Fullerene complexes bonding

Several effects can influence the electronic structure of Cjq upon metal complex formation. One is the removal of one double bond from the remaining 29 fullerene double bonds. As in any polyene system, this decreased conjugation is expected to raise the energy of the LUMO and therefore decreases the electron affinity of the system. Conversely, the d-orbital backbonding transfers electron density from the metal into n orbitals of the remaining double bonds, which also decreases the electron affinity. 

Because the fullerene hemisphere not directly involved in metal bonding is little altered by metal complexation, it is not surprising that multimetallic adducts can be formed. However, the strength of fullerene-metal bonding, the size of the addend, and the ease of dissociation also play a part in determining the likelihood of isolating such a species. 

Although complexes of Cgo have been studied most extensively, some complexes of other fullerenes have also been prepared. An example is (in -C7o)Ir(CO)Cl(PPh3)2, shown in Figure 13-37. As in the case of the known Cso complexes, bonding to the metal occurs at the fusion of two 6-membered rings,... 

Almost simultaneously, a platinum complex, in which the metal atom is bonded directly to the fullerene cage, was prepared.[Fa91 ] The site of attachment is the 6 6 double bond, as for the osmium derivative. The carbon bond lengths in the fullerene complexed with Pt are also similar to neat Ceo and to the Os derivative described above. Further details and other metal attachments are described in a review paper.[Fa92]... 

Recent additions to the family of alkene complexes are fullerene derivatives such as Rh(CO)(q -Cgo)(H)(PPh3)2 Pd(q -Cgo)(PPh3)2 (Figure 23.17b) and (q -Cp)2Ti(q2-Cgo). The Cgo cage (see Section 13.4) functions as a polyene with localized C=C bonds, and in Cgo Pt(PEt3)2 6, six C=C bonds (remote from one another) in the Cgo cage have undergone addition. Reaction 23.73 illustrates CgQ-for-ethene substitution (the 16-electron centre is retained), and reaction 23.74 shows addition to Vaska s compound (a 16- to 18-electron conversion). Equation 23.75 shows the formation of the first fullerene complex of titanium, by fullerene displacement of a coordinated alkyne. 

Fully Characterized tt-Bonded Metal-Fullerene Complexes"... 

The nature of the bonding in platinum-fullerene complexes has been the subject of many studies. 

Except for one early report [17] on the bond dissociation energies for the naked nickel-fullerene eomplex, there is little information available about relative stabilities and dissociation energies of bare metal-fullerene complexes. Why does some metals form strong bonds to fullerenes, while the exohedral metallofiillerene CoCeo has never been observed The aim of our study is to shed light on this and related issues, by exploring which factors play important roles in metal-fullerene interactions and hence determine the bond strengths. To this end we study the interaction between... 

In general, the geometry of M(PH3)2( 7 C6o) resembles precursor ethylene parent adduct M(PH3)2( 7 -C2H4) except the fact that in fullerene complexes M-P and C-C bonds as a rule are slightly longer than in ethylene compounds (Table2.1). Moreover the phosphorus in phosphine complexes prefer to stay in the same plane with the metal atom and interacting carbons of ethylene or fullerene molecule. 

Table2.6 summarizes total donation, back-donation and repulsion terms for met-allofullerenes and for the M-C2H4 parent adduct. Both ethylene and fullerene complexes have common peculiarities a donation decreases downwards the group, while 7T back-donation has the opposite trend and increases in the group with the increasing of nuclear charge of the metal. For the late transition metals studied here, except of the first-row transition metals (Co, Ni), the metal-C6o/C2H4 bond is dominated by back-donation. We found that the M-C60/C2H4 bond strength correlates with the donation and back-donation (look Tables 2.3, 2.6). BDE increases with the...

The only reported X-ray structure of a it-bonded diiodine exists in the 12/coronene associate [75], which shows the I2 to be located symmetrically between the aromatic planes and to form infinite donor/acceptor chains. -Coordination of diiodine over the outer ring in this associate is similar to that observed in the bromine/arene complexes (vide supra), and the I - C separation of 3.20 A is also significantly contracted relative to the stun of their van der Waals radii [75]. For the highly reactive dichlorine, only X-ray structures of its associates are observed with the n-type coordination to oxygen of 1,4-dioxane [76], and to the chlorinated fullerene [77]. 

A review9 with more than 37 references includes an examination of symmetry groups and chirality conditions for C60 and C70 bonded to one or two metals in rf and/or rf fashion. Palladium and platinum rf complexes of C6o and C70 are described (novel synthesis, NMR spectra, electrochemistry) as well as first optically active organometallic fullerene derivatives. 

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