Question

The complex that does NOT obey the 18-electron rule is (Given: Atomic numbers of Ti, Mn, Ta and Ir are 22, 25, 73 and 77, respectively)

• A. [($\eta^5$-C$_5$H$_5$)Ti(CO)$_4$]$^-$
• B. [Mn(SnPh$_3$)$_2$(CO)$_2$]
• C. [($\eta^5$-C$_5$H$_5$)Ir(CH$_2$)(PMe$_3$)]
• D. [TaCl$_3$(PEt$_3$)$_2$(CHCMe$_3$)]

Hint:

Use the 18-electron rule to check stability: stable organometallic complexes usually follow it. Group number + ligand electrons + charge correction = 18 indicates stability.

Answer 1

The Correct Option is D

Step 1: Apply the 18-electron rule.
Total valence electron count (VEC) = metal valence electrons + electrons donated by ligands.
For (A) Ti (group 4) = 4 e$^-$.
C$_5$H$_5^-$ = 6 e$^-$.
4 CO = 8 e$^-$.
Charge = –1 → add 1 e$^-$.
Total = 4 + 6 + 8 + 1 = 19 e$^-$ → 18-electron satisfied (approximately stable).
For (B) Mn (group 7) = 7 e$^-$.
Each SnPh$_3$ = 2 e$^-$ × 2 = 4 e$^-$.
2 CO = 4 e$^-$.
Total = 7 + 4 + 4 = 15 e$^-$.
Thus, it does NOT obey the 18-electron rule.
Step 4: Conclusion.
The complex that does not obey the 18-electron rule is (B).