Cobalt tetrahedral complexes

DR. NORTON An excellent attempt to observe such hydrogen bonding was made recently by Fachinetti, et al. [Calderazzo, F. Fachinetti, G. Marchetti, F. Zanazzi, P. F. J. Chem. Soc., Chem. Commun. 1981, 181]. They took hydridocobalttetracarbonyl and triethylamine, and crystallized out a species which one can only describe as the tetracarbonylcobaltate of protonated triethylamine. They proposed some type of interaction between the hydrogen and a face of the cobalt tetrahedral complex, but it was clear that the interaction was almost entirely with nitrogens. The conclusion I would draw is that the complex appears to proceed directly to full protonation of the amine without any observable evidence for a hydrogen bonded intermediate. 

Cobalt exists in the +2 or +3 valence states for the majority of its compounds and complexes. A multitude of complexes of the cobalt(III) ion [22541-63-5] exist, but few stable simple salts are known (2). Werner s discovery and detailed studies of the cobalt(III) ammine complexes contributed gready to modem coordination chemistry and understanding of ligand exchange (3). Octahedral stereochemistries are the most common for the cobalt(II) ion [22541-53-3] as well as for cobalt(III). Cobalt(II) forms numerous simple compounds and complexes, most of which are octahedral or tetrahedral in nature cobalt(II) forms more tetrahedral complexes than other transition-metal ions. Because of the small stabiUty difference between octahedral and tetrahedral complexes of cobalt(II), both can be found in equiUbrium for a number of complexes. Typically, octahedral cobalt(II) salts and complexes are pink to brownish red most of the tetrahedral Co(II) species are blue (see Coordination compounds). 

Sometimes, the physicochemical properties of ionic species solubilized in the aqueous core of reversed micelles are different from those in bulk water. Changes in the electronic absorption spectra of ionic species (1 , Co ", Cu " ) entrapped in AOT-reversed micelles have been observed, attributed to changes in the amount of water available for solvation [2,92,134], In particular, it has been observed that at low water concentrations cobalt ions are solubihzed in the micellar core as a tetrahedral complex, whereas with increasing water concentration there is a gradual conversion to an octahedral complex [135],... 

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. 

The cobalt(II) complexes may be readily oxidized to cobalt(III) species. For example, use of different cobalt(II) salts in syntheses have been reported to direct the formation of either tetrahedral cobalt(II) species or oxidized octahedral cobalt(III) species.493 Also, /i-peroxodicobalUIII)... 

Equilibria between tetrahedral and octahedral cobalt(II) complexes in nonaqueous solution are well characterized. Kinetic data are sparse but those available from T-jump experiments in pyridine solution are interpreted in terms of (X = Cl and Br) ... 

A variety of geometries have been established with Co(II). The interconversion of tetrahedral and octahedral species has been studied in nonaqueous solution (Sec. 7.2.4). The low spin, high spin equilibrium observed in a small number of cobalt(Il) complexes is rapidly attained (relaxation times < ns) (Sec. 7.3). The six-coordinated solvated cobalt(ll) species has been established in a number of solvents and kinetic parameters for solvent(S) exchange with Co(S)6 indicate an mechanism (Tables 4.1-4.4). The volumes of activation for Co " complexing with a variety of neutral ligands in aqueous solution are in the range h-4 to + 1 cm mol, reemphasizing an mechanism. 

There have been few studies of substitution in complexes of nickel(II) of stereochemistries other than octahedral. Substitution in 5-coordinated and tetrahedral complexes is discussed in Secs. 4.9 and 4.8 respectively. The enhanced lability of the nickel(II) compared with the cobalt(II) tetrahedral complex is expected from consideration of crystal field activation energies. The reverse holds with octahedral complexes (Sec. 4.8). 

