Ethene back-bonding

Back-bonding, with formation of a 7r-bond, from a filled metal d orbital to an anti-bonding it -ethene orbital. 

By far the best crystallographic evidence for simultaneous back-bonding is the platinum(II) complex of ethylene [ethene] PtCl3(C2HJ prepared by Zeise. The C—C... 

No new pnnciples ate involved in describing the bonding in these complexes and appropriate combinations of ihe 4p ottoilals on the diene system can be used lo construct MOs with the metal-based orbitals for donation and back donation of electron density.As with ethene, two limiting cases can be envisaged wbicb can be represented schematically as in Fig. 19.25. Consistent with... 

The energy of the SOMO of an ethene derivative which is substituted with conjugating, electron withdrawing substituents is of course much lower than that of ethene itself. An extreme case of such an anion radical is that of tetracyanoethylene (TCNE), which has been isolated as the tetrabutylammonium salt. A reasonably direct, experimental examination of the SOMO distribution of this anion radical was possible through polarized single crystal neutron diffraction studies [108]. Interestingly, the pi SOMO was found not to be centered directly around the two alkene carbon nuclei, but rather to be bent back, away from the alkene C-C bond, as is theoretically expected for an MO which is anti-bonding between these two carbons (Scheme 65). 

In the context of alternative substrates for nitrogenases, in particular alkynes, [Equation (4.27)], model compounds are of interest in which the alkyne is coordinated side-on to vanadium. The side-on or rr coordination implies a weakening of the triple bond by TT-back-donation from vanadium-d into the tt orbitals of the ligand, and hence an activation. The activated siloxyacetylene in the complex [ClVdmpe)2 i7 -(Me3SiOC= COSiMe3)] [dmpe = bis(dimethylphosphino)ethane] is reduced by hydrogen to the respective ethene, [Equation (4.39)]. The precursor acetylene complex is formed by... 

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