Reaction with Werner-Type Ligands

When a metal atom donates electron density to a bound ligand, usually by means of Ji-back bonding, electrophilic substitution reactions may be promoted. This is observed then usually with metals in low oxidation states and is therefore prevalent with organometallic complexes - and less with those of the Werner-type, where the metals are usually in higher oxidation states. Nevertheless there have been detailed studies of electrophilic substitution in metal complexes of P-diketones, 8-hydroxyquinolines and porphyrins. Usually the detailed course of the reaction is unaffected. It is often slower in the metal complexes than in the free ligand but more rapid than in the protonated form. 

Photocalorimetry offers a convenient alternative to other methods of AH determination and, in some instances, may be the only practical method. The ligand substitution reactions of robust Werner-type complexes are a case in point. Conventional thermochemical measurements are complicated by the slowness of the substitution process and/or by competing reactions. Some of these same complexes, however, undergo clean photosubstitutions with high quantum yields and thus are excellent candidates for photocalorimetry. Examples include [Cr(NH3)6]3+, [Cr(CN)6]3-and [Co(CN)6]3-.192 Photocalorimetric measurements of AH have also been obtained for isomerization and redox reactions of coordination compounds.193194... 

As noted earlier, there is an experimental distinction between the substitution reactions of labile and inert complexes. The formation of labile complexes is virtually instantaneous upon mixing of the reactants, so that there are few practical difficulties in their preparation, but three points must be remembered. First, for classical, Werner-type, complexes it is found in practice that it is difficult to prepare such complexes with several different non-ionic ligands bonded to the same metal atom, although it is much easier to prepare complexes in which an anionic species is coordinated together with a neutral ligand. Secondly, although it may be possible to isolate and characterize a solid complex, quite a different complex may be the predominant species in solution. So, the blue complex Cs2[CoCl4] crystallizes... 

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