الفهرس 
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Abstract
Vitamin B12 (Cobalt Corrinoids complexes) and its derivatives are very important from both biochemistry and coordination chemistry point of view since it is involved in the catalysis of about 15 enzymatic reactions in various organisms, including the 1.2-intramolecular rearrangements catalyzed by 5-deoxyadenosyl-cobalaminrequiring enzymes. It is of interest from the inorganic chemistry to investigate the ligand substitution reaction of the axial ligand coordination to the cobalt atom and study the Trans effect.
This work provides further evidence for hydrogen bonding between axial ligand in Co(III) corrinoid systems and solvent molecules. Polar solvents seem to favor withdrawal of electron density from cobalt ion of the corrin ring [Co3+] towards axial ligand X due to electrostatic forces or/and hydrogen bonding to X.
Nonpolar solvents prompt the opposite effect, where a better electron donation from X to [Co3+] is observed. This effect was elucidated by i) shifts in the absorbance spectra and ii) changing affinity of azoles to the corrin ring (affected by basicity of azole and elecronegativity of X).
The values of log K for the coordination of the three azoles were found to depend on the polarisability of the trans ligand (CN- > SO32- > CH3– > CH2CH3–). The values of log K increase as the basicity of the entering azoles increases in all solvents studied (pyrazole < triazole < imidazole) and as the polarity of the solvent increases (toluene < ethylacetate < acetonitrile < methanol < H2O). The calculations also confirm the dependence of log K on ligand basicity is largely electronic in origin.