Hard ligands

Coordination Complexes. The abiUty of the various oxidation states of Pu to form complex ions with simple hard ligands, such as oxygen, is, in order of decreasing stabiUty, Pu + > PuO " > Pu + > PuO Thus, Pu(Ill) forms relatively weak complexes with fluoride, chloride, nitrate, and sulfate (105), and stronger complexes with oxygen ligands (Lewis-base donors) such as carbonate, oxalate, and polycarboxylates, eg, citrate, and ethylenediaminetetraacetic acid (106). The complexation behavior of Pu(Ill) is quite similar to that of the light lanthanide(Ill) ions, particularly to Nd(Ill)... 

The i5p-titanium(IV) atom is hard, ie, not very polarizable, and can be expected to form its most stable complexes with hard ligands, eg, fluoride, chloride, oxygen, and nitrogen. Soft or relatively polarizable ligands containing second- and third-row elements or multiple bonds should give less stable complexes. The stabihty depends on the coordination number of titanium, on whether the ligand is mono- or polydentate, and on the mechanism of the reaction used to measure stabihty. 

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

Ligands which form stronger complexes with Class (a) metals are described as hard and those which form stronger complexes with Class (b) metals are called soft. Hard metals form more stable complexes with hard ligands and soft metals form more stable complexes with soft ligands. A listing of hard and soft metals and ligands is presented in Table 9-4. 

Thus it appears that the presence of two soft carbons on the palladium stabilizes the trans coordination of hard ligands and drives the selective coordination of ambidente ligands through their hardest atom. These results, as those described in the previous scheme, constitute other examples of the antisymbiotic effect which can be observed in soft palladium(II) complexes. 

Sugihara et al. in 1997.106 They utilized aqueous ammonium hydroxide as a reaction medium, which provided ammonia as a hard ligand to labilize the CO ligands and therefore enhance the rate of the PKR. The reaction of dicobalthexacarbonyl complexes of enynes and alkynes provided expected cyclopentenones via intramolecular and intermolecular modes respectively (Scheme 4.10). 

Cummins, Christopher C., Three-Coordinate Complexes of Hard Ligands ... 

Consecutive Steps of Interaction with Hard Ligands. 171... 

The models proposed lead to a number of predictions about the variations of the enthalpy and entropy changes as ligands of different character are consecutively coordinated to a metal ion. To see whether these changes really occur as expected, experimental data have been collected for hard ligand atoms (F, O Table 1) as well as for more or less soft ones (Cl, Br, I S, Se P C Table 2). 

Table 1. Thermodynamics of the stepwise coordination of hard ligands at 25 °C to... 

In their review of the classification of donors and acceptors in inorganic reactions, Williams and Hale (7) pointed out that for reactions in water, class (a) character was exhibited most strongly by lithium and least by caesium, which was indeterminate between classes (a) and (b). Here class (a) character means that the fluoride is more stable in water than the iodide. In general Group IA metals prefer hard ligands, F, O, N their interaction with sulphur and carbon is considered in para. IV. 

Higher oxidation states are stabilized by hard ligands, which often have N-or O-atoms as donor atoms. Examples... 

Fig. 2. Histogram of the formation constants of selected I 1 complexes. Formation constants generally increase with increasing metal hardness (increasing charge, decreasing ionic radius) and with increasing ligand hardness. Metal hardness increases from K+ to Zr4+ and ligand hardness from Cl " to OH". The most stable complexes are formed between hard metals and hard ligands, the weakest complexes are formed betweeen soft metals and soft ligands.

A final reason for the need for hard ligands is that it is these ligands that can form bridges to give dimeric complexes, and dimeric complexes may contribute to the catalytic activity in these reactions. The participation of dimeric species, formed for example, by... 

R = R = Cy R = 1-adamantyl, R = CfiH 5. Te2-3,5 5 ). The vanadium analogue of the latter [V N(l-Ad)(C6H3Me2-3,5) 3] " 4 has also been characterized. Furthermore, a more efficient route to [Cr N(SiMe3)2 3] and a new crystal structure determination has been described. Three-coordinate metal amides have been treated in a general review that covers three-coordinate transition metal species with hard ligands. The electronic structure and bonding in tricoordinate amido complexes of transition metals have also been detailed... 

Given the structure of InF3-3H20 (see above) and the tendency of indium(III) to be six-coordinate with hard ligands, it is reasonable that the mixed fluoro/aqua anionic complexes should also be [InF4(H20)2] and [InF5(H20)]2. This is compatible with the evidence from aqueous solution chemistry, where stability constants and thermochemical results indicate a series of [In(H20)6-nFn]l3 n)+ species.9 These complexes still await detailed systematic structural investigation. 

Hard ligands such as F" are most effectively removed by hard metal ions such as Be2 and Al3. but soft l ands such as Cl-. Br. am) are better removed by soft mcial ious such as Ag and He2 ... 

Soft buses will form complexes in many cases f hard ligands arc Absent iFryiuk. M. D. Haddad, T. S. Deri . D J. Coord. C/i.-h,. Rei 1990, 99. 137-212). 

Most of the coordination compounds discussed in this chapter involve soft ligands such as CO, PF3, arenes, and olefins. Part of the motivation for examining the photoelectron spectra of chelated compounds is that they provide an opportunity for studying the coordination behavior of hard ligands such as ketones. An additional advantage of chelated compounds is that they permit the study of a wide variety of d electron configurations within an isostructural framework. 

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