Question

The complex which does NOT obey 18-electron rule is (atomic number for Mn = 25, Fe = 26, Co = 27, Ru = 44)

• A. [Co(CO)6]8
• B. [Fe(CO)4]2-
• C. [HMn(CO)5]
• D. [(η⁷-C₅H₅)RuCl(CO)(PPh₂)]

Hint:

When calculating valence electrons in coordination compounds, consider the electrons donated by ligands and the metal's oxidation state to determine whether the 18-electron rule is obeyed.

Answer 1

The Correct Option is D

Step 1: Understanding the 18-electron rule.
The 18-electron rule states that transition metal complexes are most stable when they have 18 valence electrons, which are contributed by the metal and its ligands.

Step 2: Analyzing the complexes.
- \([Co(CO)_6]\) has 18 valence electrons (6 from CO and 12 from the metal). This obeys the 18-electron rule.
- \([Fe(CO)_4]^{2-}\) has 18 valence electrons (4 from CO and 14 from Fe). This obeys the 18-electron rule.
- \([HMn(CO)_5]\) has 18 valence electrons (5 from CO and 13 from Mn). This obeys the 18-electron rule.
- \([(η⁷-C₅H₅)RuCl(CO)(PPh₂)]\) has 17 valence electrons (1 from the \(\eta^7\)-C₅H₅, 1 from CO, and 1 from PPh₂). This complex does not obey the 18-electron rule.

Step 3: Conclusion.
The complex \([(η⁷-C₅H₅)RuCl(CO)(PPh₂)]\) does not obey the 18-electron rule, so the correct answer is (D).