7r-cyclopentadienyl ligands

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

In some cases complexes also containing (T-cyclopentadienyl groups may be formed in addition to 7r-cyclopentadienyl ligands ... 

The first acetyl group appears to deactivate the 7r-cyclopentadienyl ligand toward further electrophilic substitution. Thus, the second acetyl group enters the other ring. 

For similar reasons, n-C, W-C5 and n-C rings are able to rotate quite readily about the metal-ligand axis. For example the energy barrier to rotation of the 7r-cyclopentadienyl ligands in ferrocene is about 1 kcal./ mole. 

Another example of the exchange of a 7r-cyclopentadienyl ligand is shown in Figure 34. Possibly these exchange reactions involve an intermediate containing a bridging ir-bonded ligand system, i.e. M—... 

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. 

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. 

Linear correlations between l(L) and Hammett substituent constants for substituted jr-cyclopentadienyl and 7r-arene ligands [69], for other substituted Kgands,... 

However, its isoelectronic unsymmetrical counterpart, 7r-cyclopen-tadienyl-7r-cycloheptatrienylchromium(0) can be reduced to a stable anion radical (41). The orbital occupied by the unpaired electron has dominant ligand tt character. It appears to consist of a combination of the antibonding e2 MO s of the cyclopentadienyl (Cp) and cyclohepta-trienyl (Tr) 7r-systems with about one-third Cp and two-thirds Tr, respectively (41). Reduction of 7r-cyclopentadienyl-7r-methylcyclohepta-trienylchromium leads to an unstable radical anion that decomposes by loss of the seven-membered ligand (41). 

Recently, in a study of a large number of organo-iron compounds 120), it has been found possible to treat the isomer shift as being due to a sum of partial isomer shifts 8,. from individual ligands which for 7r-cyclopentadienyl derivatives could be correlated with proton NMR chemical shifts in the same compounds. The Mossbauer spectra of two cases of theoretical interest are worthy of mention. First, the spectra of both the mono- and binuclear iron tricarbonyl derivatives of cyclooctatetraene 115) show similar 8 and A values. The small 8 values are consistent with zero oxidation state for the iron atom and essentially complete covalent bonding between the iron and the tt electrons of the ring, so that, at least for the mononuclear... 

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... 

Several monosubstituted derivatives have been made in a similar manner, except that a unidentate ligand is used in place of the carbon monoxide, namely NH3, N2H4 (88) C2H4 (75) PPh3, AsPh3, CH3CN (231). Alternatively, substituted monooleiin derivatives have been synthesized by protonation of the metal a-allyl 7r-cyclopentadienyl tricarbonyl (55, 112). 

Also, the transfer of electronic effects was studied in the cyclopentadienyl-titanium-ligand systems, viz. the (7r-CjH5)-ligand interaction, and the interaction of the ligand with the tt-CjH,—Ti bond. 

By 1957, a variety of both 7r-cyclopentadienyl and arene-metal complexes was known, and it was interesting to determine whether mixed-sandwich complexes containing both of these ligands around the same could be produced. Two early examples of such complexes are given below ... 

Complexes of a related anionic bidentate di-carboxamido-cyclopentadienyl ligand, ZrCl3[CsI I3(COM IBur)2](TI IF) and ZrCyCsI I3(CONIIBut)2]2 (91) have also been described. This ligand chelates to Zr via the carbonyl O-atoms in a manner similar to diketonates. Structural data were consistent with 7r-delocalization of electron density in the ligand.466... 

The first known complex in this class, frans-bis(triethylphosphine)-platinum chlorohydride, was prepared by Chatt and co-workers (86). In the light of other hydrides known at that time, in particular the carbonyl hydrides of iron and cobalt and bis(7r-cyclopentadienyl)rhenium hydride, the great stability of this complex was quite unique. It rapidly became clear that tertiary phosphine ligands were markedly effective in stabilizing the metal-hydrogen bond, and ligands such as PhjP, EtjP, o-CaH PlVfe, ... 

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