Soft versus Hard Ligands

Ligands may be hard or soft depending on their propensity for ionic (hard) or covalent (soft) bonding. Likewise, metals can also be hard or soft. The favored, well-matched combinations are a hard ligand with a hard metal and a soft ligand with a soft metal hard-soft combinations are disfavored because of the mismatch of bonding preferences.  

Iodide is the softest halide because it is large, easy to polarize, and forms predominantly covalent bonds. It binds best to a soft cation, Hg , also large and easy to polarize. In this context, high polarizability means that electrons from each partner readily engage in covalent bonding. 

TABLE 1.1 Hard and Soft Acids and Bases Some Formation Constants  

Soft bases have lone pairs on atoms of the second or later row of the periodic table (e.g., Cl , Br , PPh3 or have double or triple bonds (e.g., CN , C2H4, benzene). Soft acids can also come finom the second or later row of the periodic table (e.g., Hg ) or contain atoms that are relatively electropositive (e.g., BH3) or are metals in a low ( 2) oxidation state [e.g., Ni(0), Re(I), Pt(II), 11(11)]. An important part of organometallic chemistry is dominated by soft-soft interactions (e.g., metal carbonyl, alkene, and arene chemistry). 

High-trans-effect ligands labilize the ligand located opposite to themselves. Hard ligands have first-row donors and no multiple bonds (e.g., NH3). Soft ligands have second- or later-row donors and/or multiple bonds (e.g., PH3 or CO). 

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Fig. 5. Plot of enthalpies of complex formation in the gas phase for formation of Li+ complexes versus those for the corresponding Mn+ complexes. Ligands are segregated into correlations for hard ( ) and soft (O) donors. Energies in kcal mol 1. Redrawn after Ref. (14). 

J0rgensen proposed the principle of symbiosis with respect to hard and soft acid-ba.se behavior. This rule of thumb states that hard species will tend to increase the hardness of the atom to which they are bound and thus increase its tendency to attract more hard species. Conversely, the presence of some soft ligands enhances the ability of the central atom to accept other soft ligands. In terms of the electrostatic versus covalent picture of Pearson s hard and soft or Drago s and... 

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