Preparation of Metal Carbonyls

In this case, the product is the fac isomer, in which all NH3 ligands are trans to the CO molecules. Ammonia does not form ty bonds to metals because it has no orbitals of suitable energy to accept electron density. Thus, the back donation from Cr in Cr(NH3)3(CO)3 goes to only three CO molecules, and the bond order is reduced even more than it is in Cr(CO)s, where back donation occurs equally to six CO molecules. There is, of course, an increase in Cr-C bond order and stretching frequency in Cr(NH3)3(CO)3 compared to Cr(CO)s. Based on the study of many mixed carbonyl complexes, it is possible to compare the ability of various ligands to accept back donation. When this is done, it is found that the ability to accept back donation decreases in the order 

The chemical behavior of metal carbonyls is influenced by the nature of other ligands present. A decrease in C-O bond order results from an increase in M-C bond order. If other ligands are present that cannot accept electron density, more back donation to CO occurs, so the M-C bond will be stronger and substitution reactions leading to replacement of CO will be retarded. If other ligands are present that are good iy acceptors, less back donation to the CO groups occurs. They will be labilized and substitution will be enhanced. 

As we have described previously, Ni(CO)4 can be prepared directly by the reaction of nickel with carbon monoxide. However, most of the binary metal carbonyls listed in Table 21.1 cannot be obtained by this type of reaction. A number of preparative techniques have been used to prepare metal carbonyls, and a few general ones will be described here. 

The reactions of carbon monoxide with Ni and Fe proceed rapidly at low temperature and pressure. 

For most other metals, high temperatures and pressures are required to produce the metal carbonyl. By this direct reaction, Co2(CO)8, Mo(CO)6, Ru(CO)5, and W(CO)6 have been prepared when suitable conditions are used. 

In the latter case, V(CO)6 is obtained by hydrolyzing the product with H3P04 and subliming V(CO)6 at 45-50 °C. The reduction of CoI2 can be used to prepare Co2(CO)8  

Displacement reactions. The carbonyls of some metals have been prepared by the reaction of metal compounds directly with CO because CO is a reducing agent as well as a ligand. The following equations illustrate this type of process  

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If the product is to be used for the preparation of metal carbonyl derivatives, further usable material may be obtained from the mother liquors. Removal of pentane on a rotary evaporator leaves a yellow -orangc, viscous oil that is suitable for most preparative purposes. 

Efficient Routes to Preparation of Metal Carbonyl Clusters... 

AH preparations of metal carbonyls should be carried out in well-ventilated hoods. These toxic substances must be handled with cantion and attention paid to prevent discharge of toxic effluents by using appropriate chemical treatment. 

Tungsten or molybdenum hexacarbonyls were also found by us to be readily prepared from WClf, or M0CI5, respectively, with iron pentacar-bonyl. The yield was 75% with V(CO)g 210) and 30% with Mo(CO)5 211), higher pressures of CO being necessary in the latter case. As far as I know, our discoveries of 1939 were a starting point for the industrial preparation of metal carbonyls through the carbonylation of metal chlorides accomplished in various countries. 

Preparations of metal carbonyl halide complexes containing metal-metal bonds is not a fruitful area in comparison to cleavage of metal-metal bonds by halogens. In a few cases one metal-metal bond of a cluster can be halogenated, leading to halide complexes containing metal-metal bonds. Reaction of iodine with Fc3(CO)i2 leads to a low yield of FejlCOy, ... 

This reaction is useful for preparation of metal carbonyl dimers, such as MnRe(CO)io, MnRe(CO)9L (both isomers), and Cp(OC)3WMn(CO)s ... 

Fig. 25.—Shaking apparatus for use in the preparation of metal carbonyls and certain of their derivatives.

There are several methods for the preparation of metal carbonyl hydrides ... 

Figure 7 - Preparation of metal carbonyls from salts...

The preparation of metal carbonyl anions from Lewis bases and certain neutral metal carbonyl derivatives was described above. In general, such reactions involve disproportionation of the zero-valent metal atom in the metal carbonyl to a cation, with coordination of the base to the metal atom, and to the metal carbonyl anion. A different t3rpe of reaction between Lewis bases and metal carbonyls involves displacement of carbonyl groups in the metal carbonyl by the Lewis base without change in the oxidation state of the central metal atom, e.g.,... 

The following are some of the general methods of preparation of metal carbonyls. 

Syntheses of medium- and high-nuclearity ruthenium and osmium clusters continue to be largely by thermolyses of lower nuclearity precursors, but surface-mediated methods have also been employed the use of inorganic oxides or zeolites in the preparation of metal carbonyl clusters, including pentaosmium and hexaruthenium carbido clusters, and the decaosmium complexes [H50sio(CO)24] and [Osio(/t6-C)(CO)24], has been reviewed. 

The radicals produced may react with molecules of solvent, hydrocarbon, hydrocarbon anion or may dimerize. Reaction with methyl iodide and subsequent determination of iodide anion in the aqueous extract is a recommended method for analyzing compounds like sodium-naphthalene. The etching of polytetrafluorethylene is an interesting example of the reduction of halides an active surface is produced which is then able to form strong bonds to an epoxy resin. The reducing action of hydrocarbon anions has been used in the preparation of metal carbonyls. In this it is commonly convenient to start from the salt of a metal in a +2 or +3 oxidation state, which must be reduced to the zero oxidation state both sodium-naphthalene and aluminium alkyls have been used in this connection. 

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