Metal carbonyls carbonyl ligand replacement

Although it is well known that coordinated carbon monoxide in metal carbonyls may be replaced quite easily by other coordinating ligands, only a few studies have been made of the exchange equilibria resulting from the... 

CO groups in silyl-transition-metal carbonyls can be replaced by PR3 groups or bidentate phosphorus ligands ... 

An active area of metal carbonyl chemistry involves replacement of CO with group VB donor ligands, especially phosphites and phosphines. The direct replacement of CO by the ligand is the simplest means of preparation of the substituted dimers ... 

Ligand systems containing several functional groups usually react step-wise with metal carbonyls with successive replacement of CO. An example is the photochemical reaction of Fe(CO)s with 1.5-cycloocta-diene 284,287) ... 

Acetonitrile complexes of the group-VIA metals are useful intermediates in the development of the chemistry of those metals. Three carbonyl ligands of M(CO)s, where M = Cr, Mo or W, are replaced by acetonitrile when the hexacarbonyls are refluxed in that solvent . When [M(CO)3(CH3CN)j] is treated with chelating diolefins the three acetonitrile ligands together with one carbonyl ligand are displaced to form M(diolefin)2(CO)2 ... 

Many metal carbonyls undergo easy replacement of some of their carbon monoxide ligands. Direct reaction between the olefin and the metal carbonyl frequently occurs at room temperature or at slightly higher temperatures, e.g. 

While the replacement of carbon monoxide by a tertiary phosphine ligand represents one of the most fundamental substitution reactions in metal carbonyl chemistry, it was not until 1975, some 16 years after the... 

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. 

Note that in this case, the three carbonyl ligands are staggered relative to the carbon atoms in the benzene ring (as indicated by the dotted vertical lines). Similar compounds have also been prepared containing Mo and W. Methyl-substituted benzenes such as mesitylene (1,3,5-trimethylbenzene), hexamethylbenzene, and other aromatic molecules have been used to prepare complexes with several metals in the zero oxidation state. For example, Mo(CO)6 will react with 1,3,5-C6H3(C]T3)3, 1,3,5-trimethylbenzene, which replaces three carbonyl groups. 

The dominant photochemical reaction of metal carbonyl compounds is loss of carbon monoxide, which is usually followed by substitution of another ligand to replace the expelled carbon monoxide. 

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. 

Very few such compounds are known. Three general procedures can be used for their preparation (a) addition of the olefin to a transition metal compound (b) replacement of carbon monoxide in metal carbonyls and (c) migration of organosilicon groups from metal to ligand in some reactions of silylmetal compounds with acetylenes. 

The most straightforward route to heteroaldehyde and heteroketone complexes is the substitution of a heterocarbonyl compound for another coordinated ligand. This method is naturally restricted to heteroaldehydes and heteroketones stable in the uncoordinated form, i.e., is usually restricted to thioketones and a few stable seleno- and tellurocarbonyl compounds.141718,27118 In most cases, metal carbonyls or solvent complexes of metal carbonyls were used as the complex precursors. The photochemically or thermally induced loss of the ligand to be replaced is followed by coordination of the heterocarbonyl compound [Eq. (3)]. 

This catalytic labilization of carbonyl groups has been extended to the replacement of carbonyls in metal carbonyl clusters. Metal cluster complexes are at present the subject of extensive studies, partly because of their possible relevance as models for chemisorbed metal surfaces and because of their catalytic activity. The majority of these clusters contain carbonyl ligands, and these have been prepared from the vast number of metal carbonyl precursors generally available by a variety of synthetic methods usually without recourse to designed or rational procedures. In metal isocyanide chemistry, however, suitable precursors are lacking, and as a consequence, there are few routes to homoleptic metal-isocyanide clusters, and few isocyanide clusters are known (see Section IV,A). 

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