Cobalt II Complexes

Introduction of the cobalt atom into the corrin ring is preceeded by conversion of hydrogenobyrinic acid to the diamide (34). The resultant cobalt(II) complex (35) is reduced to the cobalt(I) complex (36) prior to adenosylation to adenosylcobyrinic acid i7,i -diamide (37). Four of the six remaining carboxyhc acids are converted to primary amides (adenosylcobyric acid) (38) and the other amidated with (R)-l-amino-2-propanol to provide adenosylcobinamide (39). Completion of the nucleotide loop involves conversion to the monophosphate followed by reaction with guanosyl triphosphate to give diphosphate (40). Reaction with a-ribazole 5 -phosphate, derived biosyntheticaHy in several steps from riboflavin, and dephosphorylation completes the synthesis. 

The electronic structure of cobalt(II) complexes with Schiff bases and related ligands. C. Daul, C. W. Schlapfer and A. von Zelewsky, Struct. Bonding (Berlin), 1979,36,129-171 (76). 

But one must not be too facile with sweeping generalizations concerning concentration dependences or their absence. For example, consider the reaction between the cobalt(II) complex known as Co(sep)2+ and molecular oxygen. With these reagents only, the two-step reaction in acidic, aqueous solution is5... 

Figure 4-5. Spectrum of an octahedral cobalt(ii) complex showing a weak Tig—> M2g band.

Thus, the interaction of the ground state cobalt(ii) complex with long Co-N bonds and the ground state cobalt(iii) complex with shorter Co-N bonds initially... 

If this complex now collapses, it will be the labile Co-Cl bond which is broken, as opposed to the inert Cr-Cl bond. The labile cobalt(ii) complex reacts further with bulk water to generate [Co(H20)6] (Eq. 9.37). The key feature is that a necessary consequence of this inner-sphere reaction is the transfer of the bridging ligand from one center to the other. This is not a necessary consequence of all such reactions, but is a result of our choosing a pair of reactants which each change between inert and labile configurations. In the reaction described above, the chloride... 

The alleged preparation of the supposed cobalt(II) complex Na[Co(Et2dtc)3] described by D Ascenzo and Wendlandt (305) has been repeated by Holah and Murphy (306), who identified the product as [Co(Et2dtc)3]. Complexes of cobalt(III), nickel(II), and palladium(II) salts with cationic, dithiocarbamate ligands have been synthesized (307). Reaction of the secondary amine (Et2N(CH2)2)2NH with CS2 produces... 

Band L, Bencini A, Benelli C, Gatteschi D, Zanchini C (1982) Spectral-Structural Correlations in High-Spin Cobalt(II) Complexes. 52 37-86 Band L, Bertini I, Luchinat C (1990) The H NMR Parameters of Magnetically Coupled Dimers-The Fe2S2 Proteins as an Example. 72 113-136 Baran EJ, see MuUer A (1976) 26 81-139... 

Daul C, Schlapfer CW, von Zelewsky A (1979) The Electronic Structure of Cobalt(II) Complexes with Schiff Bases and Related Ligands. 36 129-171 Davidson G, see also Maroney MJ (1998) 92 1-66 Dawson JH, see Andersson LA (1991) 74 1-40... 

Cobalt(II) complexes of three water-soluble porphyrins are catalysts for the controlled potential electrolytic reduction of H O to Hi in aqueous acid solution. The porphyrin complexes were either directly adsorbed on glassy carbon, or were deposited as films using a variety of methods. Reduction to [Co(Por) was followed by a nucleophilic reaction with water to give the hydride intermediate. Hydrogen production then occurs either by attack of H on Co(Por)H, or by a disproportionation reaction requiring two Co(Por)H units. Although the overall I easibility of this process was demonstrated, practical problems including the rate of electron transfer still need to be overcome. " " ... 

The hexamine cobalt (II) complex is used as a coordinative catalyst, which can coordinate NO to form a nitrosyl ammine cobalt complex, and O2 to form a u -peroxo binuclear bridge complex with an oxidability equal to hydrogen peroxide, thus catalyze oxidation of NO by O2 in ammoniac aqueous solution. Experimental results under typical coal combusted flue gas treatment conditions on a laboratory packed absorber- regenerator setup show a NO removal of more than 85% can be maitained constant. 

Recent years, the authors have innovatively proposed a method by using the aqueous ammonia liquor containing hexamine cobalt (II) complex to scrub the NO-containing flue gases[6-9], since several merits of this complex have been exploited such as (1) activation of atmospheric O2 to a peroxide to accelerate the O2 solubility, (2) coordination of NO, as NO is a stronger ligand than NH3 and H2O of Co( II) complexes to enhance the NO absorption and (3), catalysis of NO oxidation to further improve the absorption both of O2 and NO. Thus, a valuable product of ammonium nitrate can be obtained. 

