Carbon monoxide backbonding

Of course, this simple bonding model, which implies a certain similarity of silenes tvith compounds of aluminium, explains inadequately the intermediate formation of a complex with carbon monoxide. As is indeed known, n backbonding is essential for the stability of CO complexes. In contrast to aluminium compotmds, silenes possess an occupied n molecular orbital which may interfere with a n state of CO (Scheme 8). Should then silenes also be qualified for intermediate formation of a- and n-complexes, respectively I will now present two reaction examples on this theme ... 

The complexes can be both oxidized and reduced reduction potentials for many of the complexes are shown in Table 6. Cyclic voltammograms of Re(a-diimine)(CO)3X show that in most cases the first oxidation is chemically irreversible at scan rates of 0.1-0.2 V s however, at much faster sweep rates (>100 V s ) a reversible wave is observed at 1.32 V (vs. SCE) in MeCN for Re(bpy)(CO)3Cl [60]. The first oxidation is metal based and is followed by the rapid loss of carbon monoxide due to the weakening of the Re 7r-backbonding... 

The second protonation step is accompanied by loss of the carbonyl ligand, since the alkylidyne complex 22 is a system (considering the alkylidyne ligand as a trianion) and there are no available electrons for n backbonding to carbon monoxide. 

From these properties it is concluded that low-valent metals do not favor water in the ligand sphere. Not that Cr(CO)6 is a stable compound because of the outstanding jr-backbonding of carbon monoxide, while Cr(FI20)6 does not exist -quite contrary to the common [Cr(H20)6]3+. On the other hand, trivalent chromium does not form the (hypothetical) cationic carbonylchromium complex Cr(CO)6]3+. This is, in short, one major reason why so little is known about typical organometallic water complexes. 

Infrared spectroscopy of olefin complexes is a less useful probe of n-bonding than infrared spectroscopy of CO complexes. Binding of an olefin to an electron-rich metal center does reduce the C-C stretching frequency, as one would expect from the reduction of the C-C bond order due to Ti-backbonding. However, the C-C stretch of a coordinated olefin is weaker than that of coordinated carbon monoxide because the vibration of the olefin creates a smaller change in the dipole moment. (Recall that symmetric vibrations are not observed in the infrared spectrum because of a lack of change in the dipole moment.) Thus, the olefin stretch is weak and lies at a frequency that overlaps with other bands. 

Keywords Asymmetric Backbonding Bent Bond order Bridging carbonyl Carbon monoxide Computation Isocarbonyl Linear Metal-metal bond Metal-metal antibond Molecular orbital diagram Symmetric Theoretical... 

A ligand of special importance is carbon monoxide. The reactivity of CO is a key difference between transition-metal chemistry and classical organic chemistry. Several of the ttansition metals, such as Mond s nickel, can even form complexes with only CO. The HOMO of CO is its a -orbital, concentrated on the carbon atom, hence CO is most commonly bonded to the metal via its carbon atom. Backbonding then occurs with electron donation from metal d-orbitals into the LUMO of carbon monoxide which is the ir -orbital (Figure 1.7). This is the case for the simple metal carbonyls including Ni(CO)4, Fe(CO)5 and Cr(CO)6. 

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