 Solution

ID: past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2021/paper-2/b7/b/solution

The arene and three carbonyl ligands are neutral, so chromium is in oxidation state zero, with a

d^{6}

metal count. The eta-six arene contributes six electrons and each CO contributes two:

6 + 6 + 3 (2) = 18 valence electrons .

The methoxy group changes the electronic properties of the arene without changing this formal electron count. The complex is a piano-stool complex, with the coordinated arene as the seat and the three carbonyl ligands as the legs.

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