Hard reagents

In principle, this constitutes a strong point for the HSAB treatment. In practice, it is its Achille s heel. The reason is that Pearson s rule (a hard reagent attacks preferentially a hard site) should apply only to the reactive species in the transition state. 

FO analysis allows us to go a little deeper into these reactions. According to STO-3G calculations, the largest LUMO coefficient is at C4 in the free (i.e. noncoordinated) conjugated carbonyl and at C2 in the corresponding ate complex. Therefore, Loupy and Seyden predicted that even a hard reagent such as LiAlH4 should be able to give... 

Corriu and Lanneau150 suggested that harder nucleophiles induce greater retention of configuration. This empirical rule can be interpreted in the following way. Hard reagents are normally small. Therefore, they have highly contracted valence orbitals which inter-... 

Metallated azoles frequently show expected properties, especially if not too many heteroatoms are present. Thus, Grignard reagents prepared from halogeno-azoles (see Section 3.4.3.9.3) show normal reactions, as in Scheme 154. 2-Lithioimidazoles react normally, e.g., with benzophenone (Scheme 155). Lithiated imidazoles are not always particularly reactive toward electrophiles and yields may be low. The nature of the quenching electrophile is the critical factor. Hard reagents like benzophenone tend to give better yields than the softer methyl iodide heteroaryl lithiums have hard carbanion centers. 

As a hard reagent, an aqueous hydrochloric acid solution of RUCI2 catalyzes the hydrogenation of ot,/l-unsaturated carboxylic acids and amides, but not that of simple soft olefins. 

Newer investigations have shown that the complex undergoes nucleophilic attack by the hard reagent water [T18], whereas formerly insertion of ethylene into a palladium hydroxo species in an intramolecular step was assmned. The soft palladium(ii) center in the hydroxyaUcyl complex D is coordinated by several hard ligands, which explains the strong tendency towards elimination with release of the final product. 

Methane Methane is one of the longest known and best-studied reagent gases. It is known and used as a hard reagent gas for general Cl applications and has been replaced by softer ones in many areas of analysis. 

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