Carbonyl ligands substitution reactions

The reactions of nucleophilic reagents with cationic and uncharged metal carbonyl complexes have received much attention in the past, and it is not surprising that these studies have now been extended to isocyanide metal complexes. Different products in these reactions can arise by three general routes these include ligand substitution, reactions involving attack at a ligand, and reduction of the metal complex. All have been observed in reactions with metal isocyanide complexes. 

Y. Harel, A. W. Adamson. Photocalorimetry. 2. Enthalpies of Ligand Substitution Reactions of Some Group 6 Metal Carbonyl Complexes in Solution. J. Phys. Chem. 1982, 86, 2905-2909. 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

It is useful to consider the reactions of carbonyl metallates separately, since their reactivity is generally concerned with the nucleophilic metal centre and will be discussed below. Simple ligand substitution reactions have already been discussed above, as have redox processes that provide access to carbonyl metallates through reduction of the metal centre. These redox or ligand addition/elimination processes are in principle no different from those encountered for classical ligands. We will now consider reactions in which the carbonyl ligand itself enters directly into the reaction and emerges transformed. 

Reaction of [Co2(ju-alkyne)(CO)6] complexes with monodentate phosphines, phosphites, and arsines results in carbonyl ligand substitution.51,52 One or both axial carbonyl groups can be displaced under thermal conditions. This has been confirmed in a number of cases by X-ray crystallography, for example, [Co2(jU-HC2H)(CO)4(PMe3)2] (Fig. 6).53 The structure inherited from the parent compound is not greatly affected by substitution. The molecule still possesses C2v symmetry and the cobalt-cobalt bond... 

Substitution of several metal-carbonyl complexes Cr(CO)6 and Mn(CO)5 (amine) show a small dependence on the nature and concentration of the entering hgand. Under pseudo-first-order conditions, the rate laws for these substitutions have two terms, as shown for Cr(CO)6 (as for some substitution reactions with 16e complexes, see equation 5). The second-order term was always much smaller than the first-order term. A mechanism that ascribes the second-order term to dissociative interchange (U) has been suggested for the Mo(CO)5Am system (Am = amine) and involves a solvent-encased substrate and a species occupying a favorable site for exchange. Thus, the body of evidence for the simple metal carbonyls indicates that CO dissociation and is the mechanism of ligand substitution reactions. 

The dinitrogen complexes are very reactive, undergoing facile ligand substitution reactions with a variety of added Lewis bases. For example, exposure to carbon monoxide gas produced the carbonyl complexes Tp °Co-CO in quantitative yield. 

Although mononuclear metal carbonyls are purportedly less effective as catalysts for this process when compared with metal carbonyl clusters (14,15), investigations of these systems will provide for a better understanding of the fundamental steps in the homogeneous metal-catalyzed water gas shift reaction. Therefore, the primary objective of this work was to examine (i) the reversible nature of the reaction of hydroxide ion with Cr(CO)e, along with the concomitant formation of /A-H[Cr-(CO)5]2" and CO2 and (ii) the ligand substitution reactions of /x-H[Cr-(CO)5]2" with CO, both thermally and photochemically (Scheme 1). 

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