The Metal Carbonyls

The metal carbonyls are of peculiar interest in the chemistry of the metals. Not only does this group of compoimds exhibit certain striking physical properties, but also their constitution has presented an unusual challenge to all theories of chemical combination. The elements that form true carbonyls are located in Groups VIA, VIIA, and VIII. of the periodic system. The individual compounds that have been prepared are the following  

In addition, the following closely related nitrosyl carbonyls and carbonyl hydrides are known  

Most of the carbonyls can be prepared by the direct combination of the metal with carbon monoxide. It is necessary that the metal be in a very active state as when freshly reduced from the oxide or a salt of the metal. While finely divided, freshly reduced nickel combines readily with carbon monoxide at room temperature and atmospheric pressure (synthesis 75) other metals require more elevated temperatures (up to 400 ) and very high pressures (up to 700 atm.). Cobalt nitrosyl tricarbonyl is produced when specially prepared cobalt is treated with a mixture of carbon monoxide and nitric oxide. 

Carbonyls also are prepared from metal salts and carbon monoxide in the presence of a variety of combinations of reagents. Always, however, one essential function of the reagents is to effect the reduction of the metal. This reduction may be accomplished in the dry way by treating a mixture of the metal salt and finely divided silver or copper with carbon monoxide 

In the last reaction, the presence of some carrier such as cysteine or cyanide is essential. Apparently, the carrier forms an intermediate complex containing carbon monoxide and cobalt, which reacts further to yield the salt of the carbonyl hydride and to regenerate the carrier. 

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The dry method for synthesizing metal carbonyls from salts and oxides has proven very usehil in a number of cases. The metal carbonyl is formed in the presence of a suitable reducing agent. In some cases CO itself is the reducing agent. Rhenium (97) and technetium (98,99) carbonyls are conveniently... 

Table 5 Hsts the metal carbonyls that are commercially available and the corresponding U.S. manufacturers or suppHers. Companies produciag these compounds on a captive basis are not iacluded.

There are only a few weU-documented examples of catalysis by metal clusters, and not many are to be expected as most metal clusters are fragile and fragment to give metal complexes or aggregate to give metal under reaction conditions (39). However, the metal carbonyl clusters are conceptually important because they form a bridge between catalysts commonly used in solution, ie, transition-metal complexes with single metal atoms, and catalysts commonly used on surfaces, ie, small metal particles or clusters. 

Much work has been undertaken to understand the steps and intermediates by which the reaction occurs on the heterogeneous catalyst surface. However, the exact mechanism is not fully established. One approach assumes a first-step adsorption of carbon monoxide on the catalyst surface followed by a transfer of an adsorbed hydrogen atom from an adjacent site to the metal carbonyl (M-CO) ... 

The 13C-NMR spectra of 4-7, 9-11 show a close similarity to the spectral data of analogous carbene complexes. The shift differences between the metal carbonyls of the silylene complexes and the related carbon compounds are only small. These results underline the close analogy between the silicon compounds 4-7, 9-11 and Fischer carbene complexes. This view is also supported by the IR spectral data. On the basis of an analysis of the force constants of the vco stretching vibration,... 

The metal carbonyls Ni(CO)4, Fe(CO)s, and Cr(CO)6 are observed to be diamagnetic. This follows from the theoretical discussion if it is assumed that an electron-pair bond is formed with each carbonyl for the nine eigenfunctions available (3d64s4p3) are completely filled by the n bonds and 2(9 — n) additional electrons attached to the metal atom (n = 4, 5, 6). The theory also explains the observed composition of these unusual sub-... 

In the metal-carbonyl catalysed hydrocarboxylation of alkynes ( Reppe reaction ) nearly exclusive cia-addition of H—COOH is found (Ohashi et al., 1952). 

Summary of Yields of Radioactive Parent Compound in the Metal Carbonyls, Without Subsequent Annealing... 

Susac et al. [33] showed that the cobalt-selenium (Co-Se) system prepared by sputtering and chemical methods was catalytically active toward the ORR in an acidic medium. Lee et al. [34] synthesized ternary non-noble selenides based on W and Co by the reaction of the metal carbonyls and elemental Se in xylenes. These W-Co-Se systems showed catalytic activity toward ORR in acidic media, albeit lower than with Pt/C and seemingly proceeding as a two-electron process. It was pointed out that non-noble metals too can serve as active sites for catalysis, in fact generating sufficient activity to be comparable to that of a noble metal, provided that electronic effects have been induced by the chalcogen modification. 

The metal carbonyls Fe(CO)5, Ru3(CO)i2, and Rh6(Co) 16 are catalysts for the deoxygenation of o-nitrostyrenes under carbon monoxide pressure to give indole derivatives (Eq. 10.59).82... 

Upon sonication in halocarbon solvents, metal carbonyls undergo facile halogenations (186). The rates of halogenation are solvent dependent, but independent of choice of metal carbonyl or its concentration, and represent the products of secondary reactions occurring from the sonolytic decomposition of the halocarbon solvent, as shown in Eqs. (16)-(20). Alkanes and other halogen atom traps suppress the halogenation of the metal carbonyls. 

While essentially all the metal carbonyl complexes for group 4B contain terminal CO ligands, only recently have some bonafide doubly bridging carbonyl complexes been reported. However, these complexes are hetero-nuclear, since the carbonyl ligand bridges a zirconium atom with the metal center of a late transition metal. 

The formulas for the metal carbonyls are determined by the number of pairs of electrons needed by the metal to reach the number of electrons in the next noble gas atom. Thus, the stable carbonyl with nickel contains four CO molecules, that with iron contains five, and that with chromium contains six. The bonding in these complexes will be discussed in more detail in Chapter 16. 

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. 

C02 and H2, presumably from the respective neutralizations of carbonate and of the metal carbonyl anions to give metal hydrides (e.g., H2Fe(C0)tf) which undergo reductive elimination of H2. 

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