GATE 2021 CY – Question 31
The Mo–Mo bond order in $[(\eta^5-C_5H_5)Mo(CO)_2]_2$, which obeys the 18-electron rule, is ______.
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Correct answer: 3 to 3
Explanation
Use neutral electron counting for each Mo: group-6 Mo contributes 6 electrons, η5-Cp contributes 5 and two CO ligands contribute 4. That gives 15 before the metal–metal bond. Each unit of Mo–Mo bond order contributes one electron to each Mo, so three are needed to reach 18. The bond order is **3**.