Stabilization with traditional ligands

Finally, the term steric stabihzation coifid be used to describe protective transition-metal colloids with traditional ligands or solvents [38]. This stabilization occurs by (i) the strong coordination of various metal nanoparticles with ligands such as phosphines [48-51], thiols [52-55], amines [54,56-58], oxazolines [59] or carbon monoxide [51] (ii) weak interactions with solvents such as tetrahydrofuran or various alcohols. Several examples are known with Ru, Ft and Rh nanoparticles [51,60-63]. In a few cases, it has been estab-hshed that a coordinated solvent such as heptanol is present at the surface and acts as a weakly coordinating ligand [61]. 

FIGURE 8.4 Dependence of water exchange rate constants for ions with no ligand field stabilization energy on the surface potential of the central metal ion, using traditional ionic radii... 

Abstract This chapter focuses on carbon monoxide as a reagent in M-NHC catalysed reactions. The most important and popular of these reactions is hydro-formylation. Unfortunately, uncertainty exists as to the identity of the active catalyst and whether the NHC is bound to the catalyst in a number of the reported reactions. Mixed bidentate NHC complexes and cobalt-based complexes provide for better stability of the catalyst. Catalysts used for hydroaminomethylation and carbonyla-tion reactions show promise to rival traditional phosphine-based catalysts. Reports of decarbonylation are scarce, but the potential strength of the M-NHC bond is conducive to the harsh conditions required. This report will highlight, where appropriate, the potential benefits of exchanging traditional phosphorous ligands with iV-heterocyclic carbenes as well as cases where the role of the NHC might need re-evaluation. A review by the author on this topic has recently appeared [1]. 

Several points are worth mentioning about the TBP family of compounds. First, the structure with axial Mn(lll) ions was obtained only in combination with equatorial Mn(II) ions, which can be explained by redox processes that take place when Mn(lII) is combined with other divalent metal ions, as well as the instability of the [Mn° CN)6] building block in the presence of protic solvents and the absence of free CN. A similar situation is observed in the case of equatorial Cr(II) ions that remain stable in the TBP cluster only when combined with axial Cr(III) ions. The only other complex with Cr(ll) was obtained in combination with [Ru (CN)5] , but the final product, [Cr (tmphen)2][Cr (tmphen)2]2[Ru (CN)6l2, contains both Cr(II) and Cr(lll) in the equatorial positions due to the reduction of axial Ru(lll) to Ru(ll) ions. Second, compounds [M(tmphen)2l3[M (CN)6l2, where M = Ru or Os, are the only structurally characterized complexes of hexacyanoruthenate(ni) and hexacyanoosmate(III) reported to date (178). Finally, the use of 7i-accepting tmphen ligands to protect the equatorial positions of the TBP cluster allows for stabilization of oxidation states that are not possible in the traditional extended structures of PB type. In the early literature, there were... 

---