Polynuclear Chromium III Complexes

B. Chromium(III) Alkyl Compounds Polynuclear Chromium(III) Complexes Polyaminocarboxylic Ligands... 

A review of recent advances in chromium chemistry (82) supplements earlier comprehensive reviews of kinetics and mechanisms of substitution in chromium(III) complexes (83). This recent review tabulates kinetic parameters for base hydrolysis of some Cr(III) complexes, mentions mechanisms of formation of polynuclear Cr(III) species, and discusses current views on the question of the mechanism(s) of such reactions. It seems that both CB (conjugate base) and SVj2 mechanisms operate, depending on the situation. The important role played by ionpairing in base hydrolysis of macrocyclic complexes of chromium(III) has been stressed. This is evidenced by the observed order, greater... 

Polynuclear chromium(III) ethylenediamine complexes have been synthesized by methods similar to those applied for the ammine systems by using the combined catalytic effect of chromium(II) and charcoal on aqueous ethylenediamine buffer solutions (pH -8) of chromium(III) (40, 42, 60). As mentioned above, the use of catalysts is important when equilibration between the mononuclear species is required, but it is unnecessary when the aim is to produce polynuclear species. In fact, identical polynuclear species are formed in approximately the same ratio when buffered chromium(III) solutions ([Cr] =0.1 M, [en = 0., i M) without catalyst are kept for a few days at 40 50 C (40). 

Hydrolysis of polynuclear hydroxo-bridged chromium (III) complexes in concentrated solutions of strong acid yields the corresponding mononuclear species. Such cleavage reactions are fast in comparison with the hydrolysis in dilute acid and proceed with retention of configuration of the mononuclear entities. A few representative examples are shown in Eqs. (46)-(49) (40, 42,161, 252). 

The reactions involved in the chrome-tanning process are those of coordination complexes. They involve the interaction between charged carboxyl groups on the collagen macromolecule and polynuclear chromium(III) coordination compounds. The most widely used chrome-tanning material is 33% basic chromium(III) sulfate produced industrially by reducing sodium dichromate with sulfur dioxide. 

The coordination chemistry of chromium(III) was first seriously investigated by Pfeiffer at the turn of the century in many ways his studies parallel the work of Werner on cobalt(III). The complexes of chromium(III) are almost exclusively six-coordinate with an octahedral disposition of the ligands. Many are monomeric ((Jeff 3.6 BM), although hydroxy-bridged and other polynuclear complexes are known in which spins on neighbouring chromiums are coupled. 

In this section the methods which have been used to gain structural information are briefly summarized. The term structure is used in this context in its broadest sense, including more qualitative observations concerning the skeleton of the bridging atoms. As a general rule, the hydroxo-bridged polynuclear complexes of chromium(III) and cobalt(III) can be isolated as well-defined crystalline salts and it is therefore quite natural that single-crystal X-ray structure analysis has... 

Only a very few polynuclear complexes containing more than two chromium(III) centers have been studied so far. However, magnetochemical and inelastic neutron scattering studies, heat capacity measurements, and emission spectroscopy have been reported for various tetranuclear species (40,142 151). Two review articles dealing with the spectroscopic and magnetic properties of chromium(III) oligomers have recently appeared (127, 128). 

Analysis of the products of these cleavage reactions has often served as proof of the structures of the polynuclear species. Cleavage of hydroxo-bridged complexes of nuclearity higher than two will in most cases yield at least two different mononuclear species. Identification of these species and determination of the relative ratio in which they are formed reduce the number of possible bridged skeletons greatly, and the studies of polynuclear ammine and amine chromium(III) made by Andersen et al. (mentioned in Section IV) provide many examples of this, one of which is shown in Eq. (48) above (see also Section II,A). 

The cleavage of polynuclear hydroxo-bridged rhodium(III) and iridium(III) complexes into the corresponding mononuclear fragments has been reported in only a few instances, but the well-established tendency of mononuclear complexes of these metal ions to undergo substitution reactions with retention of configuration indicates the possibility of analytical and synthetic applications such as described above for chromium (III). 

We have reported the synthesis and the characterisation of a new series of the polynuclear complex compounds of iron(iii) and chromium(in) where the bridging ligand is squarate dianion, coordinated in a /i-l,3-bis(monodentate) manner. 

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