Hapticity

Ferrocene Figure 2-47) provides a prime ex.ample of multi-haptic bonds, i.e, a situation where the electrons that coordinate the cyclopentadienyl rings with the iron atom arc contained in molecular orbitals delocalised over the iron atom and the 10 carbon atoms of the cyclopentadienyl rings [82. ... 

We describe here a new structure representation which extends the valence bond concept by new bond types that account for multi-haptic and electron-deficient bonds. This representation is called Representation Architecture for Molecular Structures by Electron Systems (RAMSES) it tries to incorporate ideas from Molecular Orbital (MO) Theory [8T]. 

The ideas presented above on the representation of bonding in molecular structures by electron. systems can be extended to the different t> pcs of bonding in or-ganoinetallic complexes. Such a system has not yet been fully elaborated but tire scheme is illustrated with one example, the case of multi-haptic bonds. 

Ferrocene (Figure 2-61a) has already been mentioned as a prime example of multi-haptic bonds, i.c, the electrons tlrat coordinate tire cyclopcntadicnyl rings with the iron atom are contained in a molecular orbital delocalized over all 11 atom centers [811, for w hich representation by a connection table having bonds between the iron atom and the five carbon atoms of cither cyclopcntadicnyl ring is totally inadequate. 

The X-ray structure of the dibromine complex with toluene (measured at 123 K) is more complicated, and shows multiple crystallographically independent donor/acceptor moieties [68]. Most important, however, is the fact that in all cases the acceptor shows an over-the-rim location that is similar to that in the benzene complex. In both systems, the acceptor is shifted by 1.4 A from the main symmetry axis, the shortest Br C distances of 3.1 A being significantly less than the sum of the van der Waals radii of 3.55 A [20]. Furthermore, the calculated hapticity in the benzene/Br2 complex (x] = 1.52) is midway between the over-atom (rj = 1.0) and over-bond (rj = 2.0) coordination. In the toluene complex, the latter varies from rj = 1.70 to 1.86 (in four non-equivalent coordination modes) and thus lies closer to the over-bond coordination model. Moreover, the over-bond bromine is remarkably shifted toward the ortho- and para-carbons that correspond to the positions of highest electron density (and lead to the transition states for electrophilic aromatic bromination [12]). Such an experimental location of bromine is in good agreement with the results of high level theoretical... 

The structures of the benzene/Br2 and toluene/Br2 complexes at 123 K show over-the-rim coordination with hapticities varying from about 1.5... 

A change in hapticity of the Cp ligand in 1 from q3/5 to ri1 should lead to a drastic change in the character of the molecule whereas the 7t-bonded species (I) can be regarded as a hyper-coordinated, electronically over-saturated and thus nucleophilic silylene, the a-bonded species (II) represents an example of an electron deficient and thus electrophilic silylene. 

Draw the structure for (C6H6)Cr(CO)4. What is the hapticity of C6H6 in this complex ... 

For sodium and potassium chalcogenolates, donor influence on structural pattern has been explored with a special emphasis in donor hapticity. Thus, crown ether complexation allows the isolation of monomeric species, such as [K(SCPh3)(18-crown-6)Lra] (L=thf, CgH6, hmpa, n = 0.5 L=toluene, n = l),36 [K(SMes )(dibenzo-18-crown-6)(thf)],45 [K(STrip)(dibenzo-18-crown-... 

The geometries in Figs. 4.86 and 4.87 suggest an important distinction in the multicenter hapticity character of ligand attachment to the metal atom. Hapticity refers to the number of atoms in a ligand that are coordinated to the metal. In the Ir+ diammine complex (Fig. 4.86(a)), the metal attaches to each of two nN donor lone pairs in simple monohapto (one-center, q1) fashion. However, in the Ir+ complexes with HCCH or CML the metal attaches to the face of the pi bond or three-center allylic pi system in dihapto (two-center, r 2) or trihapto (three-center, q3) fashion, respectively. The hapticity label q" therefore conveniently denotes the delocalized n -center character of the donated electron pair(s) and the geometry of the resulting coordination complex. 

The effects of altered hapticity can be seen by comparing the q3, four-electron allyl complex of Ir+ (Fig. 4.86(c)) with the corresponding complex of Au+ (Fig. 4.87(c)). Because Au+ requires only two electrons to complete its formal duodectet, the metal-allyl complexation now involves only the anionic nc center of a localized allylic H2C—CH=CH2 moiety, and the hapticity slips from q3 to q1 ... 

In other words, the gold complex is of formal MX type. Thus, the NBO description of the bonding goes considerably beyond the hapticity labels in (4.110) and suggests that only a severely distorted and reorganized form of the allyl anion exists in either of these complexes. 

Whereas changes in hapticity are normally required for changes in the number of coordinated electron pairs, acetylene and other triple-bonded ligands exhibit the... 

The Fe(Cp)2 and Ni(Cp)2 structures in Fig. 4.92 illustrate the important variation in hapticity that often distinguishes M(Cp)2 complexes for metal atoms of different acceptor characters. Thus, high-symmetry ferrocene (Sio) corresponds to highest r 5 hapticity, whereas singlet nickelocene (lowered C2v symmetry) exhibits lower r 3 coordination to each Cp ring. (The crystallographic structure of triplet nickelocene... 

A change in the allyl hapticity (q3 to p1 slippage) leading to the less substituted titanium-carbon o bond accounts for the observed y-regioselectivity. The anti diastereoselectivity stems from a pseudo-equatorial orientation of the aldehyde group. The diastereoselectivity of the reaction can be reversed through the use of a more coordinating cosolvent such as HMPA (Scheme 13.7) [14]. This reversal of anti to syn diastereoselectivity can be rationalized in terms of an open transition state. 

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