Ligand-bridged complexes

L = CO, ethylene, propene, pent-l-ene, butadiene, H2S, THF, acetone, acetonitrile, nitromethane). The moiety [Re(CO)5] + can also be added to a nucleophilic atom of a coordinated ligand, which provides a systematic way to prepare ligand-bridged complexes,12 for example,... 

Scheme 2 Photochemical and thermal transformations of the ligand bridged complex [(bpy)(CO)3ReP-PRe(CO)3(bpy)]2+ (P-P is trans- 1,2-bis-diphenylphosphinoethylene)...

The ligand-bridged complexes [Rh2(CO)4 (PPh3) (/x-RNXNR)2] (n = 1 or 2, R = aryl, X = N or CMe) undergo reversible one-electron oxidation to isolable monocations (275), which appear to be fully delocalized mixed valence complexes (276). Oxidation also leads to enhanced susceptibility to carbonyl substitution ( = 1, X = CMe), and a drastic shortening in the metal-metal bond distance implies electron loss from an anti-bonding dimetal orbital (275) (Section III,F). 

The efficiency of long-distance energy transfer and its gradual decrease with intercomponent distance is usually described by an exponential decay law of the type Fab = F) exp (-yJtAe), where y is denoted the attenuation factor and 1 ab is the spatial separation between the redox sites. In the case of ligand-bridged complexes is set as the distance between the metal centres. 

Figure 3.6.1 Outer-sphere and inner-sphere reactions. The inner sphere homogeneous reaction produces, with loss of H2O, a ligand-bridged complex (shown above), which decomposes to CrCl(H20) + and Co(NH3)5(H20). In the heterogeneous reactions, the diagram shows a metal ion (M) surrounded by ligands. In the inner sphere reaction, a ligand that adsorbs on the electrode and bridges to the metal is indicated in a darker color. An example of the latter is the oxidation of Cr(H20)5 at a mercury electrode in the presence of Cl or Br .

A number of binuclear phosphine-ligand bridged complexes have been shown to function as catalysts or catalyst precursors. Here we review these cases briefly. More detailed coverage of hydrogenation and hydroformyla-tion using rhodium catalysts will appear in the next chapter. In all cases of catalytic activity shown by binuclear complexes, there is a serious question about the true identity of the catalytically active species. As with catalysis begun by metal carbonyl clusters, the possibility exists that a small amount of highly active mononuclear compound is the true catalyst. 

Polypyridyl ligand-bridged complexes in which coupling appears to be promoted by ligand-to-metal charge transfer (hole transfer systems)... 

Light absorption by MMCT bands of heteronuclear ligand-bridged complexes is well known to induce photoredox reactions [13,60]. Suitable electron acceptors include Co(IU) compounds while cyano complexes of Fe(II), Ru(II), and Os(II) may be used as donors., e.g., equation (44) ... 

---