Metal-Bonded Cyclopentadienyl Anions

The CC bond distances in cyclopentadienyl anion, C5H5, are all equal, because the anion is aromatic (see Chapter 12, Problem 10). Electrophiles that interact electrostaticaUy with the anion, such as Na , interact equally with all five carbons, and do not disturb the anion s aromatic character. On the other hand, electrophiles that make covalent bonds, such as H , might interact more strongly with one particular carbon and destroy the aromaticity of the ring. 

Describe the similarities and differences in geometries, charge distributions and electrostatic potential maps for cyclopentadienyl sodium, cyclopentadiene and cyclopentadienyl anion. 

consider how Fe interacts with C5H5. Examine the geometry of ferrocene, Fe(C5H5)2. Are the FeC distances all the same, or does iron bond more strongly to some carbons than to others Are the CC bond distances all the same Which of the above models, the electrostatic or covalent, gives the better description  

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The 772-phosphinocarbene complexes of tungsten show ambiphilic behavior. With Lewis acids such as MeS+, electrophilic attack occurs at the metal-carbon double bond affording the dicationic tunstaphosphathiabi-cyclo[1.1.0]butane complexes 102.96,97 On the other hand, nucleophiles such as trialkyl phosphines or the cyclopentadienyl anion C5H5 add at the carbe-nic center, affording phosphoranylidene complexes 10397 or tungstaphos-... 

Alike metallocomplex anion-radicals, cation-radicals of odd-electron structure exhibit enforced reactivity. Thus, the 17-electron cyclopentadienyl dicarbonyl cobalt cation-radical [CoCp(CO)2] undergoes an unusual organometallic chemical reaction with the neutral parent complex. The reaction leads to [Co2Cp2(CO)4]. This dimeric cation-radical contains a metal-metal bond unsupported by bridging ligands. The Co—Co bond happens to be robust and persists in all further transformations of the binuclear cation-radical (Nafady et al. 2006). 

The first metal-olefin complex was reported in 1827 by Zeise, but, until a few years ago, only palladium(II), platinum(Il), copper(I), silver(I), and mercury(II) were known to form such complexes (67, 188) and the nature of the bonding was not satisfactorily explained until 1951. However, recent work has shown that complexes of unsaturated hydrocarbons with metals of the vanadium, chromium, manganese, iron, and cobalt subgroups can be prepared when the metals are stabilized in a low-valent state by ligands such as carbon monoxide and the cyclopentadienyl anion. The wide variety of hydrocarbons which form complexes includes olefins, conjugated and nonconjugated polyolefins, cyclic polyolefins, and acetylenes. 

Although most alkali metal compounds have M—C a bonds, cyclopentadienyl 7r-complexes are known for Li, Na, and K. These are commonly polymeric but solvation can break the chains and even result in cation-anion separation.44... 

In addition to substitution of the carbonyl groups, changes in the cyclopentadienyl ring have also been extensively studied. The use of pentamethylcyclopentadiene led to formation of some of the first organometallic multiply bonded complexes as discussed in Section 4.8. The tris(pyrazolyl)borate ion (Tp , see Tris(pyrazolyl)borates) is a Cp analog." It reacts with Mo(CO)6 to yield the TpMo(CO)3 anion. Oxidation yields the paramagnetic radical that shows no tendency to form a single metal-metal bond. Decarbonylation yields a triply bonded stracture. These reactions are summarized in equation (16). 

Metal 77-cyclopentadienyls somewhat resemble the rr-allyl complexes. Initially, when the nature of the metal-allyl bond was not sufficiently clear, the similarity was emphasized many times [see the review by E. O. Fischer (425)]. The similarity shows itself, for example, in the equal antisymmetric C—C stretching frequencies (1640 cm ), which indicate that the force constants, hence the bond orders, are close. The central rr-allyl proton absorbs in the same NMR region as do the protons of coordinated cyclopentadienyl. Both ligands display the symmetrical sandwichlike bond with their metals. Today, however, it is clear that the complexes differ significantly in type, the difference being associated first of all with the fact that TT-allyl complexes are much more efficient than 77-cyclopentadienyls at transforming to o-allyl or 77-olefin compounds. This may be due to the difference between the delocalization energies, 2.472 and 0.828 eV for cyclopentadienyl and allyl anions, respectively (426). 

Cyclopentadienyl—metal bonds may be synthesized by direct reaction of cyclopentadiene with certain metals and their compounds or by reaction of ionic metal cyclopentadienides with compounds of metals wherein cyclo-pentadienide anion displaces a ligand bonded to the metal. These bonds may also be formed in a limited number of instances by reaction of alkenes or alkynes with metal compounds. 

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