The cobalt(II) complex, which is optically unstable, was formed in reaction mixtures by the addition of ethylenediamine (excess) to Co . The rate law obtained from the racemization data was... 

Bis(glyoximato)cobalt(II) complexes of the types Co(DH)2B2 and Co(DH)2B2 (DH = disubstituted glyoxime, B = base, e.g. pyridine or triphenylphosphine) reduce benzyl bromide in benzene and acetone solutions ... 

Although coordination of the heterocyclic nitrogen does not occur, two cobalt(II) complexes of 3-hydroxy-5-hydroxymethyl-2-methyl-4-formylpyridine have been isolated with stereochemistry [Co(34-H)A] 2H2O (A = NO3, OAc) [173], For both complexes coordination is ONS (deprotonated phenolic oxygen), but magnetic or electronic spectral data are not included. 

Methyl-5-amino-l-formylisoquinoline thiosemicarbazone, 22, also yields cobalt(II) complexes from unheated methanol solution [202]. However, due to this ligand s added steric requirements, a complex, [Co(22)Cl2], with one ligand per metal ion center is formed. This brown solid has a magnetic moment of 4.42 B.M., is a non-electrolyte, has coordination of a neutral NNS ligand, and the electronic spectrum indicates approximate trigonal bipyramidal stereochemistry. 

To date, the only reported cobalt(II) complexes involve 2-acetylpyridine N-oxide JV-dimethylthiosemicarbazone, 34 [189]. [Co(39-H)A] (A = Cl, Br) have magnetic moments suggestive of flattened tetrahedra, but calculations based on Tj symmetry yield values of Dq of about 330 cm and B values of 743 and 707 cm for the chloro and bromo complexes, respectively. 

Cobalt(II) complexes of 4-(2-thiazolyl)-l-(2-acetylfuran) thiosemicarbazone, 44, were isolated having stoichiometries of [Co(44)2Cl2] and [Co(44-H)2] [194]. The former complex is isolated from neutral solution and is octahedral with each 44 bonding NS. The latter complex is formed in basic media, involves a tetrahedral cobalt(II) center and has NS bonding based on infrared studies. 

Nickelfll) complexes of p-anisaldehyde thiosemicarbazone, [Ni(HL)2X2] with X = Cl [212], Br [213], showed greater activity than the corresponding ironfll), manganesefll) and cobalt(II) complexes against Alternaria (Sp.), Paecil-omyces (Sp.) and Pestalotia (Sp.). All complexes were more active than the uncomplexed thiosemicarbazone. 

An unusually slow relaxation has been observed for the 2,6-pyridine-dicarboxaldimine cobalt(II) complex [Co(2,6-(CH3NH=CH)2py)2](PFg)2 in solution. Thus a relaxation time -c = 83 ns has been reported [99], the rate constants being among the lowest found. It has been suggested that nonelectronic factors such as partial ligand dissociation, steric effects or solvent interaction may be rate determining in this equilibrium. 

Cobalt(II) Complexes. Spin conversion rates greater than 10 s have been estimated for [Co(terpy)2](PFg)2 [29]. Although Ar 0.04 A is available for the [Co(terpy)2] complex [132] there is a large uncertainty concerning the magnitude of electronic coupling. Therefore, no attempt has been made to calculate the rate constant. 

The simultaneous observation of the two EPR spectra has been reported in particular for several tris(dithiocarbamato)iron(III) complexes [Fe(R2NC(S)S)3] where R = cyclohexyl [143], hydroxyethyl [144], and n-butyl [145, 146]. In addition, a considerable number of iron(III) complexes of the type [Fe" -N402] has been found which show EPR spectra of both the HS and LS isomers. These comprise [Fe(X-SalEen)2] Y2 where X-SalEen is the Schiff-base ligand obtained by condensation of X-substituted salicylaldehyde and IV-ethylethylenediamine [147] and similar compounds [100, 148, 149, 150, 151]. For the cobalt(II) complex [Co(terpy)2] (004)2, it is not completely clear whether the two observed EPR spectra are due to HS and LS states related by a spin-state transformation [152]. 

Bond length differences between HS and LS isomers have been determined for a number of iron(II), iron(III) and cobalt(II) complexes on the basis of multiple temperature X-ray diffraction structure studies [6]. The available results have been collected in Table 17. Average values for the bond length changes characteristic for a particular transition-metal ion have been extracted from these data and are obtained as AR 0.17 A for iron(II) complexes, AR 0.13 A for iron(III) complexes, and AR = 0.06 A for cobalt(II) complexes. These values may be compared with the differences of ionic radii between the HS and LS forms of iron(II), iron(III) and cobalt(II) which were estimated some time ago [184] as 0.16, 0.095, and 0.085 A, respectively. 

Table 18. Bond lengths R for equatorial and axial bonds in HS and LS Cobalt(II) complexes with salen and salen-type ligands...

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