Metals carbon monoxide

For small-molecule, metal-carbon monoxide complexes, the carbon monoxide ligand is almost always in a linear conformation and perpendicular to the metal. If one assumed bonding of CO to Hb or Mb in its normal linear, perpendicular mode, steric conflicts as illustrated in Figure 4.20 would occur and thus one might predict... 

Second, the idea of the ejection of a saturated fragment is compatible with the established view of the bond enthalpies for metal-carbon monoxide > metal-metal. Thus the formation of M-CO bonds at the expense of M-M bonds is to be anticipated. 

As with metal-carbon monoxide bonds, the MP2/6-3IG model does not lead to results of the same calibre as those from density functional models (except local density models). The model actually shows the opposite behavior as 6-3IG, in that bond lengths are consistently shorter than experimental values, sometimes significantly so. In view of its poor performance and the considerable cost of MP2 models (relative to density functional models), there seems little reason to employ them for structural investigations on organometallics. 

Metal-carbon monoxide ligand bond lengths in these same compounds have already been provided in Table 5-11. 

Iron, as found in the porphyrin derivative hemoglobin, complexes CO to form a stable metal carbonyl. Iron also forms a variety of metal carbon monoxide derivatives such as the homoleptic Fe(CO)5, Fe2(CO)9 and Fe3(CO)i2, the anionic [Fe(CO)4] and its covalent derivative Fe(CO)4Br2, [CpFe(CO)2] and its alkylated covalent derivatives CpFe(CO)2-R with its readily distinguished n (and and a (and / ) iron carbon bonds. By contrast. Mg in its chlorin derivative chlorophyll, which very much resembles porphyrin, forms no such bonds with CO nor is there a rich magnesium carbonyl chemistry (if indeed, there is any at all). 

The conversion of carbon monoxide and hydrogen into synthetic fuels occurs over metals. Carbon, monoxide and hydrogen compete for surface sites in the adsorption phase of the synthesis process. 

A third industrial use for carbon monoxide is in the refining of metals. Most metal ores exist in the form of oxides or sulfides when extracted from the earth. For example, the two most important ores of iron are magnetite (Fe304) and hematite (Fe203). After an ore has been mined, it is treated to remove the oxygen or sulfur in the ore to obtain a pure metal. Carbon monoxide is often used for this purpose with oxide ores because it combines with oxygen from the ore to form carbon dioxide, leaving the metal behind Metal oxide + CO —> Metal + C02. 

FIGURE 3.10 Orbital overlap schemes for dre-pre bonds, (a) pz-dxz in POCI3, (b) pz-dz in PF3, (c) pn-dz in P05, (d) p7t-dz -y in PO ", (e) dative bond transition metal-phosphine ti- and (f) dative n bond transition metal-carbon monoxide. 

These differences in reactions may be attributed to differences in the strength of the metal-carbon monoxide bond in these metal carbonyl derivatives. 

In addition to these two factors which influence the strength of the metal-carbon monoxide bond by clearly affecting the amount of back-bonding or partial double-bonding between the metal atom and the carbon monoxide ligand, there are two other factors which affect the strength of the metal-carbon monoxide bond. 

Comparison between various coordination numbers. The metal-carbon monoxide bonds in tetracoordinated and pentacoordinated metal carbonyls appear to be less stable than the metal-carbon monoxide bonds in the corresponding hexacoordinated metal carbonyls. For example, [Co(CO)4] reacts with allyl halides at room temperature to produce directly the 77-allyl derivative, C3H5Co(CO)3 the presumed pentacoordinate intermediate a-allyl derivative C3H5Co(CO)4 is not isolable, losing carbon monoxide rapidly even at room temperature 110). In contrast to this behavior. 

Mn(CO)5] reacts with allyl halides at room temperature to produce the hexacoordinate a-allyl derivative C3H5Mn(CO)5 which must be heated to 60° C before carbon monoxide is lost at an appreciable rate to form the ir-allyl derivative C3HsMn(CO)4 (99,100). In addition, the pentacoordinate perfluoroacyl derivatives of cobalt, RfCOCo(CO)4 (38, 111, 112), lose carbon monoxide to form RfCo(CO)4 at a lower temperature than the hexacoordinate perfluoroacyl derivatives of manganese, RfCOMn(CO)5 (38,113), form the perfluoroalkyl derivatives, RfMn(CO)s. On the basis of present data, however, it is diflicult to separate the effect of coordination number on the general stability of the molecule from the effect of coordination number on the stability of the metal-carbon monoxide bond. 

Stability of the metal-carbon monoidde bond. A low formal oxidation state, substitution of carbonyl groups by other ligands, and a hexacoordinate central metal atom of either the 3d or the 5d transition series favor a more stable metal-carbon monoxide bond. 

However, on lightly greased surfaces HCo(CO)4 decomposes more completely, producing cobalt metal, carbon monoxide, and hydrogen... 

Figure 16.1. The energy level of carbon monoxide molecules, and the formation of metal-carbon monoxide bonding [449]. (Reprinted from Grgur BN, Markovic NM, Lucas CA, Ross PN. Electrochemical oxidation of carbon monoxide from platinum single crystals to low temperature fuel cells catalysis. Part 1 carbon monoxide oxidation onto low index platinum single crystals. J Serb Chem Soc 2001 66 785-97. With permission from the Serbian Chemical Society.)...

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