Pentacyanides

Certain groups of organocobalt(III) complexes have been dealt with in previous reviews. The organo-corrinoids have been mentioned in all reviews on vitamin B, 2 since 1961, when the coenzyme form was identified as an organometallic compound [see, for example, (79, 178) and references therein]. The literature on the corrinoids is too extensive to be treated comprehensively here and for details and references readers are referred to the book on The Inorganic Chemistry of Vitamin B,2 (136)certain other aspects of the organometallic chemistry of cobalt corrinoids are treated elsewhere (137). The pentacyanides were reviewed in 1967 (105), the DMG complexes (cobaloximes) in 1968 (145), and some aspects of salen, BAE, and related complexes in 1970 (17). 

Organocobalt (III) Complexes TABLE III continued 28. Pentacyanide Complexes... 

A series of complexes in which the cyanide ligands are modified or replaced arises from the decomposition of methyl and pyridiomethylcobalt(111) pentacyanide derivatives in acid solution. The reactions include protonation of a cyanide ligand, insertion of a cyanide ligand between the organic group and the cobalt atom to produce an imine (see Section VI,D), decomposition of this imine to an acyl product, and replacement of a cyanide ligand by water 100, 101). The products are listed in Table III, 29. 

Reactions of group C, which involve interaction between two unidentate ligands in adjacent coordination positions, have so far only been reported for the pentacyanides, most of which undergo reactions to the type ... 

No j3-bromo- or iodoalkyl complexes have yet been isolated. The reaction of vicinal dibromides or diiodides with [Co (CN)j] , [Co(CN)5H], or a Co(I)-DMG complex merely gives the olefin 32, 75,105,109,161), though kinetic evidence was obtained for the intermediate formation of the j8-bromo complex in the reaction of [Co"(CN)5] with a,j8-dibromopropionate and a,/3-dibromosuccinate (75). It is interesting that the pentacyanide produced is the bromo or iodo, and not the aquo, complex 32, 75), which suggests that the decomposition may involve a cis rather than a trans elimination of Co—X. The /3-chloroethyl complex can be prepared by tbe reaction of [Co(CN)5H]3- with CICH2CH2I 105). 

Qualitative Comparison of the Stability of Pentacyanides [RCo(CNj)] " Containing Primary, Secondary, and Tertiary Alkyl Ligands"... 

Most of the examples listed are pentacyanide, corrinoid, or DMG complexes. The axial ligands are not identified in the tables, but are as follows corrinoids, 5,6-dimethylbenziminazole (cobalamins), H O or none (cobinamides), (DMG)2, usually pyridine or H2O, less frequently NHj, imidazole, benzimidazole, PBuj, etc. The nature of the axial and equatorial ligands may have a striking effect on reactivity, but few direct comparisons are available these are discussed in the next section. 

As already pointed out, the presence of a macrocyclic ligand, which occupies the four equatorial coordination sites of an octahedral complex, will tend to limit insertion and elimination reactions of the Co—C bond to those which require only that single coordination site. The presence of unidentate ligands in the pentacyanides, on the other hand, will offer greater opportunities for reactions which require a second, adjacent site. 

Catalytic hydrogenation with platinum liberates the hydrocarbon from methylcobalamin (57) and from alkyl-Co-DMG complexes (161), but not from pentacyanides with primary alkyl, vinyl, or benzyl ligands, though the cr-allyl complex yields propylene (109). Sodium sand gives mixtures of hydrocarbons with the alkyl-Co-salen complexes (64). Dithioerythritol will liberate methane from a variety of methyl complexes [cobalamin, DMG, DMG-BF2, G, DPG, CHD, salen, and (DO)(DOH)pn] (156), as will 1,4-butanedithiol from the DMG complex (157), and certain unspecified thiols will reduce DMG complexes with substituted alkyl ligands (e.g., C0-CH2COOH ->CH3C00H) (163, 164). Reaction with thiols can also lead to the formation of thioethers (see Section C,3). 

Pentaazadienes, structure, 32 171-172 Pentaborane(9) reaction of with alkynes, 26 74 Pentacarbonyltungstenhydrogen sulfide anion, reaction with hexafluoroacetone, 30 297 Penta-coordinate complexes, 4 175-178 Pentacyanides... 

The pentacyanide derivative unites with the carbon monoxide to yield sodium carbonyl ferrocyanide, Na3[Fe(CN)5.CO], which again interacts with the ferrous sulphate to yield the ferrous derivative, Fe3[Fe(CN)5CO]2, characterised by its violet colour (see p. 232). 

Pentacyanide Derivatives of Iron—Nitroso Penta-oyanoferrates—Carbonyl Penta-cyanoferrites—Nitrito Penta-cyanoferiites—Ammino Penta-cyano-ferntes—Aquo Penta-cyanoferrites—Aquo Penta-oyanoferrates—Sulphito Penta-cyanoferrites. 

Comprehensive kinetic data, involving both forward and reverse reactions and the overall equilibrium constants, have been obtained for three groups of complexes. Haim, Grassi, and Wilmarth have studied the pentacyanides their original papers have been summarized in a review article (50). Halpern, Palmer, and Blakley (86) have studied the reactions... 

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