Cyclopentadienyls other ligands

The bulk of derivatives are obtained by the displacement of CO by other ligands. These include phosphines and other group 15 donors, NO, mer-captans and unsaturated organic molecules such as alkenes, alkynes and cyclopentadienyls. 

More than twenty years ago, Nesmeyanov s group showed that chlorine can be substituted by a variety of nucleophiles in FeCp(r 6-PhCl)+ [83, 84]. Indeed the chlorine substituent in the chlorobenzene (even) ligand is 1000 times more reactive than when it is located on the cyclopentadienyl (odd) ligand [85]. The FeCp+ is a good withdrawing group which is equivalent to two nitro groups in terms of activation. The reactions proceed under ambient conditions with primary or secondary amines and have been extended to other substituted chloroarene complexes [86, 87] Eq. (22), Table 2. 

Bis(cyclopentadienyl) complexes are central to the organometallic chemistry of the early transition metals and feature in applications such as alkene polymerization chemistry. Parallels can be drawn between a porphyrin ligand and two cyclopentadienyl ligands, in that they both contribute a 2— formal charge and exert a considerable steric influence on other ligands in the same molecule. Several of the metalloporphyrin complexes discussed below have bis(cyclopentadienyl) counterparts, and authors in some ca.ses have drawn quite detailed comparisons, although these discussions will not be repeated here. 

Fig. 6 Schematic representation of cyclopentadienyl-type precursors applied in ALD processes. The dashed line represents the approximate suitability level for ALD processing as a function of ionic radius. Below this line decomposition takes place. Compounds which contain both cyclopentadienyl and some other ligands have been omitted (e.g., Cp2Zr(CH3)2 and (C5MeH4)Mn(CO)3)...

The hydridotris(pyrazolyl)borate (Tp) ligand has become well known and well established as a formal analogue of the cyclopentadienyl (Cp) ligand (43). Unlike Cp, however, when appropriately substituted, Tp can become what has been described (44, 45) as a tetrahedral enforcer ligand, producing complexes that are constrained to be tetrahedral even when other factors might allow octahedral coordination. 

The molecules selected as precursors to titanium carbide can be divided into three types those containing a fulvalene group (1, 2), those bearing a carbonyl ligand (3) and those containing chlorine substituents (4-8). In the latter family, the other ligand can be either a cyclopentadienyl (4-6) or an alkylsilyl substituted cyclopentadienyl (7, 8). 

The method involving reductive C5HS removal using alkali metals can also be applied to other i)5-cyclopentadienyl transition metal compounds. In addition, other ligands such as phosphanes, carbon monoxide or dinitrogen can be used instead of olefins. 

Osmium forms a wide variety of alkyl and aryl complexes including homoleptic alkyl and aryl complexes and many complexes with ancillary carbonyl (see Carbonyl Complexes of the Transition Metals), cyclopentadienyl (see Cyclopenta-dienyl), arene (see Arene Complexes), and alkene ligands (see Alkene Complexes). It forms stronger bonds to carbon and other ligands than do the lighter elements of the triad. Because of this, most reactions of alkyl and aryl osmium complexes are slower than the reactions of the corresponding ruthenium complexes. However, because osmium is more stable in higher oxidation states, the oxidative addition (see Oxidative Addition) of C-H bonds is favored for osmium complexes. The rate of oxidative addition reactions decreases in the order Os > Ru Fe. 

A recent development in the low-valent lanthanide area is the synthesis and X-ray structural determination of the unsolvated complex (C5Me5)2Sm 121). This species is the first structurally characterized bis(cyclopentadienyl)lanthanide species which has no other ligands in the metal coordination sphere. As such, it is the closest lanthanide analogue of the bis(ring)metallocene sandwich compounds like ferrocene. Samarocene was obtained by desolvation of (C5Me5)2Sm(thf)2 under high vacuum and sublimation of the product [Eq. (58)]. The sublimed crystals have a bent... 

Some cyclopentadienyltitanium complexes react with FeClj or iron acetylacetonate with transfer of the cyclopentadienyl ligand from titanium to iron. The yield of ferrocene also depends on the other ligands on titanium. The yield increases with decreasing number of electron-attractive halogen atoms in the molecule (see Table IV). 

Further attempts to replace alkoxy groups with other ligand systems, like cyclopentadienyl, in order to reduce the Lewis acidity of the metal atom have been unsuccessful."... 

Complexes of the 4 type catalyze the hydroboration of various olefins with catecholborane at ambient temperature [173], The proposed mechanism of the hydroboration reaction - although not within the scope of this book - parallels that of the hydrogenation and hydrosilylation reactions. The architecture of both olefins (terminal > terminal disubstituted > internal disubstituted > trisubstituted) and organolanthanides (TOF(La) 10 TOF(Sm) TOF(5) = 4 TOF(4) affects the rate of hydroboration, which for 4(La CH(SiMc3)2) and 1-hexene is TOF = 200 h , for example. The observed high regioselectivities are exclusively anti-Markovnikov. For smaller metal centers (Y, Zr, Ti) and other ligand systems (bis(cyclopentadienyl), bis(benzamidinato)) inactivation of the catalyst by catecholborane or Lewis base-metal complex induced disproportionation of catecholborane appeared to compete effectively with the catalytic conversion [174]. 

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