Cyclopentadienyl system

Further simulations have been performed. In contrast to what was observed for bis-cyclopentadienyl metallocenes, mono-cyclopentadienyl systems did reveal a significant barrier to insertion [lOj. However, for all these systems it turned out that insertion only proceeded after the formation of a relatively stable agostic interaction, an observation that clearly supports the Brookhart-Green mechanism. 

The five membered cyclopentadienyl system contrasts with cycloheptatrienyl Here the cation has four tt electrons is antiaromatic very unstable and very difficult... 

More than 80 % of all organotransition metal compounds are cyclopentadienyl complexes with Cp (C5H5) and Cp (CsMes) being the most prominent cyclopentadienyl systems used [1]. However, during the past few years functionalized cyclopentadienyl systems which do not just act as innocent spectator ligands have become very attractive. 

By considering the n MOs of the cyclopentadienyl system (C5H5) to result from an interaction between cri-butadiene n MOs and an sp1 hybridized C atom, explain the stability of the cyclopentadienyl anion and the instability of the cyclopentadienyl cation. 

Cyclopentadienyl systems bridged by a single silicon-containing unit are described in Section II.E.l. Cyclopentadienyl units which are held together by a double bridge are described in Section II.E.2. Section II.E.3. describes cooperative effects exerted by dimethylsilyl groups as bridging units, with special emphasis on polyferrocenyl compounds. 

There are several classes of Re(II) complexes that exhibit luminescence in both solution and in the solid state, the cyclopentadienyl systems Cp2Re and Cp 2Re [23] and the singular case of the homoleptic diphosphine, Re(dmpe)3 2+ (Cp is pentamethylcyclopentadienyl) [24]. Emission has been shown to be LMCT in nature (Re - P sigma bond to dn Re(II)). These complexes are also rare examples in transition metal photochemistry of doublet-double emission, i.e., a fluorescence. 

As can be seen from Scheme III, lanthanide halides are suitable precursors for the synthesis of homoleptic derivatives such as silylamides [114], cyclopen-tadienyls [115] and aryloxides [116]. Such organometallies can be readily obtained in a pure form by sublimating them from the reaction mixture. They themselves are important precursors in organometallic transformations (vide infra). Heteroleptic complexes of the type CpxLn(halide)y (x + y = 2,3) are important synthetic precursors with respect to formation of various Ln-X bonds via simple metathesis reactions [2-29]. Fig. 4 indicates the lanthanide element bonds which are involved in these ubiquitous heteroleptic cyclopentadienyl systems. 

Thulium(II) complexes are stabilized by phospholyl or arsolyl ligands that can be regarded as derived from the cyclopentadienyl group by replacing one CH group by a P or As atom. Their decreased n-donor capacity relative to the parent cyclopentadienyl system enhances the stability of the Tm(II) center, and stable complexes of the bent-sandwiched type have been isolated. 

It is interesting that the [ (CHsIskTi, 14-electron metallocene car-benoid even inserts into the hydrocarbon-like C—H bond of a penta-methylcyclopentadienyl ligand. Significantly, however, the reaction is reversible, whereas with bis(T)-cyclopentadienyl) systems the formation of complex hydrides (Sections II,A,1 and 2) seems to be an irreversible pro-... 

The chemistry of titanium and zirconium, bis(ij-pentamethylcyclopen-tadienyl) systems is essentially that of monomeric fo-CsfCH iM units. With the cyclopentadienyl systems, nearly all of the chemistry observed is that of dimers. Although the dimeric hydride fi-(tj5 tj5-C,0H8 )-/x(H2 C5H5)2Ti2 (3) is coordinatively saturated and relatively unreactive, the partially unsaturated, dimeric metallocene /t-( 1 i -C H )(,i>-CjH )3Ti1 (10) shows considerable chemical reactivity toward N2 (Section III,D) as well as interesting catalytic properties (Section VI). The behavior of dimer units in the cyclopentadienyl systems is exemplified by the unusual naphthalene ring binding in the naphthyl hydride zirconocene derivative... 

Figure 1 Plot of the C-C and C=C pair numbers (2F ) for the substituted cyclopentadienyl systems against the homo-moleoular-homodesmotio REs (E(H), in koalmol ). From D. B. Chesnutand L. J. Bartoiotti, Chem. Phys., 2000, 257, 175.

Thermal activation of alkyne-substituted clusters frequently results in the loss of one or more carbon monoxide ligands (418, 445, 446). Concomitant with this loss is an alteration in the bonding mode of the organic ligand in order to retain the electron balance within the molecule (107). Such a reaction is shown in Fig. 41, where an osmacyclopentadiene ring is transformed into a trisubstituted-f/5-cyclopentadienyl system. Metal-metal bond formation may take place in some examples (446, 447). 

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