Octahedral stereochemistry

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). 

Compounds analogous to the cobaltammines may be similarly obtained using chelating amines such as ethythenediamine or bipyridyl, and these too have played an important role in stereochemical studies. Thus ct5-[Co(cn)2(NH3)Cl] was resolved into d(+) and /(—) optical i.so-mers by Werner in 1911 thereby demonstrating. to all but the most determined doubters, its octahedral stereochemistry. More recently, the absolute configuration of one of the optical isomers of [Co(en)3] was determined (.sec Panel on p, 1125),... 

Trigonal prismatic versus octahedral stereochemistry in complexes derived from innocent ligands. R. D. Wentworth, Coord. Chem. Rev., 1972, 9,171-187 (34). 

The foregoing discussion indicates that while there are difficulties in the way of a bonding role for 3d orbitals, for certain situations at least it is possible to conceive of ways in which these difficulties may be overcome. However, it is necessary to say that even for hypervalent molecules such as SF6 which seem to require the use of d orbitals, there are molecular orbital treatments not involving the use of d orbitals. In fact, as shown by Bent in an elegant exposition12, the MO model of SF6 involving the use of d orbitals is only one of several possibilities. The octahedral stereochemistry of SF6, traditionally explained in... 

Acetylpyridine thiosemicarbazone forms [Co(8)2Cl2], which is isolated from hot ethanol [178], Based on infrared spectra the pyridyl nitrogen is coordinated and bonding is NS with two chlorines bringing the coordination number to six. The complex is a non-electrolyte in DMF, has a magnetic moment of 4.13 B.M., and the electronic spectrum has bands at about 8160 and 17 860 cm consistent with octahedral stereochemistry. 

Methyl-3-formylpyrazole thiosemicarbazone, 40, yielded [Ni(40)2]A2 (A = Cl, Br, NO3, BF4, CIO4 and 1/2 SO4) from weakly acidic solution [190]. The molecule 40 bonds as a NNS tridentate ligand and all complexes behave as 1 2 electrolytes. Their electronic spectra are consistent with an octahedral stereochemistry and Dq values range from 1111-1190 cm and B values from 629 to 741 cm" ... 

A coordination number of 6 is that most frequently found for both Fe(II) and Fe(III), giving octahedral stereochemistry although four- (tetrahedral) and,... 

The copper atom and ligand atom covalent radii in octahedral and tetragonal octahedral stereochemistries... 

Five-coordination is as abundant in copper(II) complexes as the six-coordinate elongated rhombic octahedral stereochemistry (Figure 19.1). The regular square-based pyramidal geometry with five equivalent ligands is only of limited occurrence, but does arise in... 

In general, it is not possible to predict the stereochemistry about the separate copper(II) ions in most cases they are the same and may or may not be related by a centre of symmetry. The actual stereochemistries produced are recognizably the same as those occurring in mononuclear copper(II) complexes (Figure 19.1). The most common is that of square-based pyramidal with rhombic coplanar, compressed tetrahedral, trigonal bipyramidal and elongated rhombic octahedral stereochemistries all occurring. 

Anhydrous CuCl2 has a distorted Cdl2 structure with the CuCU (366)284 environment distorted to give an elongated tetragonal octahedral stereochemistry, with the elongation axes... 

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