The Fluxionality of Fe₃(CO)₁₂

In 1989, it was shown that the lowest energy fluxional mechanism operating in Fe₃(CO)₁₂ and its derivatives is the concerted bridge-opening bridge-closing mechanism.

In this mechanism carbonyls C¹O, C¹¹O, C¹²O, C²O and C⁷O circulate around the edge of the Fe₃ triangle defined by Fe² and Fe³. There is also a concerted rotation of carbonyls C³O, C⁵O, C⁴O, C⁹O and C⁸O around the edge of the Fe₁ triangle defined by Fe².

Carbonyls C⁶O and C¹⁰O are brought into the exchange by choosing an alternative of Fe-Fe edge for the circulation axis.

The two sets of carbonyls remain synchronised as they rotate. This has been demonstrated for Fe₃(CO)₁₀(CNBut){P(OMe)₃}.

Recently the mechanism has been mapped by comparing a series of crystal structures. This has resulted in the generation of an animated graphic which shows the two sets of carbonyls rotating about the axis defined by carbonyls C⁶O and C¹⁰O. The viewpoint is looking down this axis.

Fe₃(CO)₁₂ carbonyls rotating

References

H. Adams. N. A. Bailey, G.W. Bentley and B.E. Mann, "The mechanisms of fluxionality of [Fe₃(CO)12-n{P(OR)₃}n], R = Me, n = 0 to 3; R = Pri, n = 2 or 3) and the x-ray structure of [Fe₃(CO)₁₀{P(OMe)₃}₂]", J. Chem. Soc., Dalton Trans., 1989, pp. 1831-1844.

B. E. Mann, "The application of the concerted bridge opening-closing mechanism to the fluxionality of [Fe₃(CO)₁₀(L)(CNCF₃)], L = CO, PMe₃, PEt₃, P(OMe)₃, or P(OEt)₃", Organometallics, 1992, 11, pp. 481-2.

H. Adams, A. G. Carr, B. E. Mann, and R. Melling, "An investigation of the fluxionality of [Fe₃(CO)₁₁(CNBut)] and [Fe₃(CO)₁₀(CNBut){P(OMe)₃}], and the x-ray structure of [Fe₃(CO)₁₀(CNBut){P(OMe)₃}]", Polyhedron, 1995, 14, pp. 2771-2785.

H. Adams, X. Chen, and B. E. Mann, "The crystal structures of [Fe₃(CO)₉{P(OR)₃}₃], R = Me, Pri ", J. Chem. Soc., Dalton Trans., 1996, pp. 2159-63.

B. E. Mann, "Mechanism of the low-energy fluxional process in [Fe₃(CO)12-nLn] (n = 0-2): a perspective", J. Chem. Soc., Dalton Trans., 1997, pp. 1457-1471.