Cobalt square complexes

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

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 ... 

Large numbers of low-spin cobalt II) complexes are known, both with essentially trigonal bipyramidal structures and with square pyramidal structures. 

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). 

The unsymmetrical nature of / -mercaptoethylamine should lead to geometric isomerism among its metal complexes, cis and trans isomers might be expected with the square planar nickel (II) and palladium (II) derivatives and facial and peripheral isomers with cobalt (III). However, during the course of the preparation of various samples in which the procedure and experimental conditions were varied, no evidence of such isomerism was apparent (6, 15). This is particularly evident in the case of the cobalt (III) complex, CoL3. Samples prepared by the addition of cobalt (II) chloride 6-hydrate to strongly basic aqueous solution of the ligand and by displacement of ammonia and (ethylenedinitrilo)-... 

The cobalt(n) complex (6) undergoes disproportionation in the presence of CO under basic conditions to give Co CO), where L is derived from the original ligand by introduction of a C=C bond into one of the six rings and subsequent loss of one H atom. An X-ray structure shows the complex to be square pyramidal with an axial CO group.54... 

Genuine examples of square pyramidal cobalt(II) complexes are relatively rare36,37). In the few well documented cases bands are seen at 5,000, 7,000, 11,000, 17,000 and 20,000 cm-1. The molar extinction coefficients increase on passing from the F-F to the F-P transitions. For the former e as low as 7 is observed while for the latter values as high as 320 were reported. 

The energies of the electronic transitions, the effective magnetic moment and the g and A values of some relevant five coordinate cobalt(II) complexes are given in Table 3. Genuine examples of square pyramidal chromophores are exceedingly rare. Among... 

Table 3. Spectral parameters for five coordinate cobalt(II) complexes. The energies of the electronic transitions are labelled according to the symmetry labels of the excited states in D3h symmetry for trigonal bipyramidal, and in C4v symmetry for square pyramidal complexes. F and P refer to the free ion terms which contribute mostly to the indicated state. Don.Set is the donor set of the... 

The existence of a tetracyanocobaltate(II) ion in aqueous solution has been inferred from studies at high dilution and a brown polymeric complex of composition K2[Co(CN)4] has been isolated from solutions of [Co(SCN)2] and KCN in ammonia.20 Recently, the monomeric air-sensitive complex [(PPh3)2N]2[Co(CN)4] DMF has been crystallized from DMF solution.29 The complex is essentially square planar but does feature a very weak axial interaction to a neighbouring DMF molecule (Co—O separation 264 pm). The [Co(CN)4]2 anion appears to be the sole example of a square-planar low-spin (ji = 2.15 BM) cobalt(II) complex containing solely unidentate ligands. 

There are three types of rhodium(II) complex. By far the most common are the dimeric carboxylatorhodium(II) species. Octahedral complexes may also be generated by the radiolysis of aqueous solutions of classic rhodium(lll) complexes. Square-planar complexes containing bulky tertiary phosphine ligands can be produced by carehil reduction of hydrated rhodium trichloride. The chemistry of rhodium(ll) differs very considerably from the well-known monomeric octahedral or tetrahedral cobalt(II) species because cobalt(ll) complexes are high-spin (f species while rhodium(II) complexes are all low spin. No spin reorientation is required upon oxidation to rhodium(lll), so monomeric rhodium(II) complexes are excellent reducing agents. 

Cobalt(iii) diketonate complexes generate alkyl peroxo adducts that can oxidize alkenes to oxiranes <1999IC1603>. 0-Phenylenebis(oxamate)-Iigated square-planar cobalt(iii) complexes catalyze high-yield epoxida-tions of unfunctionalized tri- and disubstituted alkenes <1997TL2377>. Low yields are obtained with terminal alkenes. Terminal alkenes can be converted smoothly to aldehydes using an epoxidation-isomerization with ruthenium(ii) porphyrin catalysts <2004AGE4950>. 

Nickel(ii) and cobalt(ii) complexes continue to be the most widely studied first-series transition metal complexes. The well resolved NMR spectra arise from the very rapid electron-spin relaxation which occurs as a result of modulation of the zero-field splitting of these ions. In the case of 4-coordinate nickel(ii), only tetrahedral complexes (ground state Ti) are of interest since the square-planar complexes are invariably diamagnetic. Many complexes, however, undergo a square-planar-tetrahedral dynamic equilibrium which can be studied by standard band-shape fitting methods (Section B.l). 

The compound Co[(sacac)2cn] is square-planar with a magnetic moment of 2.19 B.M. The red crystals dissolve readily in organic solvents such as dichlo-romethane, dichloroethane, tetrachloroethylene, chloroform, carbon tetrachloride, ether, acetone, and the lower-molecular-weight alcohols, but they are insoluble in water. The visible spectrum in C2H4CI2 exhibits a weak absorption at 980 nm (e = ca. 38) with more intense bandsat529,481,413, 346, 333, 300, and 255 nm (e = ca. lO -lO ). In addition, a lower energy band typical of square-planar cobalt(ll) complexes occurs at 1870 nm in tetrachloroethyl-... 

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