Iron-carbon bonds

The transfer of an electron to the iron atom is compatible with the electroneutrality principle. The electronegativity of iron is 1.8, leading to 12% ionic character of the iron-carbon bonds and to the satisfactory value +0.04 for the resultant charge on an iron atom that has accepted an electron and is forming nine bonds with carbon atoms. 

As we will discuss, such a symmetric profile is typical of an electron removal which does not lead to important structural changes. In fact, the 17-electron ferrocenium ion, [Fe(C5H5)2] +, generated upon oxidation, is a stable species which substantially maintains the original molecular frame (but for the fact that, because of the electron removal, the iron-carbon bonds are slightly weakened and hence elongated by about 0.04 A with respect to the neutral parent see Chapter 4, Section 1.1). 

Iron, as found in the porphyrin derivative hemoglobin, complexes CO to form a stable metal carbonyl. Iron also forms a variety of metal carbon monoxide derivatives such as the homoleptic Fe(CO)5, Fe2(CO)9 and Fe3(CO)i2, the anionic [Fe(CO)4] and its covalent derivative Fe(CO)4Br2, [CpFe(CO)2] and its alkylated covalent derivatives CpFe(CO)2-R with its readily distinguished n (and and a (and / ) iron carbon bonds. By contrast. Mg in its chlorin derivative chlorophyll, which very much resembles porphyrin, forms no such bonds with CO nor is there a rich magnesium carbonyl chemistry (if indeed, there is any at all). 

If this mechanism is correct, the aconitase reaction is an excellent illustration of the influence of the stereochemistry of the metal, as well as its charge, upon the course of a biochemical reaction. The charge on the iron is, of course, responsible for the formation of the resonating carbonium ions A and B from C, D, or E. In C and D the flow of electrons toward iron severs the bond between carbon and the hydroxyl group, whereas in E the proton is released from coordinated water and attached to one of the two ethylenic carbon atoms. The stereochemistry of the iron atom can be credited with holding the organic molecule and the hydroxide in their proper spatial relationship in A and B. It has been recently demonstrated that the complexes of the aconitase substrates with nickel have the structures postulated by Speyer and Dickman and shown in Figure 3 (19). 

Obviously, one requirement for an adhesive is that it flow easily to cover a surface. This is a more complex business than it first appears. One might naively think that the governing feature is whether we are dealing with a thin or a thick liquid, but this is not the case. If we put a drop of oil in an iron skillet, it spreads, but on a Teflon surface it beads up. The explanation revolves around surface energies, which are a measure of the relative strengths with which atoms on the surface of a material are attracted to atoms inside the bulk of the material. In a sense, this determines how much attraction these surface atoms can spare for other substances. In the case of Teflon, very little. Teflon is composed of long chains of carbon atoms, with each carbon also joined to two fluorine atoms. The fluorines, which stick out from the carbon skeleton, represent the exposed part of the molecules, the part that could potentially interact with other molecules. Fluorine, once it has bonded to carbon, is notoriously unreactive, and it is not interested in forging other... 

This average gi/es 1.225 for the carbon-carbon bond number and 0.471 for the iron-carbon (or ruthenium-carbon) bond number. The calculated average number of unshared pairs on the iron atom is 2.03. Its average formal charge is —0.79. The partial ionic character of the 4.71 Fe—C bonds (12 percent) reduces the charge to —0.22. 

We may predict the distances from the structure and radii. Carbon without doubt is quadrivalent, so that the iron-carbon bonds must have bond number. The sum of the iron radius 1.167 A and the carbon radius 0.772 A is 1.939 A, and with the correction of Equation 11-1 we obtain 2.04 A as the predicted Fe—C distance. This is in reasonably good agreement with the experimental value 2.01 A. Each iron atom forms a bond with bond number with each of two carbon atoins, using up 1 of its total valence of 6 and leaving 4 for the Fe—Fe bonds. The predicted bond numbers for ligancy 12 and 11 are 0.39 and 0.42, respectively, and the predicted Fe—Fe distances are 2.58 A and 2.56 A, which are approximately equal to the corresponding observed values,... 

There are more complex examples of metal ion catalysis. Cobalt in vitamin B12 reactions forms covalent bonds with carbons of substrates.41,42 Metals can also act as electron conduits in redox reactions. For example, in cytochrome c the iron in the heme is reversibly oxidized and reduced. 

Cyclooctatetraenyl complexes in which the metal is attached to the organic ring framework via a a bond to carbon are rare. The racemic iron compound 104 has been prepared, and its diastereoisomeric phosphine derivatives 105 (only one diastereoisomer shown) have been synthesized by photochemical replacement of a CO ligand in 104 by PR3 (180). Compound 104 exists as two... 

Iron-carbon bonds, to heteroatoms, in Fe clusters, overview,... 

For example, the (ClgTPP)Fe- anion reacts with n-BuI, which provides a measure of the lower limit for the iron-carbon bond energy ... 

In aprotic solvents, direct electrochemical reduction of C02 (—2.23 V vs. SCE) yields carbon monoxide and carbonate ion.29 The (porT)Fe dianion also reduces C02 to CO, but at a less negative potential (-1.70 V).30 Hence, the estimated iron-carbon bond energy (—AGbF) for the (porT)Fera—C(0)0 dianion is at least 50 kJ mol-1 [—AGBF > 96.5(—1.70 + 2.23)]. 

Our results indicate that the autoreduction cannot occur by a conventional outer sphere mechanism because of the gross mismatch of the electrochemical potentials. Experimental data available at this time are consistent with homolytic iron-carbon bond cleavage which may or may not involve a simultaneous nucleophilic attack on the coordinated cyanide. The homolytic metal-carbon bond cleavage may serve as a model for similar processes reported for vitamin Bi2 (26). 

Recent structural and spectroscopic investigations of organometallic complexes bonding two carbons of an allenic ligand to one rhodium 50 72> 87,95) or platinum atom 58,87,98,132) may have some pertinence to possible bridged intermediates proposed for various electrophilic additions to allenes, and the cr-iron-jr-iron complexes derived from allene and diiron... 

Using the analogy of model reactions of alkane oxidation in mixtures ofFe(II) and dioxygen in solvents, a mechanism invoking the formation of intermediate with an iron-carbon bond followed by interaction with soxygen was proposed (Waller and Fimscomb, 1996 Shilov, 1997). 

The third system is based on the scavenger formate. The dioxo-carbonate(l—) radical formed, CO) , is a strongly reducing radical, E CC /CO] ) = —1.8 V [99]. When generated from the Fenton reaction it is expected to reduce the metal and no absorbance change should result. However, we observed an intermediate with absorption maxima near 300 and 410 nm that we ascribed to a compound with iron-carbon a-bonds [2,96,123], similar to those investigated by Cohen and Meyerstein [130] ... 

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