Homoleptic iron carbonyls

A series of osmium carbonyl complexes have been prepared by the reaction of OsO-with CO or decomposition of Os3(CO)i2 [226] and a mononuclear homoleptic osmium carbonyl complex, Os(CO)5, is also known. It is a volatile, colorless liquid and is the most robust M(CO)5 type complex of the iron triad against both oxidation and heat but it gradually loses CO to form Os3(CO)i2. Multinuclear osmium carbonyl clusters such as Os3(CO),2, Os5(CO)i6, Os5(CO)i9, Os6(CO)ig, 087(00)2, and OsgfCO).. have also been reported [227]. In this section, several carbonyl complexes based on Os3(CO)j2are described. 

Homoleptic carbonyl complexes of metals in the second and third rows of the transition series form polynuclear carbonyl complexes more often than they form mononuclear carbonyl complexes. For example, Fe(CO)5 is stable and is the most common iron carbonyl, but Os(CO)j is much less stable Os3(CO)jj is more stable. 

In a paper reminiscent of an earlier entry for iron, Schaefer has described the (theoretical) structures of binuclear homoleptic nickel carbonyls, examining the possible Ni-Ni single, double, and triple bonds in Ni2(CO)x(x = 5,6,7) species. 

Various /jara-substituted styrenes have been treated with triethylsilane in the presence of iron carbonyls to give selectively ( )-P-triethylsilyl styrenes and not the hydrosilylation product (silylethyl)benzene (Scheme 4-336). Dodecacarbonyltriion shows the highest activity among the homoleptic carbonyliron complexes, whereas pentacarbonyliron is inactive. ... 

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). 

A recent review has highlighted the extensive and interesting chemistry of metal isocyanide complexes.1 Although synthetic procedures are varied, a vast number are based on substitution in metal carbonyl complexes by isocyanides. Such procedures are, however, not always successful. This is especially so in cases where multiple substitution of CO is required, as in the syntheses of homoleptic isocyanide complexes. Many of the inherent difficulties are illustrated by the reaction of iron pentacarbonyl with isocyanides. 

The classical protocol for synthesis of iron-diene complexes starts from the homoleptic pentacarbonyliron complex. In a stepwise fashion, via a dissociative mechanism, two carbonyl ligands are displaced by the diene system. However, thermal dissociation of the first CO ligand requires rather harsh conditions (ca. 140 °C). For acyclic 1,3-dienes, the diene ligand adopts an s-cis conformation to form stable q4-complexes (Scheme 1.18). 

Although additives to induce radical chemistry have allowed ligand substitutions of 18-electron complexes to be conducted under mild conditions, photochemical reactions provide a common and practical alternative. Photochemically induced dissociation of carbonyl ligands is most common, but photochemical dissociations of other dative ligands are known. Several examples are shown in Equations 5.36-5.40. These examples illustrate the dissociation of CO from homoleptic carbonyl compounds of iron - and chromium, the dissociation of CO from piano-stool carbonyl compounds, " ttie dissociation of N, and the dissociation of a carbodiimide to generate an intermediate that coordinates and cleaves the C-H bonds of alkanes. In some cases, like the formation of the two THE complexes, the products of the photochemical process are not isolated instead, they are treated in situ with a ligand, such as a phosphine, to form monosubstitution products selectively. 

Like nickel, iron can also be purified using a carbonyl compound. Iron purified this way is called carbonyl iron, and the iron has an oxidation state of zero. Briefly rationalize why iron(III) does not form a complex with carbonyls whereas Fe(0) does. Based on the EAN rule, speculate on the most likely homoleptic carbonyl complex formed by iron(O). 

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