Tt-complexes stability

A quantitative treatment of tt complex formation is, however, more complicated, since it is generally recognized that all three wave functions are necessary for an accurate description of the bond. For instance, it has been pointed out by Orgel (27) that n complex stability cannot solely be the result of n electron donation into empty metal d orbitals, since d and ions (Cu+, Ag+, Ni , Rh+, Pt , Pd++) form some of the strongest complexes with poor bases such as ethylene, tt Complex stability would thus appear to involve the significant back-donation of metal d electrons into vacant antibonding orbitals of the olefin. Because of the additional complication of back-donation plus the uncertainty of metal surface orbitals, it is only possible to give a qualitative treatment of this interaction at the present time. 

Benzene Derivative Relative <7 Complex Stability (ArH + HF—BF3) Relative tt Complex Stability with HC1° Relative Rate of Bromination, Br2 in 85% HOAc Relative Rate of Chlorination, Cl2 in HOAc... 

See Fig. 16. TT-Complex stabilization energy, in kcal/mol. Stabilization energy of the O-complex, in kcal/mol. The difference in the energies of the n-complex and O-complex. ... 

The heats of formation of Tt-complexes are small thus, — A//2soc for complexes of benzene and mesitylene with iodine in carbon tetrachloride are 5-5 and i2-o kj mol , respectively. Although substituent effects which increase the rates of electrophilic substitutions also increase the stabilities of the 7r-complexes, these effects are very much weaker in the latter circumstances than in the former the heats of formation just quoted should be compared with the relative rates of chlorination and bromination of benzene and mesitylene (i 3 o6 x 10 and i a-Sq x 10 , respectively, in acetic acid at 25 °C). 

This LSR-CSA technique (discussed in detail in ref. 76) has also been appUed to a series of sulfoxides. Nitroarylsulfoxides are also capable of a strong three-point interaction with fluoroalcohols 1, an ability that is responsible for a considerable difference in stability between the solvates. Mixtures of Id and 2,4-dinitrophenyl methyl sulfoxide are red, and the intensity of this color is inversely proportional to temperature, consistent with formation of tt-tt complexes. Crystallization of the racemic sulfoxide from carbon tetrachloride solutions of (/ )- d leaves mother liquor enriched in the (i )-sulfoxide enantiomer, that predicted by the usual solvation model (41), to form the more stable solvate. With this compound it is also apparent that the (/ , iS )-solvate may differ considerably from the predicted conformation, by population of 42. This additional interaction. 

Athene complexes of platinnm have been well established since the middle of the nineteenth centnry (see Zeise s Salt), bnt TT-complexes of gold were only discovered by Htittel in 1965, with alkyne complexes following in 1972. All of these complexes have very limited stability, and therefore their chemistry is still poorly developed, even today. 

Wakatsuki and co-workers 287) have prepared vinyl ether complexes of Pd(II) and Pt(II) of composition [CI2ML] (M = Pd or Pt L = vinyl ethers, propenyl ethers, 1,2-dimethoxyethylene) by displacement of PhCN from Pd(PhCN)2Cl2- They are fairly stable at 25°C. Since these ethers would be good a donors but poor tt acceptors, the stability of these complexes suggest that a donation is most important with tt complexes of Pd(II) and Pt(II). This conclusion is in agreement with earlier IR studies, as well as a recent NMR study of Pt olefin and acetylene complexes 35). Furthermore, a study of complex formation... 

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