Published and original data on the stability of complexes formed by bivalent ions of the first transition series have been collected and examined critically. The order Mn < Fe < Co < Ni < Cu > Zn has been found to hold for the stability of nearly all such complexes irrespective of the nature of the co-ordinated ligand or of the number of ligand molecules involved.A theoretical justification for this stability order follows from consideratioiis of the reciprocal of the ionic radii and the second ionisation potentials of the metals concerned, and it is shown why other (empirical) orders, or stability orders which include other elements, can have no general validity.The extent to which variations in characteristic co-ordination number, stereochemical considerations, and entropy factors may affect such a stability order is discussed theoretically and illustrated by examples. Changes in bonding orbitals concomitant with changes in the nature of the ligand are shown to lead to certain complexes of anomalously high stability, notably in the case of ferrous ions.

The spectroscopic and electrochemical properties of blue copper proteins are strikingly different from those of inorganic copper complexes in aqueous solution. Over three decades ago this unusual behavior was ascribed to constrained coordination in the folded protein; consistent with this view, crystal structure determinations of blue proteins have demonstrated that the ligand positions are essentially unchanged on reduction as well as in the apoprotein. Blue copper reduction potentials are tuned to match the particular function of a given protein by exclusion of water from the metal site and strict control of the positions of axial ligands in the folded structure. Extensive experimental work has established that the reorganization energy of a prototypal protein, Pseudomonas aeruginosa azurin, is approximately 0.7 eV, a value that is much lower than those of inorganic copper complexes in aqueous solution. The lowered reorganization energy in the protein, which is attributable to constrained coordination, is critically important for function, since the driving forces for electron transfer often are low (approximately 0.1 eV) between blue copper centers and distant (>10 A) donors and acceptors.

No abstract

No abstract

No abstract