Carbon monoxide ligand, isoelectronic

All of these metal complexes are isoelectronic and isostructural with each other. Upon coordination of the carbon monoxide to the nickel(O) centre, the metal ion acquires negative charge. The carbon monoxide ligand possesses empty 7r -orbitals, and back-donation from the metal to the ligand reduces the electron density on the metal. 

For this compound, which is closely related to the isoelectronic Hg[Fe(CO)3NO]2 (45) (Section IX) and to Hg[Mn(CO)5]2 (VII, 79), a convenient method of preparation in aqueous solution was discovered in conjunction with E. O. Fischer (46). Moreover, we could make IIg[Co(CO)4]2 react, like Co2(CO)8, with ligands either with substitution of carbon monoxide and formation of Hg[Co(CO)3L]2 (L = PR3, AsR3, SbR3), or by valence disproportionation of cobalt (VII, 80) using nitrogen bases or isonitriles, e.g.,... 

The adoption of two different geometries for these isoelectronic Au6Pt clusters is probably due to electronic differences between the carbon monoxide and acetylide ligands, as neither ligand is sterically demanding in these compounds. However, both cluster compounds adopt metal cage geometries that are best described as hemispherical, as expected from their valence electron counts. 

Insertion reactions involving metal alkoxides are also known. For example, carbon dioxide is known to react with some metal alkoxides as shown in equation (12). The formation of a bidentate ligand is a significant thermodynamic driving force for some of these reactions. The isoelectronic aryl and alkyl isocyanates and carbodiimides can react similarly. Insertion reactions involving alkenes and carbon monoxide are known for platinum alkoxides. 

Lewis bases such as R3N, RC=N, H2O, and R3P are also two-electron donors, as are carbon monoxide ( C=0) and the isoelectronic isocyanides ( C=NR). (When CO is attached to a metal it is called a carbonyl group.) The bond between a Lewis basic ligand and the Lewis acidic metal is sometimes called a dative bond. 

In place of carbon monoxide, isocyanides are often used as the isoelectronic compound. In 1986, Jones et al. reported that the low-valent ruthenium phosphine complex catalyzed intramolecular insertion of isocyanide into the sp3 C-H bond under thermal conditions (Eq. 33) [60,61 ]. Their finding provided a new route for synthesis of indole. An interesting feature of their reaction is that C-H bond cleavage occurs even in the presence of an excess of the trapping ligand, i.e., isocyanide. 

In contrast to the dioxygen, carbon-monoxide, and nitric-oxide ligands, the isocyanide and nitroso functions bear an organic tail. Moreover, nitroso ligands are isoelectronic with dioxygen. 

Just like the isoelectronic carbon monoxide, an isocyanide is an excellent ligand to metal ions. The chemistry of metal isocyanide complexes has been reviewed by Singleton and Oosthuizen. Only a few examples will be given here. Insertion of an isocyanide into a metal-carbon bond frequently occurs. It is not always clear whether the key step is electrophilic or nucleophilic attack on the coordinated isocyanide or whether the reaction is concerted. Insertion into metal-carbene and metal-carbyne complexes have been reviewed by Aumann. Coordination to the metal considerably affects the chemistry of the isocyanide. If the metal is electron-donating, as in nitrogenase-like centres, the coordinated isocyanide is apt to electrophilic attack at nitrogen cf. Section III. 

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