7r-cyclopentadienyl complexes

P. Jutzi and N. Burford (1999) Chemical Reviews, vol. 99, p. 969 - Structurally diverse 7r-cyclopentadienyl complexes of the main group elements . 

Sandwich complexes Mcp2 are thermally very stable. They melt at relatively high temperatures, usually without decomposition. The resistance of metallocenes to oxidation varies. Some are completely air stable, for example, ferrocene and ruthenocene. However, some are pyropophoric, such as chromocene and vanadocene which spontaneously inflame in air. Often during controlled oxidation, oxo 7r-cyclopentadienyl complexes are obtained. 

These complexes also undergo many of the reactions shown for ferrocene, particularly the aromatic substitution reactions. An additional outline of some reactions of mono-7r-cyclopentadienyl complexes are shown in Fig. 6-5. These compounds also undergo carbonyl substitution reactions. 

The sandwich bis-7r-cyclopentadienyl complexes are thermally rather stable, and many melt without decomposition at about 173X. They are stable to hydrolysis, and the C5H5 rings resist catalytic hydrogenation. Their stability to oxidation, however, varies greatly with the nature of the metal. At room temperature the 18-electron complex ferrocene is inert to molecular oxygen, whereas chromocene (which possesses a 16-electron configuration) is pyrophoric in air. 

The five molecular 7r-orbitals of an isolated cyclopentadienyl group are constructed from the five 2/77i-atomic orbitals and are pictorially represented in Figure 22. Since in the bis-7r-cyclopentadienyl complexes, the metal atom... 

X-ray studies show to contain a yr-cyclopentadienyl system [75] as is shown in 8.21. In the preparation of this complex a hydrogen migration must have occurred. Related complexes with 7r-cyclopentadienyl ligands are formed from phenylacetylene [52c] and fulvenes (see Figure 81). Other 7r-cyclopentadienyl complexes and some olefin-iron complexes formed from fulvenes are also shown in Figure 81. 

Modification of the acceptor properties of the metal atom may be achieved by using complexes containing 7r-cyclopentadienyl ligands. 

Several mixed 7r-benzene-jr-cyclopentadienyl complexes have been investigated in UPS in order to compare the properties of these groups as ligands. The ds system Cr(cp)(bz) was investigated by Evans etal (57) together with the d6 complexes Cr(bz)2, Cr(tol)2, and Mn(cp)(bz). The spectmm of Cr(cp)(bz) shows in the region... 

An approach other than steric hindrance has been used to overcome the previously mentioned instability of the actinide homoalkyls. It was found that the inclusion of jT-bonding ligands in the coordination sphere considerably enhanced the stability of the alkyl complex. Recently, the same line of reasoning has also yielded a new series of 7r-cyclopentadienyl lanthanide alkyls (C5H5)2LnR where Ln =Gd, Er, Yb and R = C=C, and CH3 120,121). The infrared data for these complexes are consistent with u-bonded structures and the room temperature magnetic susceptibilities are very close to the free ion values. The actinide complexes (75,... 

The Lever s model has also been extended to sandwich and half-sandwich complexes with 7T-cyclopentadienyl or 7r-arene ligands [66, 67, 69]. The parameter for the 7r-ligands has been defined [69] on the basis of the low spin Fe hH redox couple, by Eqs. (21) or (22), for homolep-tic sandwich [Fe( 7r-L)2] or mixed sandwich ]Fe( 7r-Li)( 7r-L2)] complexes, respectively. 

The ESR spectra of a series of triarylboron anion radicals have been determined and show selective line-broadening effects (95). The radical anions of diborane, generated in an argon matrix at 77 K by irradiation with a-rays have been studied by ESR (121). The vanadium-aluminum complex obtained on mixing dichlorobis(7r-cyclopentadienyl)vanadium with ethylaluminum dichloride, a system that catalyzes the polymerization of ethylene to give a polymer with good characteristics, has been characterized by ESR and ultraviolet studies (44). 

The failure of various 7r-cyclopentadienyl-metal and 7r-arene-metal complexes to undergo aromatic-type substitution reactions does not necessarily mean... 

The types of compounds formed by gold(I) and gold(III) often differ from those of other metals due to the constraints imposed by coordination number and electron count at the metal. Thus, for example, whereas 7r-bonded cyclopentadienyl complexes of palladium and platinum are numerous (336), and a copper(I) species of this type is known (337), cyclopentadienyl complexes of univalent (94, 96, 97) and trivalent (228) gold have invariably been found to be fluxional behavior, similar to that in dicyclopentadienylmercury, was involved (228). 

A number of readily reversible cr-7r rearrangements have been observed wherein a labile ligand such as carbon monoxide is lost by pyrolysis or photolysis, producing a coordinatively unsaturated metal center, which then regains coordinative saturation by means of a tr-n rearrangement. For example, irradiation of o--alkyl-7r-cyclopentadienyl-molybdenum tricarbonyl (15) produces the rr-allene complex (16) (25). These... 

Similarly protonation of diphenylfulveneiron tricarbonyl generates a substituted 7r-cyclopentadienyl cation (241, 242). Hydride abstraction from cyclopentadieneiron tricarbonyl releases the 77-cyclopentadienyl cation complex (172). The Mossbauer spectra of the [CpFe(CO)3]1 cation and related iron carbonyl cations have been determined (121). 

A number of cyclopentadienyl complexes of Pd are known. In most cases, the other groups are 7r-allyl,... 

Greco, A., Cesca, S., and Bertolini, G. (1976) New 7r-cyclooctateTraenyl andiT-cyclopentadienyl complexes of cerium. Journal of Organometallic Chemistry, 113, 321. 

Similarly, with spiro[4.4]nona-1,3-diene and spiro[2.4]hepta-4,6-diene, Fe(CO)5 produces the 7r-cyclopentadienyl-type binuclear complexes (248, reference 3), while Fe2(CO)fl affords the tt complexes (90) and (91), respectively. With the latter olefin, the complex of structure... 

Wilkinson had observed an associative exchange of CjH, rings analogous to that of (77-allyl)(7r-cyclopentadienyl)palladium and other related complexes discussed below. 

The reactivity of such compounds has been studied in detail 201-212). Substitution of the 7r-cyclopentadienyl group has been investigated for monocyclopentadienyl halogen and alkoxytitanium derivatives, dicyclo-pentadienyltitanium halides and alkoxides 201-211), and for (77-a llyl) (77-cyclopentadienyl)palladium 212a). The stereochemistry of these complexes has been confirmed unequivocally by physical methods including X-ray studies. The structures of (77-allyl)(7T-cyclopentadienyl)palladium 213) and of (77-cyclopentadienyl)titanium trichloride 214) are shown. 

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