Nonclassical coordination compounds

As mentioned above, [M(OH2)3(CO)3] can be considered as a normal aqua ion with only three available coordination sites. Substitution of the water molecules with almost any type of chelator (classic/nonclassic) forms kinetically stable coordination compounds. This holds true for mono-, bi-, and tridentate ligands, regardless of their hardness or softness. This behavior also represents the distinct feature of [M(OH2)3(CO)3] as compared with other technetium and rhenium metal centers, and is one of its major advantages for the labeling of molecules for imaging and therapeutic purposes. 

The chemistry of tetravalent lanthanide compounds is mainly restricted to cerium. Perhaps the most important material in this field is cerium dioxide (ceria), especially in the form of nanoparticles. Coordination compounds of cerium in the oxidation state -1-4 include halogeno complexes and complexes of 0X0 acids, /3-diketonates and related Schitf-base complexes as well as porphyrinates and related complexes. An extension of this chemistry to nonclassical tetravalent lanthanide ions such as Pr +, Nd +, Tb" +, or Dy + certainly presents one of the major challenges for future work in this area. 

The Une that divides classical and nonclassical metal complexes, organometallic complexes, and coordination compoimds is imperfect. For instance, what constitutes a coordination reaction and a condensation reaction Piperidine is an organic compound, that can react with metal-containing sites in two different ways. If the N-proton is retained, then the reaction is a coordination reaction and the resulting product a coordination compound. In this case, the nitrogen donates its lone pair of electrons to the metal site. However, if the proton is lost, then the reaction is described as a condensation reaction with the product called a condensation product. Polymers formed from such condensation reactions are covered in this volume. Schemes 1 and 2 depict reactions that illustrate the difference between coordination and condensation reactions using 1,6-hexanediamine. ... 

In the past several years, it has become apparent that with proper tuning of ligation, it is possible to prepare organome-tallic compounds of actinide elements with very high coordinative unsaturation and very high chemical reactivity (JJ./Jj2, J 3) In regard to exploring "nonclassical" modes of carbon monoxide acti-... 

Hydrogen complexes form by reaction of transition metal compounds with molecular hydrogen or by protonation. The hydrogen in a transition metal complex may be bonded in the classical or nonclassical way. The complexes may interconvert, may be deprotonated, or may lose molecular hydrogen, generating vacant coordination sites. Thus, the picture of transition metal hydrogen complexes to-day is one of considerable complexity [1, 20, 24, 32, 33]. 

Evidence has now been presented that indicates that the above compound behaves as a carbocationic polymerization initiator for styrene, W-vinylcarbazole, vinyl ethers, and isobutylene. The mechanism of initiation and polymerization of these monomers by such metallocene complexes is still being investigated. It was suggested by Wang et al. [53], that the mechanism of carbocationic polymerization of such olefins by the above complex would involve coordination of the olefins, as shown below, in a nonclassical p -fashion, with the metal-olefin. This interaction is stabilized by a complementary borate-olefin interaction. The next step in the polymerization process by this mechanism, then involves attack on the carbocationic centers of the metal ions-activated olefin molecules by secondary olefin monomers, followed by chain growth [53] ... 

Carbonium ions Compounds containing high coordinate carbon bearing a positive charge with multicenter bonding. Also called nonclassical cations. 

The isotopic perturbation method is very useful for studying fractional bonding and hypercoordination in coordinatively unsaturated and electron deficient compounds such as transition metal complexes or carbocations. The 2-norbornyl cation and the bicyclobutonium cation are the most prominent examples of carbocations whose structures have led to much controversial discussion (the so-called classical-nonclassical ion controversy). The application of the isotopic perturbation methed is likely to be the most decisive piece of nmr evidence for the hypercoordinated structure of these two cations in solution. 

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