Ethylenediamine complexes of cobalt

Since the reviews have stressed the numerous stereochemical studies which have been conducted on the octahedral bis(ethylenediamine) complexes of cobalt(III), those results will be reiterated only briefly. 

When, however, the ligand molecule or ion has two atoms, each of which has a lone pair of electrons, then the molecule has two donor atoms and it may be possible to form two coordinate bonds with the same metal ion such a ligand is said to be bidentate and may be exemplified by consideration of the tris(ethylenediamine)cobalt(III) complex, [Co(en)3]3+. In this six-coordinate octahedral complex of cobalt(III), each of the bidentate ethylenediamine molecules is bound to the metal ion through the lone pair electrons of the two nitrogen atoms. This results in the formation of three five-membered rings, each including the metal ion the process of ring formation is called chelation. 

The importance of metal coordination compounds in biological systems has led to the study of polydentate Schilf base complexes of cobalt(II), nickel(II), and copper(II) (204, 205). Dimers have been observed in the spectra of complexes of both tri- and tetradentate ligands [e.g., salicylaldehydeand A,A-bis(salicylidene)ethylenediamine]. The parent ions form the base peaks, and the spectra are characterized... 

Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

The procedure described here is based on the observation that amine monohydroxo complexes of cobalt(III), rhodium(IIl), and iridium(III) react directly with carbon dioxide to form the corresponding carbonato complexes,2 3 without effect on the configuration of the amine ligands.4 The amine monoaqua complex is allowed to react with lithium carbonate or carbon dioxide gas at room temperature at pH 8.0 for a few minutes, and the carbonato complex is isolated by adding alcohol. The procedure has been used to prepare salts of the following cations pentaammine(carbonato)-cobalt(III),2 ds-ammine(carbonato)bis(ethylenediamine)cobalt(III),5 trans-... 

Although the existence of circular dichroism and anomalous optical rotatory dispersion for the visible d—d transitions of transition metal complexes was discovered by Cotton (7), the first resolution of an octahedral complex was achieved by Werner (2), for the chloroamminebis-(ethylenediamine)cobalt(III) ion (I, X = C1, Y = NHs). In the course of a few years he resolved (3) the trisethylenediaminecobalt(III) ion (II), a number of bis- and tris-chelated octahedral complexes of cobalt, chro-... 

Very little is known about the metabolism of metal complexes, though a number of ammine complexes of cobalt(III) were used as nitrogen sources for aspergillus niger nearly forty years ago (150), and much more recently, both tris(ethylenediamine)cobalt(III) ions (757) and amino acid complexes (152) of cobalt(III) have served as nitrogen sources for species of pseudomonas. Correlations through enzymic selectivity are therefore as yet not... 

In the case of the octahedral robust complexes of cobalt (III) and chro-mium(III), substitution in the first sphere is hindered. This type of complex ion is, therefore, especially suitable for studying association in the second sphere. The hexammine and tris(ethylenediamine) cobalt(III) ions have especially been used for this kind of study. For the association of these ions with anions, such as sulfate and thiosulfate, the ion-pair constant is of the order of magnitude of 10 at 7 = 0, somewhat smaller for Coena" than for Co(NH3)6 21)y but strongly dependent on the ionic strength. Thus Posey and Taube 37) y from spectrophotometric measurements in the ultraviolet, obtain the following expression for the association constant of the ion pair [Co(NH3)6]S04 in solutions with y/Jvarying from 0.04 to 0.3 ... 

Treatment of an aqueous solution of the cobalt(III) bis(ethylenediamine) complex of 1-aminocyclopropanecarboxylate 1 with ammonium sulfide followed by hydrochloric acid gave cobalt(II) sulfide and the hydrochloride salt of 1-aminocyclopropanecarboxylic acid (2). ... 

The kinetics of stereoselective deuteration of malonate hydrogens in bis(malonato)-cobalt(III) complexes [Co(mal)2L2] containing L2 = en, pn, N,N -Me2en, phen, c/5 -(NH3)2, or c/5-(py)2 have been monitored. Both acid and base hydrolysis are observed, and there is a reversal of stereoselectivity with solution pH. There are some kinetic differences between the amine ligands on the one hand and py and phen on the other, as competition for OD" between malonate and amine is possible, between malonate and py or phen not. The rate law for deuteration of a hydrogens in a-aminocarboxylato complexes of cobalt(III) containing various combinations of glycine, sarcosine, or alanine with ammonia, ethylenediamine, or diaminopropane is simple second order. 

Balthis and Bailar6 obtained tris (ethylenediamine) chromium-(III) complexes by the oxidation of chromium(II) solutions, using a procedure somewhat similar to that used for the synthesis of cobalt (III) com plexes. Mori7 described the preparation of hexaamminechromium(III) salts from the oxidation of chromium (II) salts in the presence of ammonia. The results obtained in both syntheses have been erratic.8,9 Berman noted that the foregoing syntheses are rendered dependable by the use of a catalyst of activated platinum on asbestos. Schaeffer,100 in a subsequent study, independently used colloidal platinum as a catalyst but reported some difficulty in separating it from the product.106 The procedures recommended and described here are based on the use of platinized asbestos as the catalyst. 

In other reactions, the metal may be oxidized as the complex is formed. Numerous complexes of Co(III) have been prepared by the oxidation of solutions containing Co(II). This technique is particularly useful in the case of cobalt because Co3+ is a strong oxidizing agent that reacts with water if it is not stabilized by complexation. In the following reactions, en is ethylenediamine, H2NCH2CH2NH2 ... 

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