Iodo complexes, mononuclear

Reactions with Iodine and Bromine. All the reactions discussed above proceed at limiting rates that are independent of the concentration and nature of the reactant. However, reactions of iodine with Mn2(CO)j0 (6) and Re2(C0)1g (30) proceed by paths that are first order in [I2] as well as by the [I2]-independent homolytic fission paths. P-donor substituents increase the rates of reaction with I2 by several orders of magnitude (31) so that they proceed rapidly even at room temperature. Thus reaction of Mn2(C0)g P(C6Hu53)2 is estimated to occur over 108 times faster than Mn2(C0)jQ by a path first order in [I2] at 25°C in cyclohexane. In all cases the reactions proceed with fission of the metal-metal bonds to form the mononuclear iodo complexes. 

Substitution at polynuclear aqua-ions is often much slower than at the respective parent mononuclear aqua-ions, as has been demonstrated for, e.g., aluminum(III), iron(III), zirconium(IV), and hafnium(IV). A further example has been provided by aluminum, where the Al,3 polynuclear oxo-aqua-species takes several months to equilibrate with lactate. Very different rates of complex formation are used to sort out speciation in multicomponent polynuclear/mononuclear aqua-cation systems. The problems involved in this approach are well illustrated by a Al NMR study of the numerous reaction pathways in the Al " /ferron [ferron = 7-iodo-8-hydroxyquinoline-5-sulfonic acid, (18)] reaction. The slowness of reaction of polynuclear species can cause difficulties in the determination of total metal content, e.g., of aluminum by ferron or Alizarin S (19), in natural waters. In certain situations these... 

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