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In which of the following oxoanions are the M-O bond lengths NOT all identical?

1. MnO4⁻
2. MnO4²-
3. Cr2O7²-
4. CrO4²-

Correct answer: Cr2O7²-

Solution

MnO4⁻: permanganate, tetrahedral, Mn in +7, all four Mn-O bonds are equivalent by symmetry. MnO4²-: manganate, also tetrahedral, all Mn-O bonds equivalent. CrO4²-: chromate, tetrahedral, all Cr-O bonds equivalent. Cr2O7²-: dichromate, two CrO4 tetrahedra sharing one oxygen vertex. The bridging O (Cr-O-Cr) has a longer, weaker bond than the terminal Cr=O bonds. Therefore in Cr2O7²-, all Cr-O bond lengths are NOT identical — the bridging Cr-O bond is longer than the terminal Cr-O (or Cr=O) bonds.

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