7i-back-donation

As a typical case, olefin-metal complexation is described first. Alkene complexes of d° transition metals or ions have no d-electron available for the 7i-back donation, and thus their metal-alkene bonding is too weak for them to be isolated and characterized. One exception is CpfYCH2CH2C(CH3)2CH=CH2 (1), in which an intramolecular bonding interaction between a terminal olefinic moiety and a metal center is observed. However, this complex is thermally unstable above — 50 °C [11]. The MO calculation proves the presence of the weak metal-alkene bonding during the propagation step of the olefin polymerization [12,13]. 

The M-N-N-M moiety in these complexes could be linear, M( n -N2)M, (for weak back donation interaction) or zigzag, or even rhombic, M(t 2-N2)M, depending on the strength of the 7i-back donation (see Figure 1). 

Given these statements, it is not surprising that NHC complexes of almost all the transition metals have been prepared. In particular, metals incapable of 7i-back-donation such as titanium were only involved in Schrock-carbene complexes until the stable Fischer-type complexes were prepared from TiCU and imidazol-2-ylidenes (IV). The electronic properties of these NHC are also well illustrated in metallocene chemistry (a) 14-electron chromium(II) complexes have been isolated, (b) the displacement of a Cp ligand of chromocene and nickellocene can be achieved by imidazol-2-ylidenes (IV), giving bis(carbene) complexes (Scheme 8.26). 

Sodium methoxide is a catalyst for addition of alcohols across double bonds of fluorinated olefins. The reagent in this reaction is methoxide anion, which adds to the carbon of the double bond that has lower electron density. Because of a strong electron 7i-back-donation by fluorine, the lower electron density is at the carbon linked to two fluorines. 

The activation of C—C multiple bonds essentially involves electron donation via a coordinate bond to the empty orbital on the metal and a 7i-back donation from the metal to empty n orbitals on carbon atoms resulting in the well-known s)mergetic effect (13). As a result of these interactions, C—C multiple bonds are activated. 

Figure 3 depicts this correlation with additional values of Heidemann. The authors had restricted their values to tensors measured from nuclei with a local surrounding of virtual Csv symmetry, and they correlated the values against the lengths of Si-O bonds lying along the parallel tensor direction. What we observe in this case is the change in the parallel CS tensor contribution caused by a redistribution of ti electrons between the parallel bond and the other three Si-0 d-p-7i bonds. Silicon can acquire d electrons from 7i back-donation. 

Co-condensation of arsenic atoms and CO in an argon matrix formed AsCO (vCO 1919.3 cm" ) and AsCO" (1756.4 cm" ). The bonding is believed to involve 7i-back-donation from excited states of As and As". ... 

The researchers noted that the theoretical model of the end-on terminal binding through the N (rather than 0)-atom is more consistent with the experimental spectroscopic data mode. The studies also revealed that Ru"-N20 bond is dominated by 7i back-donation, which, however, is weak compared to that found in the corresponding known ruthenium-nitrosyl complex. See further discussions below for possible bridge binding of N2O in a copper enzyme. 

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