Octahedral cobalt complexes

Bimolecular nucleophilic substitutions of octahedral cobalt complexes. S. C. Chan and J. Miller, Rev. Pure Appl. Chem., 1965, 15, 11-22 (21). 

It is noteworthy that, as early as 1929, Shibata and Tsuchida reported a kinetic resolution of rac-3,4-dihydroxyphenylalanine by selective oxidation of one enantiomer using a chiral cobalt complex [Co(en)3NH3Cl]Br2 as a catalyst [46,47]. Figure 12 shows a highly enantioselective addition of diisopropy-Izinc to 2-(ferf-butylethynyl)pyrimidine-5-carbaldehyde via an autocatalytic process in the presence of a chiral octahedral cobalt complex with ethylenedi-... 

Examples of coordination complexes of pyridine with metals are legion and only a few can be mentioned here. Pyridine is a ligand in square complexes of gold (AuEt2Br-py) and copper (Cupy2-Cl2). The cobalt complex CoCLrpy2 has an octahedral structure. Nickel and platinum can coordinate with four pyridine molecules (30). 

Figure Cl Energy levels in the distorted octahedral cobalt complexes...

Co59 Shifts in some Octahedrally Coordinated Cobalt Complexes (35)... 

Geometrical isomerism Geometrical isomerism is possible only in hexacoordinate complexes and in the case of 2 1 metal, e.g. chromium and cobalt, complexes arises from coordination of the ligand in a meridional (81) or a facial (82) mode in an octahedral complex. In the former case only an enantiomorphic pair of isomers is possible, but in the latter the possibility exists of four enantiomorphic pairs and a centrosymmetric isomer (Figure 1). 

The term cobaloximes refers to that class of cobalt complexes in which two dimethylglyoximato or other glyoximato monoanions occupy four coordination positions about cobalt and in which the four N donors are in essentially the same plane. A neutral octahedral low-spin d6 Co(III) cobaloxime is shown in Figure 1. Cobaloximes with this oxidation state have been studied most extensively since these are more stable than the oxidation state I, II, and IV compounds. The chemistry of these latter oxidation state compounds will be reviewed... 

From the viewpoint of stereochemistry the most interesting metal complexes are the octahedrally coordinated 1 2 chromium and cobalt complex dyes, which are medially metallized azo and azomethine compounds with functional groups in the o- and o -positions. Three types of isomerism can be discriminated geometrical, N-a, 3, and that arising from azo-hydrazone tautomerism. 

Among metal-complex dyes only the planar 1 1 copper complexes exhibit sufficient substantivity to dye paper, whereas the octahedral 1 2 chromium and cobalt complexes are unsuitable for this purpose. Cationic substantive dyes, such as the bis-copper complex 26 [45] provide the most colorless effluent waters and superior fast paper dyeings that do not bleed out upon contact with milk, fruit juice, or alcohol [46],... 

Finally it should be noted that although one might not find a compound listed uhder an incorrect name, there is no guarantee that it will be listed under its correct name either. In consulting a journal index for information about a compound, one may often save time by selecting what appears to be the most important element in the compound and looking under the section devoted to that element. Thus each of the octahedral cobalt complexes pictured above would be likely to be listed under Cobalt Compounds, whereas the two platinum complexes might very well be listed under Platinum Compounds. ... 

The analysis has subsequently been extended to other Co(L)3 complexes [331] as well as M(bipy)32+ and M(phen)32+ complexes [332]. In the former study, L represents the unsaturated ligands acetylacentonate (acac), oxalate (ox), malonate (mal), and dithioxalate (diox). In the latter study, M represents the metals iron, ruthenium, and osmium. For the cobalt complexes, BP86/TZP computed spectra were in acceptable agreement with their experimental counterparts. The unsaturated ligands cause more complicated spectra due to the presence of both a and % orbitals, and therefore the analysis was somewhat more challenging. The distortions of the geometry away from an idealized octahedral metal environment resulted not... 

It was her thesis work with Alfred Werner34 that was to enter her in the annals of the history of chemistry. Among the compounds she made was cis-bis(ethylenediamine)dinitro-cobalt(III) bromide. This was the very first synthesis of a chiral octahedral cobalt complex, though at the time the significance of her synthesis was overlooked.31(a) Werner was so impressed with her work that, for her last year, he took her on as his personal assistant, the first women to be chosen for this prestigious post.35 More important for the impoverished Humphrey, she at last had some income in very expensive Switzerland. 

Optically active organometallic complexes have been used to study stereochemical reactions. Substituted cobalt nitrosyl complexes are interesting chiral see Chiral) complexes because they exhibit tetrahedral structures, whereas most optically active organometallic complexes are half-sandwich structmes with octahedral geometries. Diastereomeric cobalt complexes of the type Co(CO)(NO)(L)( L) (L = phosphite or phosphane L = optically active phosphane or isocyanide) have been synthesized from (4) via substitution (Scheme 6). ... 

Cobalt complexes in which the metal can be considered to have the formal oxidation state (III) may be six-coordinate (octahedral and perhaps trigonal prismatic), five-coordinate (square pyramidal or trigonal bipyramidal), or four-coordinate (square planar or tetrahedral). 

Two perfectly reversible one-electron reduction steps are observed for Co.S ", at —0.53 V (Co +/+) and 1.205 V (Co+/ ). Comparison of the electrochemical properties of the cobalt catenate with the previously reported data for cobalt bpy or phen complexes [31, 32] shows here again a strong stabilization of the reduced states, Co and Co". Other sterically constrained polyimine ligands also lead to stabilized monovalent cobalt complexes [33-37]. Remarkable also is the drastic structural effect of the catenate on the Co +/-+ redox potential whereas bpy or phen complexes of cobalt(II) can easily be oxidized to octahedral cobalt(III) [36, 38], the redox potential value of the Co + -+ couple being close to 0 V, no oxidation peak is observed for Co.5 + prior to ligand oxidation (Ep > 1.6 V). The destabilization effect of Co " due to tetrahedral environment provided by the entwined and interlocked structure of 5 is thus very large (>1.5 V). 

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