Cobalt complexes oxygen ligands

It has been noted (2) that many oxygen-carrying cobalt(II) complexes involve ligands that provide four donor atoms lying in the same plane as the cobalt atoms, such as salen , porphyrin and dimethylglyoxime, DMG, and related ligands. 

Hemocyanin, 40 362-363 as cobalt complex ligands, 44 291 oxygen-carrying properties, 40 366 XAS, 36 326-327... 

Some complexes are known in which carbon dioxide is bonded to a transition metal. Aresia era/. [19) isolated the nickel complex (1)in which the CO ligand is coordinated througli the carbon atom and one of the oxygen atoms. Fachinetti et al. [20] found the polymeric cobalt complex (2), in which carbon dioxide is C bonded to cobalt and 0-bonded to two different potassium ions. Lappert isolated tlie remarkably stable niobium complex (3). which was the first authentic CO complex of an early d-block metal [21 j. 

So far, for the highly oxygen-sensitive Co -forms of cobalt-corrinoids (such as Co -cobalamin, (6)), information from X-ray analysis on their three-dimensional structure is not available. However, in these strongly nucleophihc, highly reduced cobalt complexes, the diamagnetic Co -center (a d metal ion, isoelectronic with a Ni"-ion) presumably is ligated by the corrin ligand only, in a tetracoordinated, nearly square-planar fashion. As a consequence, the Co -forms of complete corrins would have to be base-off, that is, the nucleotide function would not be attached to the central cobalt ion. 

For transition metals, such migrations are accompanied by ligand replacement. The chemistry and catalysis of transition metal complexes in zeolites have been recently reviewed by Lunsford (156). In these catalysts, it can be desirable to replace, for example, ammonia by pyridine. Zeolite-encaged cobalt complexes, for example, exhibit high potential for oxygen activation. 

Co(in) complexes promote similar reactions. When four of the six octahedral positions are occupied by amine ligands and two cis positions are available for further reactions, it is possible to study not only the hydrolysis itself, but the steric preferences of the complexes. In general, these compounds catalyze the hydrolysis of N-terminal amino acids from peptides, and the amino acid that is removed remains as part of the complex. The reactions apparently proceed by coordination of the free amine to cobalt, followed either by coordination of the carbonyl to cobalt and subsequent reaction with OH or H2O from the solution (path A in Figure 12-15) or reaction of the carbonyl carbon with coordinated hydroxide (path B). As a result, the N-terminal amino acid is removed from the peptide and left as part of the cobalt complex in which the a-amino nitrogen and the carbonyl oxygen are bonded to the cobalt. Esters and amides are also hydrolyzed by the same mechanism, with the relative importance of the two pathways dependent on the specific compoimds used. 

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