Soft ligands

The redox behaviour of transition metal complexes with soft ligands. E. Uhlig, Comments Inorg. Chem., 1981,1,169-182 (43). 

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. 

Hg " complexes are common, but complexes of Na, Mg", or Al are rare. Chromium complexes are also common, but in such complexes the chromium is in a low or zero oxidation state (which softens it) or attached to other soft ligands. In another application, we may look at this reaction ... 

The thiocyanate ion provides an interesting test of these ideas. In the SCN ion, the sulfur atom is considered to be a soft electron donor, whereas the nitrogen atom is a much harder electron donor. Accordingly, Pt2+ bonds to SCN- at the sulfur atom, whereas Cr3+ bonds to the nitrogen atom. Uncharged metal atoms are considered to be soft electron acceptors, and they form complexes with soft ligands such as CO, 11, and PR3. We will see many examples of such complexes in later chapters. On the other hand, we would not expect complexes between uncharged metal atoms and NH3 to be stable. 

Late transition metals are found on the right of the Periodic Table (e.g. Ni, Pd, Pt, Co, Fe, Ru, Rh, Cu active in low oxidation states) and are more tolerant of oxygen functionalities in the monomer and the polymer. This behavior stems from the preference of late transition metals for soft ligands (polarizable ligands, typically... 

A tremendous amount of work has been reported on the synthesis of arylthiophenes and heteroarylthiophenes utilizing the Stille reaction approach. In one case, 2-tributylstannylthiophene was coupled with p-acetoxyphenyl iodide to give thienylphenol 114 after hydrolysis [93]. In another, the union of 2-tributylstannylbenzo[fc]thiophene and p-acetyliodobenzene provided arylbenzothiophene 115 using inexpensive Pd/C as a heterogeneous catalyst, Cul as a co-catalyst, and AsPh3 as a soft ligand [94], Moreover, Kennedy et al. coupled 2-tributylstannylthiophene and 2-chloro-4-bromobenzylphosphonate (116) to make heterobiaryl... 

The effectiveness of dimethyl sulfide as an additive for the selective formation of anti-product 22 from propargyl epoxide 20 may be due to the formation of stabilized copper species, which are less prone to undergo electron transfer processes. In this respect, other soft ligands which bind strongly to copper, in particular phosphines and phosphites [8h-j, 25, 28], have been used even more frequently. These additives also serve to suppress the formation of a common side product, i.e. an allene containing a hydrogen atom instead of the carbon substituent which should... 

The conductivity of pyridine is considerably increased by addition of iron carbonyl and corresponds to that of potassium picrate in the same solvent 91 Likewise, DMSO, alcohols, or ammonia95,as well as soft ligands such as triphenylphosphine, may serve as donor molecules 10°1 ... 

The lower oxidation states are stabilized by soft ligands e.g. CO (Prob. 3). The aquated vanadium ions represent an interesting series of oxidation states. They are all stable with respect to disproportionation and labile towards substitution. They undergo a number of redox reactions with one another, all of which have been studied kinetically. Many of the reactions are [H ]-dependent. There has been recent interest in the biological aspects of vanadium since the discovery that vanadate can mimic phosphate and act as a potent inhibitor (Prob. 4). 

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