THE ELECTRON-DEFICIENT PLANAR TETRAIRON CLUSTER FE4(CO)8PYRIDINE-4

A 56-electron cluster with formula Fe4(CO)8(pyridine)4 (1) is formed by addition of FeCl2 and Na2Fe(CO)4 in tetrahydrofuran containing a small excess of pyridine. The compound, which has a magnetic moment of 3.8 μB, has been structurally characterized. Crystal data for C28H20N4O8Fe4: a = 21.511 (3) Å, b = 16.766 (3) Å, c = 19.253 (3) Å, β= 117.94 (2)°, monoclinic P21/a, Z = 8, R = 0.06 for 2848 observed reflections. The structure consists of a triangulated parallelogram of iron atoms, two of which are coordinated, at opposite vertices, each by four carbon monoxide molecules and the other two are each coordinated by two pyridine molecules. The Fe-Fe separations along the periphery are an average of 2.534 (4) Å, a length attributable to an Fe-Fe single bond. The length of the shortest diagonal is 2.759 (3) Å, raising the question whether there is residual Fe-Fe bonding along that direction. The question is examined by extended Hückel calculations. Simple qualitative MO arguments describe the interactions between opposite pairs of L2Fe and L4Fe fragments. Although they do not give a uniquely defined electronic ground-state configuration, significant pieces of information on the Fe-Fe trans-diagonal bond and on the electron distribution over the four metal atoms may be attained.