Hard donor ligands

N-heterocyclic compounds containing six-membered rings (pyridine and analogues) behave as excellent -acceptors and in turn they provide a rather soft site for metal ion coordination. The 7r-excessive five-membered pyrazole is a poorer 7r-acceptor and a better 7r-donor and it acts as relatively hard donor site. Inclusion of six- and five-membered N-heterocycles like pyridine and pyrazole in one ligand system leads to very attractive coordination chemistry with variations of the electronic properties.555 The insertion of a spacer (e.g., methylene groups) between two heterocycles, which breaks any electronic communication, makes the coordination properties even more manifold. 

A combination of P- and N- donors is another useful approach to potentially reactive (and cataly-tically active) Ni species. Similar to O- donors, N is a hard donor capable of stabilizing metal ions in higher oxidation states, whereas the soft donor P is best suited to stabilize medium or low oxidation states. A neutral bidentate P,N ligand combining a hard dimethylamino and a soft phosphine donor in /V,/V -dimethyl-2-(diphenylphosphino)aniline (241) affords the neutral trigonal bipyramidal and the... 

Formally, in each of these cases the disproportionation produces a positive metal ion and a metal ion in a negative oxidation state. The carbonyl ligands will be bound to the softer metal species, the anion the nitrogen donor ligands (hard Lewis bases) will be bound to the harder metal species, the cation. These disproportionation reactions are quite useful in the preparation of a variety of carbonylate complexes. For example, the [Ni2(CO)6]2 ion can be prepared by the reaction... 

For complexes formed between hard acceptors and donors, the expected decrease of ASn for each consecutive step obviously occurs (Table 1). Even if the acceptor coordinated to the hard donor is a borderline case, like Cu2+, or even mildly soft, like Cd2+, the same rule applies. The only exceptions are the In + fluoride and Zn + acetate systems where mild reversals are observed on the formation of the third and second complexes, respectively. These are possibly connected with a change of the coordination figure which causes an especially large number of water molecules to be set free at these particular steps. More marked reversals are shown by the same acceptors at the same steps in connexion with soft ligands (Table 2). The phenomenon will therefore be further discussed together with the material presented in Table 2. 

These results clearly indicate that the chelate ligation is driven primarily by the enthalpic factor and the entropy plays merely a trivial role in determining the complex stability. This is quite reasonable since the structures of these chelate complexes are strictly defined by the number and direction of the coordination sites of given heavy/transition metal ions, and therefore there is little room for the entropic term to adjust flexibly the complex structure and stability. On the contrary, alkali and alkaline earth metal ions also have the formal coordination numbers, but the actual number and direction of ligand coordination are highly flexible in the weak interaction-driven ligation by hard donors like glyme and crown ether. 

Once parameter values have been determined for particular bond distances, we need to choose a distance dependence. For hard donors like N and O, an inverse sixth power is normally chosen while for softer donors, 1/r5 or 1/r4 is used. However, the particular power dependence is not critical given the subsequent optimization of Morse and ligand-ligand repulsion terms. It also turns out that a distant-dependent AOM term delivers values which work reasonably well for any metal. For example, even... 

Metal complexes that contain a weakly basic hard donor ligand coordinated to a soft transition metal center exhibit high reactivity. Poor ligands such as... 

The alkaline earth metal cations follow similar trends to the alkali metal cations in their complexation reactions with monodentate ligands. Hard , class V donor atoms are preferred, and beryllium and magnesium show a greater tendency to form complexes than do their larger congeners. 

Figure 10-4. The structure of the calciumbinding protein troponin from chicken skeletal muscle. Although this is an exceptionally complicated ligand to a coordination chemist, the binding of calcium ions is to the hard donor sites that might be predicted. Binding of the calcium triggers a conformational change.

Figure 9.1 Chiral ligands and metal complexes with hard donor atoms. The complexes of these ligands with metal ions in relatively high oxidation states are used in asymmetric epoxidation, cyclopropanation, and nitroaldol condensation reactions.

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