The number of IR-active vibrational modes of several prototypical metal carbonyl complexes.

Direct reaction of metal with carbon monoxide:-

Nickel tetracarbonyl and iron pentacarbonyl can be prepared according to the following equations by reaction of finely divided metal with carbon monoxide:

Ni + 4 CO → Ni(CO)4 (1 bar, 55 °C)

Fe + 5 CO → Fe(CO)5 (100 bar, 175 °C)

Nickel tetracarbonyl is formed with carbon monoxide already at 80 °C and atmospheric pressure, finely divided iron reacts at temperatures between 150 and 200 °C and a carbon monoxide pressure of 50 to 200 bar. Other metal carbonyls are prepared by less direct methods.

Reduction of metal salts and oxides:-

Some metal carbonyls are prepared by the reduction of metal halides in the presence of high pressure of carbon monoxide. A variety of reducing agents are employed, including copper, aluminum, hydrogen, as well as metal alkyls such as triethylaluminium. Illustrative is the formation of chromium hexacarbonyl from anhydrous chromium(III) chloride in benzene with aluminum as a reducing agent, and aluminum chloride as the catalyst:

                CrCl3 + Al + 6 CO → Cr(CO)6 + AlCl3

The use of metal alkyls, e.g. triethylaluminium and diethylzinc as the reducing agent leads to the oxidative coupling of the alkyl radical to the dimer:

WCl6 + 6 CO + 2 Al(C2H5)3 → W(CO)6 + 2 AlCl3 + 3 C4H10

Tungsten, molybdenum, manganese, and rhodium salts may be reduced with lithium aluminum hydride. Vanadium hexacarbonyl is prepared with sodium as a reducing agent in chelating solvents such as diglyme.

                VCl3 + 4 Na + 6 CO 2 diglyme → Na(diglyme)2[V(CO)6] + 3 NaCl

                 [V(CO)6]− + H+ → H[V(CO)6] → 1/2 H2 + V(CO)6

In aqueous phase nickel or cobalt salts can be reduced, for example, by sodium dithionite. In the presence of carbon monoxide, cobalt salts are quantitatively converted to the tetracarbonylcobalt(-1) anion:

Co2+ + 1.5 S2O42− + 6 OH− + 4 CO → Co(CO)4− + 3 SO32− + 3 H2O

Some metal carbonyls are prepared using CO as the reducing agent. In this way, Hieber and Fuchs first prepared dirheniumdecacarbonyl from the oxide:

                  Re2O7 + 17 CO → Re2(CO)10 + 7 CO2

If metal oxides are used carbon dioxide is formed as a reaction product. In the reduction of metal chlorides with carbon monoxide phosgene is formed, as in the preparation of osmium carbonyl chloride from the chloride salts. Carbon monoxide is also suitable for the reduction of sulfides, where carbonyl sulfide is the byproduct.

Reactions

Metal carbonyls are important precursors for the synthesis of other organometalic complexes. The main reactions are the substitution of carbon monoxide by other ligands, the oxidation or reduction reactions of the metal center and reactions of carbon monoxide ligand.

CO substitution:-

The substitution of CO ligands can be induced thermally or photochemically by donor ligands. The range of ligands is large, and includes phosphines, cyanide (CN−), nitrogen donors, and even ethers, especially chelating ones. Olefins, especially diolefins, are effective ligands that afford synthetically useful derivatives. Substitution of 18-electron complexes generally follows a dissociative mechanism, involving 16-electron intermediates.

Substitution proceeds via a dissociative mechanism:

M(CO)n → M(CO)n-1 + CO

M(CO) n-1 + L → M(CO)n-1L

The dissociation energy is 105 kJ mol−1 for nickel tetracarbonyl and 155 kJ mol−1 for chromium hexacarbonyl.

Substitution in 17-electron complexes, which are rare, proceeds via associative mechanisms with a 19-electron intermediates.

M(CO)n + L → M(CO)nL

M(CO)nL → M(CO)n-1L + CO

The rate of substitution in 18-electron complexes is sometimes catalysed by catalytic amounts of oxidants, via electron-transfer.

Reduction:-

Metal carbonyls react with reducing agents such as metallic sodium or sodium amalgam to give carbonylmetalate (or carbonylate) anions:

Mn2(CO)10 + 2 Na → 2 Na[Mn(CO)5]

For iron pentacarbonyl, one obtains the tetracarbonylferrate with loss of CO:

Fe(CO)5 + 2 Na → Na2[Fe(CO)4] + CO

Mercury can insert into the metal-metal bonds of some polynuclear metal carbonyls:

Co2(CO)8 + Hg → (CO)4Co-Hg-Co(CO)4

Nucleophilic attack at CO:-

The CO ligand is often susceptible to attack by nucleophiles. For example, trimethylamine oxide and bistrimethylsilylamide convert CO ligands to CO2 and CN−, respectively. In the "Hieber base reaction", hydroxide ion attacks the CO ligand to give a metallacarboxylic acid, followed by the release of carbon dioxide and the formation of metal hydrides or carbonylmetalates. A well-known example of this nucleophilic addition reaction is the conversion of iron pentacarbonyl to hydridoirontetracarbonyl anion:

Fe(CO)5 + NaOH → Na[Fe(CO)4CO2H]

Na[Fe(CO)4COOH] + NaOH → Na[HFe(CO)4] + NaHCO3

Protonation of the hydrido anion gives the neutral iron tetracarbonyl hydride:

Na[HFe(CO)4] + H+ → H2Fe(CO)4 + Na+

Organolithium reagents add with metal carbonyls to acylmetal carbonyl anions. O-alkylation of these anions, e.g. with Meerwein salts, affords Fischer carbenes.

With electrophiles:-

Despite being in low formal oxidation states, metal carbonyls are relatively unreactive toward many electrophiles. For example, they resist attack by alkylating agents, mild acids, mild oxidizing agents. Most metal carbonyls do undergo halogenation. Iron pentacarbonyl, for example, forms ferrous carbonyl halides:

Fe(CO)5 + X2 → Fe(CO)4X2 + CO

Metal-metal bonds are cleaved by halogens. Depending on the electron-counting scheme used, this can be regarded as oxidation of the metal atom:

Mn2(CO)10 + Cl2 → 2 Mn(CO)5Cl

Application:- 

1.Metallurgical uses:-

Metal carbonyls are used in several industrial processes. Perhaps the earliest application was the extraction and purification of nickel via nickel tetracarbonyl by the Mond process (see also carbonyl metallurgy).By a similar process carbonyl iron, a highly pure metal powder, is prepared by thermal decomposition of iron pentacarbonyl. Carbonyl iron is used inter alia for the preparation of inductors, pigments, as dietary supplements, in the production of radar-absorbing materials in the stealth technology, and in thermal spraying.

2.Catalysis:-

Metal carbonyls are used in a number of industrially important carbonylation reactions. In the oxo process, an olefin, dihydrogen, and carbon monoxide react together with a catalyst (e.g. dicobaltoctacarbonyl) to give aldehydes. Illustrative is the production of butyraldehyde:

                      H2 + CO + CH3CH=CH2 → CH3CH2CH2CHO

Butyraldehyde is converted on an industrial scale to 2-ethylhexanol, a precursor to PVC plasticizers, by aldol condensation, followed by hydrogenation of the resulting hydroxyaldehyde. The "oxo aldehydes" resulting from hydroformylation are used for large-scale synthesis of fatty alcohols, which are precursors to detergents. The hydroformylation is a reaction with high atom economy, especially if the reaction proceeds with high regioselectivity.