Q4(b). 18-electron rule: [(η⁶-C₆H₆)Cr(CO)₃] and [(CO)₂Rh(μ-Cl)₂Rh(CO)₂]
What is 18-electron rule? Confirm that the Cr centre in [(η⁶-C₆H₆)Cr(CO)₃] obeys the 18-electron rule, but the Rh in [(CO)₂Rh(μ-Cl)₂Rh(CO)₂] does not.
Answer in brief
A stable organometallic complex has 18 valence electrons filling its nine valence orbitals; Cr in the benzene complex has 18 e, but each Rh in the chloro-bridged dimer has only 16 e.
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Covers18-electron ruleelectron countingbenzene chromium tricarbonyl16-electron complexrhodium carbonyl chloride dimer
All parts of Question 4
- 4(a) Wilkinson’s catalyst; hydrogenation cycle15 marks
- 4(b) 18-electron rule: [(η⁶-C₆H₆)Cr(CO)₃] and [(CO)₂Rh(μ-Cl)₂Rh(CO)₂]10 marks
- 4(c) Magic numbers; stability of ²⁰⁸₈₂Pb5 marks