Metallocene structures, bent

Fig.5 Examples of simple metallocene structures a parallel sandwich, b multi-decker sandwich, c half-sandwich, d bent/tilted sandwich and e compounds with differently bonded cyclopentadienyl ligands...

The organosamarium(II) complex Sm( / 5-CsMc5)2 has the bent-metallocene structure, which can be attributed to polarization effects its THF solvate Sm( / 5-C5Mc5)2(TI If )2 exhibits the expected pseudo-tetrahedral coordination geometry. Sm( j5-C5Me5 )i] is a very powerful reductant, and its notable reactions include the reduction of aromatic hydrocarbons such as anthracene and of dinitrogen, as illustrated in the following scheme ... 

Fig. 12. Valence bond interpretation of the bending in Group 14 metallocenes. The bent structure stems from a stereochemically active lone pair of electrons, which is approximately sp1 hybridized.

The two substituted cyclopentadienyl rings in hexakis(trimethylsilyl)-magnesocene and -ferrocene are nonparallel with a Cp-M-Cp angle of 171.5° [average for M = Mg (149)] and 173.9° [M = Fe (99)]. These bent metallocenic structures (Fig. 15a) are in contrast to parallel Cp arrangements observed otherwise for normal of even bulky Cp s [cf. octaphenyl-ferrocene (45)]. The structure of normal magnesocene shows it to be isomorphic with ferrocene (184,185). 

Ab initio calculations on hypothetical organometallic cations Cp2Ln+ (Ln = Sc, La) show bent metallocene structures are favoured due to the covalent a bonding contributions involving the totally symmetric metal d orbitals which favor bent structures. Although the contribution of da is lower than dn, its involvement is sufficient to give bent structures [119]. 

Lanthanoid(II) metallocenes have been known for Sm, Eu and Yb since the 1980s and are stabilized by using the bulky [C5Me5] ligand (equations 24.26-24.28). The products are obtained as solvates the desolvated metallocenes have bent structures in the solid state (Figure 24.4c) rather than a ferrocene-like structure. For each of Sm, Eu and Yb, the most convenient route to (q -C5Me5)2Ln starts from LnU nTHF. 

Harrod and coworkers have found that metallocene catalysts such as Cp2ZrMe2 and Cp2TiMc2 have been found to catalyze the conversion of primary arylsilanes such as PhSiHs at room temperatures to afford short-chain polysilanes H-(PhHSi)n-H which contain Si-H terminal groups (Fig. 7.16) [56-57]. These Group 4 metallocenes have bent structures in contrast to ferrocene where the iron is sandwiched between the two cyclopentadi-enyl rings. 

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