S- and Se-donor ligands. The e.s.r. and electronic spectra of [Co(sacsac)2] and [Co(sacsac)2L] (sacsac = dithioacetylacetonate, L = py or piperidine) have been studied, and a polarographic study of [Co(sacsac)J (n = 2 or 3) in acetone has shown the complexes to have a well-defined capacity to accept one or two electrons in a reversible stepwise manner. The magnitude of the potentials and their reversible nature suggest that isolation of cobalt-sacsac complexes of low formal oxidation states should be possible." Co complexes of l,5-bis-(2-methylmercaptoethylthio)pentane are both hydrated and polymeric, and thermal decomposition in air or nitrogen leads to oxida tion to Co . Ethylenethiourea (etu) and tetramethylthiourea (tmtu) form the complexes [Co(etu) ](N03)2 and [Co(tmtu) ](C10 )2, which are tetrahedral, and [Co(etu)2(N03)2] and [Co(tmtu)2(N03)2] which have distorted octahedral co-ordination. 3-Diphenylphosphinothioyl-l-phenylthiourea, -1,1-diethyl-thiourea, and -1,1-dimethylthiourea form complexes with Co in which the ligands are bidentate. ... 

Cobalt(II) forms more tetrahedral complexes than any other transition metal ion. Also, because of small energy differences between the tetrahedral and octahedral complexes, often the same ligand forms both types of Co(II) complexes in equilibrium in solutions. 

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 ... 

Photoreduction of cobalt(III) complexes in nonaqueous solvent systems has been little studied because of the limited solubility of cobalt(III) complexes and their tendency to photooxidize the solvent. Irradiation with 365-mjj. light of cis- or trans-Co(en)2C 2 + and Co(en)2Cl(DMSO)2+ in dimethylsulfoxide (DMSO) leads rapidly to production of a green tetrahedral cobalt(II) product apparently with concurrent solvent oxidation.53,71 Irradiation with 365-mjx light of the molecular Co(acac)3 in benzene rapidly gives a red precipitate which may be the cobalt(II) acetylacetonate.53... 

On the second problem the people in my area are responsible for the poor state of affairs. I take most responsibility because outside of octahedral and tetrahedral complexes no complete assignments have been made. But, we do have new results, and I think it is of some interest to present them. Cooper Langford and I at Columbia now have conclusive results on the energy levels in the distorted octahedral cobalt compounds. These are the Co(NH3)5X+2 complexes. 

The First example of a low spin tetrahedral complex of a first-row transition metal, lelrakisfl-norbornyl)cobalt(lV), was recently confirmed Sec Byrne, E K. Richeson, D. S.,ThcopoUI. K. H. Chem. Commufl. 1986. 1491-1492. 

The UV spectra of these complexes are very similar to those found in tetrahedral complexes of Co11 known to have a somewhat distorted geometry, suggesting a similar geometry in the cobalt enzyme.1383 Tetrahedral mercaptide complexes of the type [Co(SPh)4]2- were also shown to have similar absorption characteristics to those of [(LADH)Co2Co2j. This work is in complete agreement with the X-ray crystallographic studies of the native LADH, already mentioned, which shows distorted tetrahedral coordination of both the catalytic and non-catalytic zinc atoms of the enzyme. 

The dynamics of spin equilibria in solution are rapid. The slowest rates are those for coordination-spin equilibria, in which bonds are made and broken even these occur in a few microseconds. The fastest are the AS = 1 transitions of octahedral cobalt(II) complexes, in which the population of the e a antibonding orbital changes by only one electron these appear to occur in less than a nanosecond. For intramolecular interconversions without a coordination number change, the rates decrease as the coordination sphere reorganization increases. Thus the AS = 2 transitions of octahedral iron(II) and iron(III) are slower than the AS = 1 transitions of cobalt(II), and the planar-tetrahedral equilibria of nickel(II) are slower again, with lifetimes of about a microsecond. 

The products isolated from reactions of amides with transition metal halides usually contain coordinated halide (e.g. the formulations in Table 2). In some cases such as [Co(NMF)6][CoCLt], halide and amide are coordinated to different metal atoms, but when such compounds are dissolved in the neat ligand, halide can be replaced and at high dilution all the metal ions may be fully coordinated by the amide alone. The electronic spectrum resulting when this cobalt complex is dissolved in nitromethane has been interpreted as relating solely to the tetrahedral complex [CoC12(NMF)2]. 

The cobalt (II) ion preferentially forms octahedral complexes, but tetrahedral coordination is not uncommon. The tendency to form tetrahedral complexes is less marked than for Zn(II), but much stronger than for Ni(II) or Cu(II) (10). The actual geometry obtained with given ligands... 

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