Question

Which of the following neutral molecule does not obey 18 electron rule?

• A. $({\eta }^5-C_5{H}_5)Fe(CO{)}_2$
• B. $({\eta }^5-C_5{H}_5{)}_{2}Fe$
• C. $({\eta }^6-C_6{H}_6)Mo(CO{)}_3$
• D. $({\eta }^5-C_5{H}_5)Re({\eta }^6-C_6{H}_6)$

Answer 1

The correct option is A $({\eta }^5-C_5{H}_5)Fe(CO{)}_2$

According to $18e^{-}$ rule, in a complex the sum of valence electrons and the electrons donated by the ligands equals to 18. It provides a closed and stable configuration, $n{s}^2(n-1)d^{10}n{p}^6$

$\left(i\right).\left[\right({\eta }^5-C_5{H}_5\left)Fe\right(CO{)}_2]\to 8+5+(2\times 2)=17(ii).({\eta }^5-C_5{H}_5{)}_{2}Fe\to 8+(5\times 2)=18\left(iii\right).({\eta }^6-C_6{H}_6)Mo(CO{)}_3\to 6+6+(3\times 2)=18(iv).({\eta }^5-C_5{H}_5\left)Re\right({\eta }^6-C_6{H}_6)\to 7+5+6=18$

Hence, compound (a) does not obeys $18e^{-}$ rule.