Identify the thermodynamically stable structure of [(η⁵-C₅H₅)Fe(µ₂-CO)(NO)]₂

Concept:

• Complexes following 18e^-  rule are considered to be thermodynamically stable
• Nitrosyl ligand combines with metal in two forms : 1) Bent Nitrosyl and 2) Linear Nitrosyl.
• Neutral Nitrosyl contributes 3e^- in linear form and 1e  in bent form

Explanation:

Electron contribution by

• n⁵ cyclopentadiene = 5
• Fe = 8
• CO=2 (in bridging as well as non-bridging form)
• bent NO = 1
• Linear NO =3

(a) 

Electron count at metal center = 8 (Fe)+ 5(cyclopentadienyl) + 3(Nitrosyl) + 2 (µ2-CO) +1 (Fe-Fe) = 19e^-

Given structure is not following 18e^- rule, therefore, it is not thermodynamically stable.

(b)

Electron count at metal center = 8 (Fe)+ 5(cyclopentadienyl) + 3(Nitrosyl) + 2 (µ2-CO) = 18

It follows the 18e^- rule and is thermodynamically stable.

(c) 

Electron count at each metal center = 8 (Fe)+ 5(cyclopentadienyl) + 1(Nitrosyl) + 2 (µ2-CO) + 1 (Fe-Fe) = 17

Given structure is not following 18e- rule, therefore, it is not thermodynamically stable.

(d)

Electron count at each metal center = 8 (Fe)+ 5(cyclopentadienyl) + 1(Nitrosyl) + 2 (µ2-CO) +  = 16

Given structure of organometallic compound is not following 18e- rule, therefore, it is not thermodynamically stable.

Conclusion:

Hence, the thermodynamically stable structure of

[(η5-C5H5)Fe(µ2-CO)(NO)]2