Cobalt electron distribution

In isolation the electron distribution in the trivalent chromium (III) ion consists of three unpaired electrons in the d shell, as indicated in line (a) of Table 5.1. In line (b) the six electron pairs donated to the central chromium atom by oxygen atoms of water molecules give rise to sp3d2 hybridisation. This is characteristic of an octahedral structure. A similar situation arises with the trivalent cobalt(III) complex in line (e), where each of the three t2g levels is doubly occupied by an electron pair from each cyano ligand. 

One bond between carbon and cobalt, while predominantly covalent, has some ionic character. The oxidation-reduction potentials of cobalt are modified by its incorporation into the corrin ring system, being able to exist in either a mono-, di-, or trivalent state. In the cobalt I derivatives (Schrauzer, 1976), the metal is in spin-paired d8 configuration. Two electrons occupy the weakly binding 3d orbital, and the resulting electron distribution is responsible for the high nucleophilic reactivity of cobalt I. Indeed, such cobalt derivatives have been termed super nucleophiles. 

Electron paramagnetic resonance (continued) cobalt-thermolysin complex, 28 334, 335 exchange reactions, 31 106-107 glutamine synthetase, 28 358-364 invisible oxygen species, 31 94-95 metalloenzymes, 28 324, 326 metal particle size distribution, 36 99-100, 104... 

Scheme 6.27 considers other, formally confined, conformers of cycloocta-l,3,5,7-tetraene (COT) in complexes with metals. In the following text, M(l,5-COT) and M(l,3-COT) stand for the tube and chair structures, respectively. M(l,5-COT) is favored in neutral (18-electron) complexes with nickel, palladium, cobalt, or rhodium. One-electron reduction transforms these complexes into 19-electron forms, which we can identify as anion-radicals of metallocomplexes. Notably, the anion-radicals of the nickel and palladium complexes retain their M(l,5-COT) geometry in both the 18- and 19-electron forms. When the metal is cobalt or rhodium, transition in the 19-electron form causes quick conversion of M(l,5-COT) into M(l,3-COT) form (Shaw et al. 2004, reference therein). This difference should be connected with the manner of spin-charge distribution. The nickel and palladium complexes are essentially metal-based anion-radicals. In contrast, the SOMO is highly delocalized in the anion-radicals of cobalt and rhodium complexes, with at least half of the orbital residing in the COT ring. For this reason, cyclooctateraene flattens for a while and then acquires the conformation that is more favorable for the spatial structure of the whole complex, namely, M(l,3-COT) (see Schemes 6.1 and 6.27). 

---