Discussion
The major product obtained by the treatment of (η5-C5H5)2Ni with Na/Hg in ethanol is
Answer: C
The product is (η5-C5H5)(η3-C5H7)Ni.
Nickelocene has 20 valence electrons (Ni(II) d8 plus two 6-electron Cp− rings), two more than the 18-electron ideal, and readily reacts to lose that excess. Sodium amalgam in ethanol reduces one Cp ring and ethanol protonates it, adding two hydrogens to give a cyclopentenyl ligand, C5H7, bound as an η3-allyl. The product, CpNi(η3-C5H7), is an 18-electron complex (Ni 10, Cp 5, allyl 3 in the neutral count).
(η5-C5H5)(η3-C5H5)Ni and (η3-C5H5)2Ni only change hapticity and add no hydrogen, so they are not reduction products. (η3-C5H7)2Ni would require both rings to be reduced, giving a 16-electron complex rather than the observed product.
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