Chromium complexes octahedral configuration

The neutral complexes of chromium, molybdenum, tungsten, and vanadium are six-coordinate and the CO molecules are arranged about the metal in an octahedral configuration as shown in stmcture (3). Vanadium carbonyl possesses an unpaired electron and would be expected to form a metal—metal bond. Steric hindrance may prevent dimerization. The other hexacarbonyls are diamagnetic. 

It has already been stated that chromium complexes of tridentate metallizable azo compounds occupy their position as the single most important class of metal complex dyestuffs because of their high stability. It should be noted, however, that in this context the term stability is not used in the thermodynamic sense but relates to the kinetic inertness of the complexes.25 Octahedral chromium(III) complexes have a tP electronic configuration and the ligand field stabilization energy associated with this is high.26 Ligand replacement reactions involve either a dissociative... 

In 1939, Drew83 proposed structures for 2 1 chromium complexes of o,octahedrally coordinated to the central chromium ion (81). Pfitzner71 independently arrived at the same conclusion and 2 1 chromium complexes having this equatorial or meridional (mer) configuration have been described84 as Drew—Pfitzner types. This configuration permits only an enantiomeric pair of isomers. 

The bivalent metals, as usual, combine with two molecules of biguanide to form 4-coordinated planar complexes, while the trivalent cobalt and chromium combine with three molecules of the ligand to produce a 6-coordinated octahedral configuration. The only exception is the trivalent silver which yields, however, a 4-coordinated planar complex. The preparation of the free tris(biguanidato) chromium, Cr(C2N5H6)s, in the anhydrous state,6 as well as of the corresponding anhydrous cobalt(III),8 copper(II), cobalt(II), palladium(II), and nickel(II) compounds, provides indisputable evidence for the structure proposed. Similar anhydrous metallic complexes with numerous substituted biguanides also have been included in the above-mentioned studies. 

The -1-3 oxidation state is the most stable for chromium in its compounds and octahedral coordination dominates for Cr(III) centres. Table 20.3 shows the large LFSE associated with the octahedral configuration, and Cr(lll) complexes are generally kineticaUy inert (see Section 25.2). 

We now turn to an octahedral MLe complex with a electronic configuration, which we shall consider as the starting point for inorganic fragments. This complex obeys the 18-electron rule, just as methane obeys the octet rule. To be a Utde more concrete, though this choice is in no way unique, we shall consider a chromium complex [CrL ], with six neutral L-type ligands (PR3, CO, etc.), each of which supplies a pair of electrons to the metal, fn this complex, chromium is in the oxidation state zero, and the electronic configuration is indeed d. ... 

Dibenzenechromium(O) has an octahedral configuration consistent with d sp hybridization of the chromium atomic orbitals, (Fig. 3-2). If we accept this formulation of the electronic configuration of dibenzene chromium(O), this TT-complex can be classified as a coordination compound. The electronic configuration of most metal 7i-complexes thus can be accounted for by VBT. Therefore, metal r-complexes can be regarded as a new class of coordination compounds. 

Term labels appropriate for d3 and d6 configurations in octahedral ligand fields will be used in this review to designate the ligand field or d-d excited states of cobalt(III) and chromium(III) complexes. This... 

After the resolution of 1-2-chloro-ammino-diethylenediamino-cobaltie chloride many analogous resolutions of optically active compounds of octahedral symmetry were carried out, and active isomers of substances containing central cobalt, chromium, platinum, rhodium, iron atoms are known. The asymmetry is not confined to ammines alone, but is found in salts of complex type for example, potassium tri-oxalato-chromium, [Cr(Ca04)3]K3, exists in two optically active forms. These forms were separated by Werner2 by means of the base strychnine. More than forty series of compounds possessing octahedral symmetry have been proved to exist in optically active forms, so that the spatial configuration for co-ordination number six is firmly established. 

---