Discrete ligand approach

One example of this approach has been published by Sposito and coworkers (Sposito and Holtzclaw, 1977 Sposito et al., 1977), who have published proton binding data and a discrete ligand model for fulvic acids derived from sewage sludge. The experimental data, which appear to have been carefully obtained, contain some peculiar anomalies that are difficult to explain. For instance, when low fulvic acid concentrations are titrated with strong base, the low pH region of the titration curve indicates that some functional groups are reprotonated as base is added. Sposito and co-workers have attributed this phenomenon to counterion condensation. The same experimental observation was also reported by Perdue et al. (1980). 

The problems associated with the in situ approaches can be avoided by using a discrete catalyst. The presumed structure of the monometallic palladium catalyst contains the sulfonated phosphine ligand chelated to the palladium and a palladium-carbon bond (polymer), most probably in cis geometry with respect to the phosphorous (Fig. 8). 

Most of the mixed-valence systems mentioned by Robin and Day and by Hush were in the solid state. The problem of creating discrete chemical systems for which experiments could be carried out either in solution or in the solid state was first attacked experimentally by Creutz and Taube (6). Their approach was to link together the two metal sites through a ligand bridge, which led to dimers and oligomers. 

In coordinately unsaturated molecular metal complexes, carbon-hydrogen bonds of the peripheral ligands may, if the stereochemistry allows, closely approach a metal center so as to develop a three-center two-electron bond between the carbon, the hydrogen, and the metal atoms, C-H-M. In some instances, the interaction is followed by a scission of the C-H bond whereby the metal is effectively oxidized and discrete M-H and M-C a bonds are formed. This class of metal-liydrogen-carbon interactions and reactions is shown to be a common phenomenon in metal surface chemistry. 

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