Metal carbonyls, ligand site exchange

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. 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

A common feature of metal clusters is their stereochemical nonrigidity, in which carbonyl and hydride ligands exchange their coordination sites. Mixed-metal clusters are ideally suited for studies of the fluxional processes in clusters because of the low symmetry inherent in their metal framework. In such clusters, the majority of the ligands are in chemically nonequivalent positions and should thus be distinguishable by NMR... 

Although the number of structurally characterized clusters is now immense, the study of their reactivity has been mostly limited to smaller clusters. [21] Huttner et al., for example, were able to show that trinudear, /I3-ER bridged carbonyl metal clusters (E = N, P, As, Sb, Bi) are suitable for kinetic and mechanistic studies. [22] The peripheral ligands on such clusters are susceptible to exchange by addition and subsequent elimination. It is notable that the Nig clusters 2 and 3 also lend themselves to studies of ligand substitution and addition reactions. The Cl ligands can be substituted in 2, and neutral molecules can be added to the open coordination sites in 3. Scheme 3-14 summarizes the reactions carried out so far. 

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