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Penta cyano ferrate ii complexes of amino acids






Journal of the American Chemical Society 104(26): 7509-7515

Penta cyano ferrate ii complexes of amino acids

The electronic spectra and the kinetic and thermodynamic properties of an extensive series of (amino-acid)-pentacyanoferrate(II) complexes were investigated in aqueous solution. The spectra of the complexes containing aminocarboxylate ligands are similar to that of Fe(CN)5NH33-, with Dq estimated to be .apprx. 0.233 .mu.m-1. For the methionine, histidine, and methionine sulfoxide complexes, Dq values are 0.239, 0.248, and 0.286 .mu.m-1, respectively. The rates of formation of the complexes are 240-370 M-1 s-1 for zwitterionic amino acids and 18-30 M-1 s-1 for mononegatively charged and 9 M-1 s-1 for dinegatively charged .alpha.-aminocarboxylate ligands (25.degree. C and .mu. [ionic strength] = 0.10 M). The specific rates increase as the number of C.sbd.C bonds separating the coordination site from the carboxylate group is increased. The kinetic and thermodynamic properties (dissocation rate, stability constant, reduction potential) of each (amino acid)pentacyanoferrate depend strongly on the donor/acceptor nature of its amino acid ligating group. The imidazole group of histidine is unique in the sense that it stabilizes both Fe(II) and Fe(III) states (KII = 5.9 .times. 105; KIII = 1.1 .times. 106 M-1).


Accession: 006079085



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