Hard, donor

The chemistry of Th(IV) has expanded greatly since the mid-1980s (14,28,29). Being a hard metal ion, Th(IV) has the greatest affinity for hard donors such as N, O, and light haUdes such as F and CF. Coordination complexes that are common for the t7-block elements have been studied for thorium. These complexes exhibit coordination numbers ranging from 4 to 11. 

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... 

The identity of the hard donor group and how it is incorporated in a molecular structure has a bearing on the affinity of a siderophore for iron(III). An analysis of siderophore structure and its relationship to iron(III) binding affinity as expressed by the thermodynamic stability constant is useful in understanding structure/function relationships and in the design of siderophore mimics for specific applications. 

Solvent molecules with hard donor atoms, like H20, MeCN, DMF, and DMA, have exchange rates on Pd2+ which differ less than a factor of 15. 

It seems worth pointing out, that 137 and human semm albumin contain no pendant phosphines and the donor atoms in the complexes formed with rhodium can be only O (137) or O, N and perhaps S (HSA), which are not the most suitable for stabilizing low oxidation state metal ions. Still these macroligands gave active and stable catalysts with rhodium, which shows that perhaps in the high local concentration provided by the polymer even these hard donor atoms are able to save the metal ion against hydrolysis or other deterioration. 

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. 

On the whole, the thermod3mamic functions found for the stepwise formation of complexes in aqueous solution agree very well with the models proposed for complexes of different character. Thus for interactions between hard acceptors and hard donors, postulated to be mainly electrovalent, the expected stepwise decrease of ASn generally occnrs, often accompanied by a similarly expected decrease of AHn- With interactions between soft acceptors and soft donors, postulated to be mainly covalent, virtually constant values of AHn from step to step are often found, while in other cases values of AHn becomes step by step less exothermic. Both modes of behaviour are compatible with the current model. 

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... 

As hard metal centers, lanthanide(III) ions have a general preference for hard donor atoms (33,34). Much of their early coordination chemistry involved anionic oxygen donors and it is well established that carboxylates and (3-diketonates are very good at coordinating lanthanide ions. 

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

Rais, J.,Tachimori, S. 1994. Extraction separation of tervalent americium and lanthanides in the presence of some soft and hard donors and dicarbollide. Sep. Sci. Technol. 29 (10) 1347-1365. 

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