Iron carbonyl anions properties

The main absorption band of benzoquinones appears around 260 nm in nonpolar solvents and at 280 nm iu water. Extinction coefficients are 1.3-1.5 x 10 M Upon reduction to hydroquinones, a four times smaller band at 290 nm is found. The most important property of quinones and related molecules is the relative stability of their one-electron reduction products, the semiquinone radicals. The parent compound 1,4-benzoquinone is reduced by FeCl, ascorbic acid, and many other reductants to the semiquinone anion radical which becomes protonated in aqueous media (pk = 5.1). Comparisons of the benzaldehyde reduction potential with some of the model quinones given below show that carbonyl anion radicals are much stronger reductants than semiquinone radicals and that ortho- and para-benzoquinones themselves are even relatively strong oxidants comparable to iron(III) ions in water (Table 7.2.1). This is presumably caused by the repulsive interactions between two electropositive keto oxygen atms, which are separated only by a carbon-carbon double bond. When this positive charge can be distributed into neighboring n systems, the oxidation potential drops significantly (Lenaz, 1985). 

For a metal with eight valence electrons, electron precise carbonyl structures see Electron Precise Compound) are M(C0)5 and [M(CO)4]3, which are known for all three metals of the iron group, albeit with quite different properties for the different metals. The next member, [M(CO)3]6, is not formed since a four-connected vertex requires more metal-metal bonding orbitals than can be arranged on the surface of a sphere. Bonding orbitals directed towards the inside of the cluster, however, form MOs that will accommodate more electrons than would be predicted by an 18 valence electron count for each metal see Eighteen Electron Compounds), that is, 86 instead of 84 for a six-vertex closo cluster see Closo Cluster). This situation is realized in the iron group by the anions [M6(CO)i8], M = Fe, Ru, Os (isolectronic to M6(CO)i6, M = Co, Rh, Ir). 

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