Carbon monoxide as ligand

There is often a striking similarity between olefins and carbon monoxide as ligands, and one of the most common ways of preparing olefin complexes is by replacement of one or more carbon monoxide ligands in a metal carbonyl by olefins. Both olefin and carbonyl complexes frequently obey the simple E.A.N. rule, each CO ligand and C C bond contributing two ir-electrons to the metal atom, to enable it to attain the electronic configuration of the next inert gas in the Periodic Table. 

Most transition metals form complexes known as carbonyls with carbon monoxide as ligands. Examples include Fe(CO)s, Fe2(CO)9, Cr(CO)6, and Rh6(CO)i6, in all of which the metal is ostensibly in the oxidation state of zero, and many mixed-ligand carbonyls such as Mn(CO)sI, CH3Mn(CO)s, and (C6H6)Mo(CO)3 are known. Such compounds have an organiclike chemistry, being essentially covalent (see Section 8.2 and Chapter 18), and the simple carbonyls such as Ni(CO)4 are volatile liquids that can be purified by fractional distillation. Of all these, however, only Ni(CO)4 (bp 43 °C) forms rapidly (and reversibly) from the elemental metal and CO gas... 

P°—Metal forms neutral carbonyl derivative(s) with only carbon monoxide as ligands, p-—Metal forms anionic carbonyl derivative(s) with only carbon monoxide as ligands. 

Abstract This chapter focuses on carbon monoxide as a reagent in M-NHC catalysed reactions. The most important and popular of these reactions is hydro-formylation. Unfortunately, uncertainty exists as to the identity of the active catalyst and whether the NHC is bound to the catalyst in a number of the reported reactions. Mixed bidentate NHC complexes and cobalt-based complexes provide for better stability of the catalyst. Catalysts used for hydroaminomethylation and carbonyla-tion reactions show promise to rival traditional phosphine-based catalysts. Reports of decarbonylation are scarce, but the potential strength of the M-NHC bond is conducive to the harsh conditions required. This report will highlight, where appropriate, the potential benefits of exchanging traditional phosphorous ligands with iV-heterocyclic carbenes as well as cases where the role of the NHC might need re-evaluation. A review by the author on this topic has recently appeared [1]. 

Since 1948-50, by using as reactants isonitriles, phosphorus trihalides, and tertiary phosphines, we have gained important insight into the dependence of reactions of metal carbonyls with bases upon the nature of the ligand. Organophosphines were introduced into carbonyl chemistry even prior to 1948 by Reppe and Schweckendiek (7). In general, these ligands react only by substitution of CO, and do not cause disproportionation. Thus nickel carbonyl frequently reacts with complete displacement of carbon monoxide, as we were first able to demonstrate in the reaction with phenyl isonitrile (8). 

This chapter is concerned with carbon monoxide as a ligand. By the end of the chapter you should be familiar with ... 

The first compound to be synthesized containing carbon monoxide as a ligand was another platinum chloride complex, reported in 1867. In 1890, Mond reported the preparation of Ni(CO)4, a compound that became commercially useful for the purification of nickel. Other metal CO (carbonyl) complexes were soon obtained. 

Aryl-, alkenyl- and alkynylpalladium species readily undergo carbonylation reactions because carbon monoxide as a loosely bonded ligand can reversibly insert into any palladium-carbon bond [110]. Thus, 2-allyl-l-iodocyclopentene (148), under palladium catalysis, reacts with carbon monoxide in two modes, depending on the excess of carbon monoxide and the catalyst cocktail (Scheme 3-39) [110a]. With a slight excess (1.1 atm of CO) in the presence of [Pd(PPh3)4] in tetrahydrofuran, 148 cyclized with one CO insertion to yield 3-methylenebicyclo[3.3.0]oct-l(5)-en-2-one (152), and under 40 atm of CO with [Pd(PPh ,)2Cl2] in benzene/acetonitrile/methanol, methyl 2- 3 -(2 -oxobicyclo[3.3.0]oct-1 (5 )-enyl) acetate 149 after two CO insertions (Scheme 3-39). 

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