Mechanism of the low-energy fluxional process in [Fe3(CO)12−nL n] (n = 0–2): a perspective

The methods of Bürgi and Dunitz, using crystal structures to map out an exchange pathway for the carbonyls, have been applied to [Fe 3 (CO) 12 ] and its derivatives. It has been shown that the exchange pathway follows that predicted by the concerted bridge-opening and bridge-closing mechanism. The data are inconsistent with other proposed mechanisms. It has also been shown that the concerted bridge-opening and bridge-closing mechanism can equally be described as a rotation of either the Fe 3 triangle or the carbonyl icosahedron about a S 10 axis of the icosahedron through the equatorial ligands. This mechanism has been used to explain the lowest-energy fluxional mechanism in [Fe 3 (CO) 12 ], both in solution and in the solid state, in [Fe 3 (CO) 11 L] [L = PMe 2 Ph, P(OR) 3 , CNBu t , CNCF 3 ] in solution, in [Fe 3 (CO) 10 L 2 ] [L = P(OR) 3 (R = Me, Et or Pr i )] in solution and in [Fe 2 Os(CO) 12 ] in the solid state. The other mechanisms examined are the C 2 libration mechanism due to Johnson, the rotation of either the Fe 3 triangle or the carbonyl icosahedron about a S 10 axis of the icosahedron through a bridging ligand and an axial ligand on the unbridged iron due to Lentz, and the 60° rotation about the C 3 axis of the Fe 3 triangle due to Hanson. It is argued that the Ligand Polyhedral Model, albeit very elegant, is misleading and has led to improbable mechanisms. The approach using the original Local Bonding Model is recommended.