Co-axial bonding

Fig. 1. Structure of methyl manganesepentacarbonyl, with bond lengths in A (S3). A Mn-CO (axial) bond length of 1.82 A was assumed.

Since their first discovery by Iijima in 1991 [1], carbon nanotubes have attracted a great deal of interest due to their very exciting properties. Their structure is characterized by cylindrically shaped enclosed graphene layers that can form co-axially stacked multi-wall nanotubes (MWNTs) or single-walled nanotubes (SWNTs). Like in graphite, carbon atoms are strongly bonded to each other in the curved honeycomb network but have much weaker Van der Waals-type interaction with carbons belonging to... 

Mechanisms involving axial coordination of the optically active amine have also been invoked, and crystal structure data on RCo(DMG)2B complexes, where R is alkyl or (R)-l-(methoxycarbonyl)ethyl, and B is (R )-a-methylbenzylamine, were obtained (316, 317). Because deuteration of the (R)-methoxycarbonyl complex gave (5)-methylpropionate-2-d, it was concluded that Co—C bond cleavage occurred with inversion of configuration at the carbon (317). It would be useful to know the mechanistic details of this step, which could involve attack by D+, DCo(III), or coordinated D, as well as D2, for it is an unusual, if not unique, observation [contrast with the usual retention mechanism outlined in Eq. (39)]. 

Control experiments establish that the initial process converting the dibromide 275 to 276 takes place in two steps with E1/2 for extrusion of the first and second axial ligands occurring at —0.36 and —1.08 V, respectively [75]. After reaction with the alkyl halide, the resulting octahedral complex 277 is further reduced in the range of -1.4 to -1.7 V to form a cobalt (II) complex which decays via the addition of an additional electron, cleavage of the C-Co and Co-Y bonds, and reaction with the Michael acceptor. 

Many Com(Con) complexes are photosensitive and cleavage of axial bonds to Co is induced by visible light.254,267 For alkyl, mercapto, sulfinato and selenocyanato Co111 corrins, homolytic fission (photoreduction) proceeds, whereas heterolytic dissociation (photoaquation) occurs for amino and cyano complexes. These photoprocesses are considered to be initiated by n -> n excitations. 

---