Crystal structure showing cation

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

The next landmark was the synthesis of the germylium, 22, and the stannylium ion, 23, by one-electron oxidations from the corresponding stable radicals with trityl TPFPB by Sekiguchi and co-workers. As in the case of the allyl cleavage to generate the mesityl-substituted cations, the reaction, in this case the oxidation, occurs at the periphery of the molecule and gives the possibility for efficient steric protection of the incipient cation. Both trivalent cations were obtained as their TPFBP salts and the crystal structure show well separated anions and cations. 

An analysis of metal binding to peptide carbonyl groups (Chakrabarti, 1990), mainly calcium ions in protein crystal structures, shows that the cations tend to lie in the peptide plane near the C=0 bond direction. Generally, this binding occurs in turns in proteins or in regions with no regular secondary structures. Ca---0 distances range from 2.2 to 2.5 A, and metal ions do not deviate by more than 35° from the peptide plane. Thus, metal ions in proteins do not, Chakrabarti observed, bind in lone-pair directions. 

Octamethylpyrophosphoramide (ompa) has been shown to loosen ion pairs in solution178 and a 1 3 complex has been isolated with Mg(C104)2. The crystal structure shows that the ligand is bidentate and chelates to the cation through the phosphine oxide oxygens.179... 

Figure 3.40 X-ray crystal structure showing the wrapping of the podand Kryptofix-5 (3.60) around an Rb+ cation.

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