value of K2 for SV agreed with the value determined earlier by potentiometric titration. The method was successful in cases where the absorption curves of the two complexes showed small differences in shape and/or Xmax but failed where the two curves differed only in amplitude. The values of Kj and K2 for the acetoacetonitrile dyes are smaller by several orders of magnitude than those of related dyes; the spectra and molecular models suggest that bohd angles and lengths are distorted in the chelates. The advantages and disadvantages of the approach are discussed.
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