Square-planar cobalt complexes

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. 

In the other two mechanisms (homolytic splitting and insertion) hydride formation is accompanied by formal oxidation of the metal, and reactivity is closely linked to the susceptibility of the latter to oxidation. Thus, the high reactivity of Co(CN).-, toward Ho, compared with that of Co(CNCH i)- reflects the tendency of CN" to stabilize preferentially the higher oxidation state, and CNCH the lower oxidation state, of cobalt. For square planar complexes the expected order of the tendency toward oxidation, and hence of reactivity toward Ho, is (subject to some modification by ligand variation),... 

The phosphine complex Ru(dmpe)2 has been studied in matrices [62], Ru(diphos)2 (diphos = depe, dppe, (QFs P F P Fs ) has similarly been formed by photolysis of Ru(diphos)2H2 in low-temperature matrices. They probably have square planar structures and undergo oxidative addition with cobalt, C2H4 and hydrogen [63]. 

Enikolopyan et al.til found that certain Co11 porphyrin complexes (eg. 87) function as catalytic chain transfer agents. Later work has established that various square planar cobalt complexes (e.g. the cobaloximes 88-92) are effective transfer agents.Ij2 m The scope and utility of the process has been reviewed several times,1 lt>JM ns most recently by Hcuts et al,137 Gridnev,1 3X and Gridnev and Ittel."0 The latter two references1provide a historical perspective of the development of the technique. 

Splitting of d-orbitals in square planar complexes of copper(II), nickel(II) and cobalt(II). Y. Nishida andS. Kida, Coord. Chem. Rev., 1979, 27, 275-298 (94). 

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. 

Tinnemans et al.132 have examined the photo(electro)chemical and electrochemical reduction of C02 using some tetraazamacrocyclic Co(II) and Ni(II) complexes as catalysts. CO and H2 were the products. Pearce and Pletcher133 have investigated the mechanism of the reduction of C02 in acetonitrile-water mixtures by using square planar complexes of nickel and cobalt with macrocyclic ligands in solution as catalysts. CO was the reduction product with no significant amounts of either formic or oxalic acids... 

Pentacoordinated Pc complexes with planar macrocycle are very scarce. Very recently, Galezowski and Kubicki [25] reported a o-bonded ComPc(CH3CH2) (2) with an alkyl group as axial ligand. Investigation of its crystal structure revealed that the solid consists of centrosymmetric face-to-face dimers. The cobalt atom in 2 is pentacoordinated and has a distorted square pyramidal geometry. The cobalt atom... 

Cobalt complexes with square planar tetradentate ligands, including salen, cor-rin, and porphyrin types, all catalyse the reduction of alkyl bromides and iodides. Most preparative and mechanistic work with these reactions has used cobalamines, including vitamin-B,. A generalised catalytic cycle is depicted in Scheme 4.10 [219]. At potentials around -0.9 V vs. see, the parent ligated Co(lll) compound un-... 

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)-... 

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