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General Chemistry Principles, Patterns, and Applications, 2011

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When vitamin B12 (the form present in vitamin tablets) is ingested, the axial cyanide lig<strong>and</strong> is replaced by<br />

a complex organic group.<br />

Figure 23.27 Vitamin B12<br />

In the body, the axial cyanide lig<strong>and</strong> found in the vitamin is replaced by a complex organic unit.<br />

Heterolytic cleavage of the Co–C bond in the resulting organometallic complex generates an<br />

organic radical for the catalysis of rearrangement reactions.<br />

The cobalt–carbon bond in the enzyme-bound form of vitamin B12 <strong>and</strong> related compounds is unusually<br />

weak, <strong>and</strong> it is particularly susceptible to homolytic cleavage:<br />

Equation 23.22<br />

CoCH2R Co2 +× + ×CH 2R<br />

Homolytic cleavage of the Co 3+ –CH2R bond produces two species, each of which has an unpaired electron:<br />

a d 7 Co 2+ derivative <strong>and</strong> an organic radical, ·CH2R, which is used by vitamin B12-dependent enzymes to<br />

catalyze a wide variety of reactions. Virtually all vitamin B12-catalyzed reactions are rearrangements in<br />

which an H atom <strong>and</strong> an adjacent substituent exchange positions:<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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