Bent metallocenes structure

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 ... 

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]. 

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