Structure cyclopentadienyl

Its aromaticity cannot, of course, be tested by attempted electrophilic substitution, for attack by X would merely lead to direct combination with the anion. True aromatic character (e.g. a Friedel-Crafts reaction) is, however, demonstrable in the remarkable series of extremely stable, neutral compounds obtainable from (15), and called metallocenes, e.g. ferrocene (16), in which the metal is held by n bonds in a kind of molecular sandwich between the two cyclopentadienyl structures ... 

In each case, as the temperature was raised to 200 K, a markedly changed VEEL spectrum was observed, which was attributed to the formation of cyclopentene by dehydrogenation (see Section VI.B). Avery s study of the adsorption of cyclopentane was continued to 260 K, whereby a much simpler spectrum was obtained, convincingly attributed to the formation of the 175-C5H5 (i75-cyclopentadienyl) structure adsorbed flat on the surface. The 200 K spectrum of the species on Ru(0001) may even contain some features characteristic of this species (strong bands at 758 and 3057 cm 1). 

Figure 2.43 Tl -TlICeoPhsl-a complex with fivefold coordination of the thallium atom which is Tl

Cr, Mo and W - The gas-phase ion chemistry of the chromium cycloheptatriene complexes [Cr(T -C7H8)(CO)3] was studied272 by Fourier Transform Ion Cyclotron Resonance Spectroscopy. The presence of a fragment ion [C5H6Cr]+ with the hydrido-ri -cyclopentadienyl structure was detected. Reaction of the same complex [Cr(Ti -C7H8)(CO)3] with the carbene complex [Cr(CO)5(=C(OMe)Me)] afforded273 an isomeric mixture of bicyclo[4.2.1]nonanone products. 

Figure 31 shows the proposed Longuet-Higgins loop for the cyclopentadienyl cation. It uses the type-VI Ai anchors, with the type-VII B structures as transition states between them. This situation is completely analogous to that of the radical (Fig. 23). Since the loop is phase inverting, a conical intersection should be located at its center—as required by the Jahn-Teller theorem.

The structure of the metallocene cation energy minimised with the Car-Parrinello method agrees well with the experimentally obtained crystal structures of related complexes. Typical features of the structure as obtained from X-ray diffraction on crystals of very similar neutral complexes (e.g., the dichlorides), such as small differences in distances between C atoms within a cyclopentadienyl (Cp) ring, as well as differences in distances between the C atoms of the Cp ring and the Zr atom, were revealed from the simulations. 

Representation of such a system by a connection tabic having bonds between the iron atom and the five carbon atoms of either one of the two cyclopentadienyl rings is totally inadequate. A few other examples of structures that can no longer be adequately described by a standard connection table are given in the Section 2.G.2. 

Write resonance structures for cyclopentadienyl anion suffi cient to show the delocalization of the negative charge over all five carbons J... 

Section 14 14 Transition metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity Organic ligands can be bonded to a metal by a ct bond or through its it system Metallocenes are transition metal complexes m which one or more of the ligands is a cyclopentadienyl ring Ferrocene was the first metallocene synthesized Its electrostatic potential map opens this chapter... 

Fig. 4. Representative structures for compounds of molybdenum(IV) (a) bis(diaIkyldithiocarbamato)oxomolybdenum(IV), MoO(S2CNR2)2, where R = alkyl (b) / Jtetracyanodioxomolybdenum(IV), Mo02(CN) (c) M03SJ3 (d) Mo3S4(SCH2CH2 )1 (e) Mo304(H20)g" (f) the Mo M S thiocubane core stmcture (g) bis(cyclopentadienyl)dichloromolybdenum(IV), CP2M0CI2, where Cp = cyclopentadienyl.

Fig. 5. Representative structures for compounds of molybdenum(III) (a) hexacholoromolybdenum(III) ion, MoClg (b) bexabis(dimethylamiHo)dimo1ybdeniim (TTT), Mo (N(CH ) ) (c) the Mo S thiocubane core stmcture (d) dichlorocyclopentadienyl triaIkylphosphinedichloromolybdenum(III), CpMo(PR2)Cl2, where Cp = cyclopentadienyl and R = alkyl.

Figure 5.10 Cyclopentadienyl derivatives of beryllium showing (a) the slruclure of fBe(ij -C5H5)Y] and (b) (he structure of crystalline IBe(/j -C5H5)(ij -C5H5)] at —120 (see text).

Tlie importance of bis(cyclopeniadienyl)irou (Fe(jj -C5H3)2( in the developmenl of organo-metallic chemistry has already been alluded to (p. 924). Tile compound, which forms orange crystals, mpl74°, has extraordinary thermal stability (>500°) and a remarkable structure which was unique when first established. It also has an extensive aromatic-lype reaction chernisiry which is reflected in its common name ferrocene The molecular structure of ferrocene in the ciysialline slac features two parallel cyclopentadienyl rings at one lime Ihese... 

Cationic cyclopentadienyl complexes are not common in this group, but recent examples whose structures have been determined include [Nb ( j -C5H5)2Cl2]BF4< > and [Nb( j -C5H5)2L2](BF4)2 (L = CNMe and NCMe),< > which have pseudo-tetrahedral symmetry. 

At about 159°C it turns pink and adopts the sandwich structure, expected for [M(C5H5)2] compounds, and this is retained in the gaseous phase and in hydrocarbon solutions. Using substituted cyclopentadienyls a variety of analagous sandwich compounds have been prepared and their magnetic properties indicate that the... 

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