Cyclopentadienyl compounds of group

Fig. 3.22 Structures of some cyclopentadienyl compounds of Group II elements.

Within compounds containing metals belonging to the same vertical sequence, that is, normally isostructural compounds with the same number ofvalence electrons for the central metal atom (for example, the hexacarbonyls of group 6), the order of reactivity is often 3d < 4d > 5d, with the exception of the cyclopentadienyl derivatives of group 5, MCp(CO)4 (M = V, Nb, Ta), for which the order of reactivity is V > Nb > Ta. Relevant to these findings are the data provided in Table 6 showing that the calculated first CO dissociation energy for the hexacarbonyls and the tetracarbonyls is lowest for the 4d term. Thus, at least for dissociative mechanisms, the order of... 

Note that metallocene terminology (Section IR-10.2.6) is not recommended for bis(cyclopentadienyl) compounds of the main group metals (see Examples 6 and 7). 

Cyclopentadienyl compounds of the form CpCo(CO)2 are also known to convert l,n-enynes into the tranditional 1,3-diene cycloisomerization products (486, 487). Remarkably, the metathesis reaction is independent of tether length between both unsaturated functional groups, and leads to five-membered ring carbocycles exclusively (Scheme 60). 

Although the cyclopentadienyt group, C5H5, has long been known to form compounds with metal or metalloidal atoms, no such compound was considered noteworthy until the first cyclopentadienyl compound of a transition metal was reported in 1951. This compound was bis(pema/mptocyclopen-tadienyl)iron, (7 5-C5H5)2Fe, commonly called ferrocene. X-ray studies, as well as a variety of other physical and chemical evidence, showed that ferrocene has a sandwich structure (23-XII). The remarkable structure of this molecule as well as its extraordinary thermal stability (to above 500° C)... 

A small number of self-assembled cyclopentadienyl compounds containing Group 15 elements have been reported in the literature. Thermolabile, light-sensitive (CsH5)SbCl2, 54, was prepared according to Eqs(2) and (3) [56-58]. The route via trimethylstannylcyclopentadiene gives quantitative yield. 

Cyclopentadienyl compounds of this group are most commonly obtained by the reaction of Nacp with metal halides MC1 . Some properties of cyclopentadienyl complexes of the vanadium group are presented in Table 9.10. The properties of [Mcp(CO)4] are given in Chapter 2. 

In cyclopentadienyl compounds of copper such as [Cucp(CO)] and [Cucp(PR3)] (R = Et, Bu, Ph) the C5H5 group is a pentahapto ligand. Formally these compounds have 18 valence electrons. However, because of a considerable lowering of the energy of M copper orbitals and a substantial increase in the energy difference between the Ap and As orbitals, the orbitals of copper probably interact weakly with the orbi-... 

Synthesis of cyclopentadienyl compounds of metals from cyclopentadiene depends on the strongly acidic properties of cyclopentadiene. This acidity may be attributed to the ready formation of the highly resonance-stabilized cyclopentadienide anion, CsHs , in which there are six delocalized ir electrons. This method is limited primarily to the active metals of Groups lA and IIA and to their strongly basic compounds. The compounds produced are principally the ionically bonded metal cyclopentadienides. 

Addition to the cyclopentadienyl ring or to the metal atom is a frequently used synthetic method, especially for the compounds of Group VIII elements. An example of an addition reaction (94) is... 

Synthetic routes to mixed cyclopentadienyl/guanidinato complexes of Group 4 metals have also been developed. Scheme 78 illustrates two typical reactions leading to such compounds. 

Catenated Organic Compounds of the Group IV Elements, 4,1 Conjugate Addition of Grignard Reagents to Aromatic Systems, 1, 221 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365 Diene-Iron Carbonyl Complexes, 1, 1... 

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