Intramolecular site-exchange

Soon after the discovery of H2 complexes facile intramolecular site exchange of H atoms between H2 and hydride ligands was found to occur [61]. The NMR signals of the cis H2 and hydride in [Ir(H2)H(bq)(PPh3)2]+ coalesce at 240 K because of exchange, and even the hydride trans to H2 in [Fe(H2)H(dppe)2]+ exchanges positions with the H atoms of... 

TABLE 5. Activation parameters for intramolecular site exchange of fluorine atoms in chelated fluorosilanes... 

In addition to the ReHg- ion there is a technetium analog. Both of these exhibit a single, sharp pmr signal despite the existence of two different proton environments. This has been attributed to a rapid intramolecular site-exchange process, already described (page 44). The ReH ion is obtained by reduction of NaRe04 with an excess of sodium in ethanol the isolation of various pure salts requires careful workup procedures.66 The ReH ion... 

Low-valent Fe Fe complexes of the type (/i-SRS)[Fe(CO)3]2 are very attractive as active-site synthetic analogues, in that they are associated with fundamental properties related to the structure and the chemistry of [Fe]H2ase active site that can be summarized as follows (i) they undergo CN substitution reactions with remarkable regioselectivity, to result in the dicyanide derivatives (ii) they show fluxionality or intramolecular site exchange at Fe(CO)3 units and (iii) they have a highly reactive Fe—Fe bond that reacts with electrophiles such as FI or SMe" " to form Fe (/r-Fl)Fe or Fe (/r-SMe)Fe complexes, respectively. 

Intramolecular coordination site exchange reactions in square-planar platinum(II) complexes [Pt(tbte)Cl]+ of the potentially four-donor-atom tripodal ligand tris[2-(t-butyl-thio)ethyl]amine (tbte) N,S,S-bonded occur through a trigonal bipyramidal five-coordinate intermediate... 

With the cylindrical cryptands, each macrocycle may bind one cation so that both mono- and dinuclear cryptates may be formed. Although the 12-membered (N202) macrocycles of ligand 5 are too small to bind two cations within each of the macrocycles, variable temperature 13C-NMR measurements have revealed intramolecular cation exchange between identical sites at the top and bottom of this cryptand, for Ca2+, Sr2+, and Ba2+. Cation jump between the two sites is fast with respect to intermolecular cation exchange, modeling the elementary jump processes of cations between binding sites in membrane channels (91). 

The hexacoordinate silicon complexes discussed here are highly flexible compounds, like some other hypercoordinate silicon compounds,1 6 and undergo a variety of fluxional reorganization reactions, observable by temperature-dependent NMR spectroscopy. The dimethylamino-coordinated bis-chelates 30-38 are particularly suitable for an NMR study of ligand-site exchange processes (inter- or intramolecular), because of the pairs of diastereotopic V-methyl groups, which under certain... 

Variable-temperature H NMR studies of the reactions between alkyne-substituted clusters and protic acids indicate that protonation takes place initially at a metal center in the cluster framework with inter- and intramolecular hydride exchange. If a second protonation takes place, as happens in some cases, the site of the reaction is the organic ligand (118, 456, 457). 

The intramolecular electron exchanges among the different metal sites of polynuclear /3-diketonate-metal complexes are a matter of intrinsic interest, especially as far as the stabilization of mixed-valent oxidation states is concerned " . Obviously, the pertinent electrochemical pattern, while becoming richer and richer, also tends to become more... 

These observations offer possible explanations for some site exchanges or isomerizations that have either been explained in other ways or lack a convincing explanation at all. The intramolecular phosphine scrambling in diamagnetic [NiH(PR3)s]+, rationalized in terms... 

In the absence of a dissociative process, the remaining mechanistic options are only intramolecular nondissociative of the pseudorotation family. The nondissociative site-exchange reactions commonly discussed for octahedral complexes are the Ray-Dutt [10] and Bailar twist mechanisms [11], by which three ligands are rotated relative to the other three via a trigonal prism transition state. None of these processes effects a topomerization in one step. At least two consecutive such twists are necessary in order to bring about the observed topomerizations. However, with these mechanisms it would be difficult to rationalize the observation of two distinct barriers in 1-4. 

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