Rr back-donation

Trialkyl phosphates form volatile 1 1 adducts with acids such as nitric and chloroacetic, from which the esters are recovered by base treatment. I.r. and n.m.r. spectral data suggest that these are hydrogen-bonded complexes. At low temperatures, in FSOaH-SbFj, trialkyl phosphates were shown (by n.m.r.) to give protonated species in which there appears to be considerable pir-d-rr back-donation from oxygen to phosphorus. These species are not stable the tri-n-butyl ester decomposing over the course of two days to MeaC+ and (HOiP. ... 

Experimental data on the ir-accepting ability of alkylphosphines are ambiguous. Photoelectron spectroscopy indicates that aliphatic phosphines are poor Tr-acceptors, ° but one study on a complex with an electron-rich (P metal center suggested that the relative energies of electronic states could be influenced by rr-back-donation into the cr-bonding orbitals of an aliphatic phosphine. The NBO analysis concluded that alkylphosphines are effective ir-acids. Tlius, further debate is likely to ensue about the ir-accepting ability of alkylphosphines, but other phosphorus ligands are clearly ir-acceptors. 

Arenes are usefril 7c-ligands of transition metals. The metal-arene bonding comprises both rr-donation from ligand to empty d-orbital of metal and rr-back-donation from filled metal d-orbital to 7i -orbital of ligand, and... 

Carbene is a liable intermediate in organic reactions however, if it coordinates to a transition element, the coordination bond is stable and it is able to be isolated as a complex. Singlet carbene ( CX2) forms an sp hybrid orbital, has six electrons with two p-orbitals, and a third orbital remains as vacant (p orbital). When the carbene forms a complex, lone pair electrons of one of the sp hybrid orbitals a-donates to the metal atom and 7T-electrons of the metal back-donate to the vacant orbital p, of the carbon atom (a-donation rr-back donation). Therefore, this MC bond should thus exhibit partial double bond character. However, as the p, vacant orbital which is used for back donation didn t have enough acceptor orbitals available for the back-donation process, the double bond character is thought to be small [53]. 

In extreme cases, such as Pl(Ph3P)2(CH2 CH2) in which the metal is very electron rich, rr back donation is significant and as a result, the carbon-carbon bond lengthens considerably (to 143 pm) and the hydrogen atoms are hent considerably uul of the plane ... 

Metal complexation causes a slight weakening of the bonds, as a result of it back-donation into the fullerene rr MOs, and a concomitant shift to lower frequencies is observed. 

The electronic structures of the metal carbonyls have been investigated vigorously over the past few years because the interaction between CO and metals (76, 137) is essential for an understanding of both organometallic chemistry and CO chemisorption phenomena. There is continuing debate concerning the relative importance of o-donation from the carbon lone pair of carbon monoxide and back donation from metal d electrons into vacant rr ligand orbitals. The most frequently cited data in favor of n back donation are... 

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... 

Table 26.4 illustrates the remarkable sensitivity of the reduction potential of the Co"VCo couple to different ligands whose presence renders Co unstable to aerial oxidation. The extreme effect of CN can be thought of as being due, on the one hand, to the ability of its empty rr orbitals to accept back-donated" charge from the metal s filled t2g orbitals and, on the other, to its effectiveness as a cr donor (enhanced partly by its negative charge). The magnitudes of the... 

Voorhoeve et al. (14,30) have also stressed that the catalytic activity of perovskites is influenced by their stoichiometry. A simple way of varying the oxidation state of the ion at the position B is by substitution of the A ion by a different ion with an oxidation state other than 3. This method has been used by several authors (9, 62, 88, 96, 179-181) to understand the role of the 3orbital occupancy in the LaM03 series on the catalytic oxidation of CO. For M = Co the appearance of Co2+ ions by introduction of Ce4+ in position A enhances the rate of oxidation of CO, whereas the presence of Co4+ ions by substitution with Sr2+ reduces the rate. The explanation for this behavior has been given by assuming that CO is bonded to the transition-metal ion as a carbonyl, as occurs on metals (182), with donation of the carbon lone pair into the empty 3dzi orbital of M to form a cr-bond accompanied by back-donation of the f2g electrons of the metal to the antibonding rr-orbital of CO. It should be noted that the dz2 orbital is the lowest et level for the M3+ ions at the surface, and in order to have a partially empty dzi level, the occupation of all the et levels must be below unity. 

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