PART 17: TRANSITION METAL ALKENE COMPLEXES FOR CSIR NET/GATE

Описание: In this lecture no. 17 I have explained the detailed concepts of transition metal alkene complexes. Synthesis, structural and bonding properties have been discussed in great details along with previous year csir net questions. Though the first metal olefin complex dates back a long time to the beginning of 19th century, its formulation was established only a century later in the 1950s. While reacting K2PtCl4 with EtOH in 1827, the Danish chemist Zeise synthesized the famous Zeise’s salt K[PtCl3(C2H4)]∙H2O containing a Pt bound ethylene moiety and which incidentally represented the first metal−olefin complex. The metal−olefin bonding interaction is best explained by the Dewar−Chatt model, that takes into account two mutually opposing electron donation involving σ−donation of the olefinic C=C π−electrons to an empty dπ metal orbital followed by π−back donation from a filled metal dπ orbital into the unoccupied C=C π* orbital. Quite understandably so, for the d0 systems, the formations of metal−olefin complexes are not observed. The extent of the C=C forward π-donation to the metal and the subsequent π−back donation from the filled dπ orbital to the olefinic C=C π* orbital have a direct bearing on the C=C bond of the metal bound olefinic moiety in form of bringing about a change in hybridization as well as in the C−C bond distance. If the metal to ligand π−back donation component is smaller than the ligand to metal σ−donation, then the lengthening of the C−C bond in the metal bound olefin moiety is observed. This happens primarily because of the fact that the alkene to metal σ−donation removes the C=C π−electrons away from the C−C bond of the olefin moiety and towards the metal center, thus, decreasing its bond order and increasing the C−C bond length. Additionally, as the metal to ligand π−back donation increases, the electron donation of the filled metal dπ orbital on to the π* orbital of the metal bound olefin moiety is enhanced. This results in an increase in the C−C bond length. The lengthening of the C−C bond in metal bound olefin complex can be correlated to the π−basicity of the metal. For example, for a weak π−basic metal, the C−C bond lengthening is anticipated to be small while for a strong π−basic metal, the C−C lengthening would be significant. Another implication of ligand−metal π−back donation is in the observed change of hybridization at the olefinic C atoms from pure sp2, in complexes with no metal to ligand π−back donation, to sp3, in complexes with significant metal to ligand π−back donation, is observed. The change in hybridization from sp2 to sp3 centers of the olefinic carbon is accompanied by the substituents being slightly bent away from the metal center in the final metalacyclopropane form. This change in hybridization can be conveniently detected by 1H and 13C NMR spectroscopy. For example, in case of the metalacyclopropane systems, which have strong metal to ligand π−back donation, the vinyl protons appear 5 ppm (in the 1H NMR) and 100 ppm (in the 13C NMR) high field with respect to the respective position of the free ligands. An interesting fallout of the metal to ligand π−back bonding is the tighter binding of the strained olefins to the metal center as observed in the case of cyclopropene and norbornene. The strong binding of these cyclopropene and norbornene moieties to the metal center arise out of the relief of ring strain upon binding to the metal. Lastly, in the metal−olefin complexes having very little π−back bonding component, the chemical reactivities of the metal bound olefin appear opposite to that of a free olefin. For example, a free olefin is considered electron rich by virtue of the presence of π−electrons in its outermost valence orbital and hence it undergoes an electrophilic attack. However, the metal bound olefin complexes having predominantly σ−donation of the olefinic π−electrons and negligible metal to ligand π−back donation, the olefinic C becomes positively charged and hence undergoes a nucleophilic attack. This nature of reversal of olefin reactivity is called umpolung character.
#ZEISESALT #METALLACYCLOPROPANE #DEWARCHATT #METALALKENECOMPLEX
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