Cobalt complexes geometric isomerism

Cobaltate, tris(l, 2-ethanediamine)-racemization solid state, 466 Cobaltate, tris(oxalato)-racemization solid state, 467 Cobaltates sepulchrates, 22 Cobalt complexes geometric isomerism, 11 hexaammine... 

Azopyrazolone, o-hydroxy aryl-chromium complex geometrical isomerism, 70 cobalt complexes stability, 52... 

In contrast, chromium (ITT) and cobalt(III) form 2 1 dye metal complexes that have nonplanar stmctures. Geometrical isomerism exists. The (9,(9 -dihydroxyazo dyes (22) form the Drew-Pfitzner or y rtype (23) (A = C = O) whereas o-hydroxy—o -carboxyazo dyes (24) form the Pfeiffer-Schetty or fac type (25), where A = CO 2 and C = O. 

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)-... 

In contrast, chromium)III) and cobalt(IH) form 2 1 dye metal complexes that have nonplanar structures. Geometrical isomerism exists. 

Geometrical isomerism Geometrical isomerism is possible only in hexacoordinate complexes and in the case of 2 1 metal, e.g. chromium and cobalt, complexes arises from coordination of the ligand in a meridional (81) or a facial (82) mode in an octahedral complex. In the former case only an enantiomorphic pair of isomers is possible, but in the latter the possibility exists of four enantiomorphic pairs and a centrosymmetric isomer (Figure 1). 

From the viewpoint of stereochemistry the most interesting metal complexes are the octahedrally coordinated 1 2 chromium and cobalt complex dyes, which are medially metallized azo and azomethine compounds with functional groups in the o- and o -positions. Three types of isomerism can be discriminated geometrical, N-a, 3, and that arising from azo-hydrazone tautomerism. 

Cyclopropanation of C=C bonds by carbenoids derived from diazoesters usually occurs stereospecifically with respect to the configuration of the olefin. This has been confirmed for cyclopropanation with copper s.si.eo.ss) palladium and rhodium catalysts However, cyclopropanation of cw-Dj-styrene with ethyl diazoacetate in the presence of a (l,2-dioximato)cobalt(II) complex occurs with considerable geometrical isomerization Furthermore, CuCl-catalyzed cyclopropanation of cis-2-butene with co-diazoacetophenone gives a mixture of the cis- and rrans-1,2-dimethylcyclopropanes... 

Pt(NH3)2Cl2], for example, the chloro ligands can be either adjacent to or opposite each other ( Figure 23.21). (We saw an earlier example of this type of isomerism in the cobalt complex of Figure 23.8, and we will return to that complex in a moment.) This form of stereoisomerism, in which the arrangement of the atoms is different but the same bonds are present, is called geometric isomerism. The isomer on the left in... 

Although all of these examples involve cobalt(III) complexes, reactions involving other metal ions could be cited as examples. This paper will deal mainly with the complexes of cobalt(III) and chromium(III) and will be limited to the anation, racemization, and geometric isomerization processes. Linkage isomerization will not be discussed since such reactions are fairly familiar and have been recently reviewed elsewhere. " Ligand exchange will be discussed only as it is encountered in racemization and geometric isomerization. 

William E. Colley, Chui Fan Liu and J. C. Bailar, Jr., The Stereochemistry of Complex Inorganic Compounds. XXIII. Double Optical Isomerism and Optical-Geometric Isomerism in Cobalt(III) Complexes, J. Am. Chem. Soc. 81 4189 (1959). 

Linkage isomerization to the N-bonded isothiocyanato-complex /w j-[Co(acac)2-(NCS)py] was observed to have a half-life of ca. 4 h in chloroform solution. In the solid state linkage isomerization at 341 K has a half-life of ca. 12 h, and this is followed by geometric isomerization with a half-life of ca. 2 days. The latter process is suspected to be catalysed by cobalt(ii). 

Cobalt(II) forms more tetrahedral complexes than any other ion except zinc(II). Draw the structure(s) of the tetrahedral complex [CoCl2(en)]. Could this complex exhibit geometric or optical isomerism If one of the CD ligands is replaced by Br, what kinds of isomerism, if any, are possible in the resulting compound ... 

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