LEONOR MICHAELIS 



Fe++ + 2 CN- > Fe(CN)2 (a) 



However, the combining power of the ferrous ion is not exhausted by 

 this reaction based on opposite charges. In fact, the molecular species 

 Fe(CN)2 has never been shown to be capable of existence. More 

 than two cyanide ions are attached to the iron due to the fact that 

 each CN~ ion has one pair of electrons not used for chemical bonding. 

 Each of such electron pairs can be shared with the iron to fill up its 

 outermost incomplete electron shell to a complete shell, as in a noble 

 gas. In addition to the two CN~ ions of equation (a), four more can 

 be attached, which contribute four negative charges to the complex 

 molecule, which is a ferrocyanide ion: 



Fe(CN)2 + 4 CN- > Fe(CN)J~ (b) 



The six CN groups are arranged around the central iron atom as the 

 corners of an octahedron. Fe is said to possess "six coordination 

 places" which may be occupied by atoms or atom groups. In analogy, 

 when ferrous ion combines with cysteine, which we may write, briefly, 

 as RSH, * we may imagine that, primarily, a saltlike compound 

 between iron and two molecules of cysteine is formed in such a way 

 that the two hydrogen atoms of the sulfhydryl groups are replaced 

 by an iron atom: 



Fe++ + 2 RSH » Fe(RS)2 + 2 H++ (c) 



This scheme accounts, so far, for the saturation of two (of the six pos- 

 sible) coordination places. Now atom group R contains another 

 atom with an unused electron pair, viz-, the nitrogen atom of the 

 amino group. Thus, the two molecules of RSH will occupy four 

 coordination places. Probably because of the large size of the RSH 

 molecule and steric hindrance involved in it, the two remaining 

 coordination places cannot be occupied by a third molecule of RSH. 

 In fact, no ferrous complex of cysteine can be prepared with more 



* SH is a sulfhydryl group, and R represents the rest of the molecule, 

 which is, altogether: 



SH NHj 



1 I 

 H2C— C— COOH 

 H 



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