Cobalt coordination with metals

Vitamin Bjj (8.50, cobalamin) is an extremely complex molecule consisting of a corrin ring system similar to heme. The central metal atom is cobalt, coordinated with a ribofuranosyl-dimethylbenzimidazole. Vitamin Bjj occurs in liver, but is also produced by many bacteria and is therefore obtained commercially by fermentation. The vitamin is a catalyst for the rearrangement of methylmalonyl-CoA to the succinyl derivative in the degradation of some amino acids and the oxidation of fatty acids with an odd number of carbon atoms. It is also necessary for the methylation of homocysteine to methionine. 

The catalyst (cat) loses one CO ligand to form Co(CO)3 (cat ) and then binds the olefin (ole) as a ir-complex (X,) at the vacated coordinative site. Addition of H2 produces a trihydride (X2), which loses H2 to form a cobalt alkyl with metal-carbon (j-bond (X3). Next, a new CO ligand adds to the newly vacated coordinative site (X4). 

Metal ion complexation rates have been studied by the T-jump method. ° Divalent nickel and cobalt have coordination numbers of 6, so they can form complexes ML with monodentate ligands L with n = 1—6 or with bidentate ligands, n = 1-3. The ligands are Bronsted bases, and only the conjugate base form undergoes coordination with the metal ion. The complex formation reaction is then... 

The basis for the toxicological activity of this substance is the reaction of cobalt ion with cyanide ion to form a relatively nontoxic and stable ion complex. The hexacyanocobaltate ion contains a Co2+ central metal ion with six cyanide ions as ligands. This coordination complex involves six coordinate covalent bonds whereby each cyanide ion supplies a pair of electrons to form each covalent bond with the central cobalt ion. The formation constant for the hexacyanocobaltate ion is even larger than for dicobalt EDTA,3 and thus the cobalt ion preferentially exchanges an EDTA ligand for six cyano ligands ... 

The liquid-phase oxidation of acrolein (AL), the reaction products, their routes of formation, reaction in the absence or presence of catalysts such as acetylacetonates (acac) and naphthenates (nap) of transition metals and the influence of reaction factors were discussed in an earlier paper (22). The coordinating state of cobalt acetylacetonate in the earlier stage of the reaction depends on the method of addition to the reaction system (25, 26). The catalyst, Co(acac)2-H20-acrolein, which was synthesized by mixing a solution of Co(acac)2 in benzene with a saturated aqueous solution, decreases the induction period of oxygen uptake and increases the rate of oxygen absorption. The acrolein of the catalyst coordinated with its cobalt through the lone pair of electrons of the aldehyde oxygen. Therefore, it is believed that the coordination of acrolein with a catalyst is necessary to initiate the oxidation reaction (10). 

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). 

Other reactions of dienes with metal atoms are only of a limited synthetic use. Dibenzylideneacetone (PhCH=CH—CO—CH=CHPh DBA) reacts with palladium vapor to afford Pd2(DBA)3, a complex in which the coordination is through the two C=C units and does not involve the C=0 (5, 92). Cobalt vapor undergoes an extremely complicated reaction with 1,4-pentadiene, producing pentenes, C5H6, and various polymers as well as the organometallic product, HCo( 1,3-pen tadiene)2, which involves isomerization from a nonconjugated to a conjugated diene (104, 110). 

In general, then, metal ions in solution form complexes (frequently six coordinate) with the solvent molecules, their counterions, and other donor molecules that happen to be in the solution. For example, in ammo-niacal aqueous solution, Ag+ forms Ag(NH3)2+ (as noted above), Cu2+ forms a series of aquaammines but most notably the royal blue trans-Cu(NH3)4(OH2)22+, and cobalt(II) forms Co(NH3)i(H20)6-i2+ complexes which react quite rapidly with oxygen in air to give the strawberry-red cobalt(III) complex Co(NH3)5OH23+ or (if much chloride ion is present) the Co(NH3)5C12+ ion mentioned above. 

Reaction 11 involves hydrogen atom transfer as proposed by Halpern et al. (13) in the mechanism of formic acid oxidation by cobalt (III) in aqueous solutions. In this reaction one could consider that as peracetic acid approaches the coordination sphere of Co111 and transfers the hydrogen atom to the coordinated acetate, the Co111 atom is transformed into a Co11 complex of peracetoxy radical (or Co111 complex of peracetate anion). Complexes of free radicals with metal ions have been postulated by Kochi (16). The substitution rate in this complex could be intermediate between the rate of substitution of cobalt (III) and cobalt (II) complexes owing to the contribution of the resonance structures ... 

Porphyrins with metals other than iron (and cobalt) are not of particular direct relevance to the present section, although it should be noted that porphyrins have been extracted from coal and oil shales with metals such as gallium and the early transition metals coordinated. The biological significance of these observations is questionable. Nevertheless, detailed studies on a range of metalloporphyrins have contributed substantially to our appreciation of the bonding in such molecules, and the influence of cis and trans ligands on their structure and reactivity.599 609... 

Complexes of thieno[2,3-d]pyrimidines 100 (R1 = NH2 R2 = NHEt, NEt2, NEtPh, or morpholino R3 = C02Me) with copper(II), nickel(II), and cobalt(II) have been obtained. The metals were found to coordinate with N-l of the thieno[2,3-d]pyrimidines (88MI2 94MI4). 

The ability of the hydrophilic PAA core of the amphiphilic core-shell brushes to coordinate with different metal cations can be used for the synthesis of novel nanosized organic/inorganic hybrids or for the generation of gold clusters or cobalt nanowire [126]. 

To observe the B12 function, the equatorial ligand must have either — 1 or zero charge (cobaloxime) when coordinated with cobalt so that the higher oxidation state of cobalt is not too stabilized. However, if the equatorial ligand involves a lower 7r level that can accept an electron from the metal orbital, the univalent cobalt state is stabilized to the extent that the cobalt is not reactive. 

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