Donor-acceptor complexes chelation

They acknowledge that the exact mechanism of the adsorption of heavy metal chelates is quite complex but do not hesitate to propo.se the formation of an electron donor-acceptor complex of the chelate and the active sites (e.g., carbonyl groups) and possible beneficial effect of hydrogen bonding between the... 

Reactivity studies of two-coordinate chelate group 13 species have Just begun and appear particularly interesting because such species may exhibit both a Lewis acid character (due to the sextet configuration at the metal center) and a Lewis base one (due the lone pair of electrons at the metal center) see Lewis Acids Bases). For instance, the two-coordinate Ga chelate HC(CMeNAr)2 Ga reacts with B(C6F5)3 to form the gallium-boron donor-acceptor complex HC(CMeNAr)2 Ga... 

The reader may wonder why the calculated geometries of the chelate complexes are in good agreement with experiment, even though the geometry optimization was carried out only at the HF level. In the beginning, we said that donor-acceptor complexes need to be optimized at a correlated level. Nevertheless, strong donor-acceptor bonds may be calculated with reasonable... 

Silatranes 1 are meanwhile classical cage compounds with donor-acceptor interactions and represent examples of hypercoordinated silicon [2], The donor-acceptor contact in 1 is formed by an interaction of the Lewis-basic amino group with the Lewis-acidic silicon center favored by the chelate effect. Numerous examples show that electron-rich transition metal complexes also possess Lewis-basic properties [3, 4]. Isolobal replacement of the NR3-unit in 1 by a d ML4-unit [5] leads to compounds of type 2 [1,6, 7]. These Si/Ni-cages 2 can be regarded as metallosilatranes. Here we report on the s5mthesis, structure and bonding of 2. 

This is not to say that there are no SHAB problems of donor-acceptor incompatibility. Several groups have adopted the strategy of synthesizing ligands that have hard and soft moieties, such as (8). For example, Fryzuk and coworkers use ligands with a hard center to anchor to the hard metal, in this case Zr or Hf enabling the phosphorus chelating moieties to form a stable complex (9). 

---