Co-ordination Numbers other than Six

The great majority of the complexes whose reactions have been mentioned in this chapter have been of co-ordination number six and of octahedral stereochemistry. For the benefit of readers who are interested in the role of co-ordination number in determining reactivity or mechanism we have collected together in Table 17 some references to kinetic or mechanistic studies on complexes of co-ordination number five, seven, or above. 

Recent reviews have included a discussion by Wilkins of his extensive work on the mechanisms of ligand replacement in octahedral nickel(n) complexes. Hunt has reviewed the water-exchange studies in labile 

2 Complex Formation Involving Unsubstituted Metal Ions 

In Wat . —There has been renewed interest recently in ultrasonic relaxation in aqueous salt solutions with the appearance of several papers on bivalent and tervalent sulphates. Jackopin and Yeager have presented some new data on manganese(n) sulphate and have critically re-examined the question of the number of relaxation peaks observed with this species. 

Two preliminary accounts have appeared which report new approaches to the problem of the mechanism of complex formation in water. The reaction between Nd and S04 was investigated by ultrasonics in H2O and D2O and it was concluded from the fact that the reaction was 2.3 times slower in D2O than in HgO (whereas the dissociation step was accelerated) that solvent exchange at the metal ion cannot be the controlling step in lanthanoid complexation reactions. A high-pressure cell with spectrophotometric detection was used in conjimction with a laser temperature-jump apparatus to measure the volumes of activation of the reaction of Co + and Ni + with the bidentate ligand pyridine-2-azo-p-dimethylaniline (1), and of Ni + with NH3. In all cases the value of AV was ca. 8 cm mol, a value which corresponds to a considerable fraction of the molar volume of water  

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