TRIIRON DODECACARBONYL - AN ANALYSIS OF ITS STEREOCHEMISTRY

The statistically disordered crystal structure and molecular architecture of Fe3(CO)12 have been unambiguously determined from a three-dimensional, single-crystal X-ray photographic analysis. Anisotropic least-squares refinement of 15 independent atoms has yielded discrepancy factors of R1 = 7.1% and R2 = 7.6% based on 539 independent reflections. The molecular structure of Fe3(CO)12 is comprised of an isosceles triangular arrangement of iron atoms in which an Fe(CO)4 group is symmetrically coordinated by only iron-iron bonds to an Fe2(CO)8 residue containing two identical Fe(CO)3 groups connected to each other by two bridging carbonyl groups and by an iron-iron bond. This configuration can be simply considered as that of Fe2(CO)9 with one of the three bridging carbonyl groups replaced by an Fe(CO)4 unit. The crystal-disordered model successfully utilized in the least-squares refinement assumes a random distribution of each Fe3(CO)12 molecule in one of two orientations related to each other by a crystallographic center of symmetry such that on the average one-half of an atom occupies each atomic site. The icosahedral-like molecular arrangement of 12 carbonyl groups enclosing the triiron cluster results in a near-superposition of the corresponding half-weighted carbon and oxygen atoms for the two orientations and thereby produces a “Star of David” arrangement for the two isosceles triangles of six iron half-atoms. Either orientation of the triangle of iron half-atoms together with the twelve “whole” carbonyl groups (of which only six are crystallographically independent) yields the same molecular configuration. Although the resulting disorder-averaged configuration of Fe3(CO)12 precludes the accurate determination of molecular parameters for the carbonyl groups, there is definite evidence that the two bridging carbonyl groups are somewhat unsymmetrical such that the idealized geometry of C2v-2mm symmetry based on two symmetrical bridging carbonyl groups is reduced by their asymmetry to C2-2 symmetry. The structural features of Fe3(CO)12 are correlated with those of the HFe3(CO)11 anion and Fe3(CO)11P(C6H5)3. A structural interrelationship between Fe3M(CO)12 and Co4(CO)12 is shown to exist in that the removal of any of the three equivalent basal metal atoms from the tetrahedron of cobalt atoms in Co4(CO)12 along with a concomitant deformation of the quasi-regular carbonyl icosahedron surrounding the cobalt tetrahedron leads to the geometry of Fe3(CO)12. To explain the infrared solution spectra of Fe3(CO)12, it is proposed that an intramolecular rearrangement of Fe3(CO)12 occurs on dissolution to give an instantaneous geometry of C3v-3m symmetry consisting of three identical Fe(CO)3 groups positioned at the vertices of an equilateral triangle and linked in pairs to one another by a bridging carbonyl group and an iron-iron bond. Both the established solid-state and postulated solution geometries are shown to be stereochemically analogous to the established molecular configurations of the two solid-state isomeric forms of the electronically equivalent RH3C5H5)3(CO)3 complex. © 1969, American Chemical Society. All rights reserved.