(METAL-METAL)-NONBONDING [FE2(CO)6(MU-2-PPH2)2]2-DIANION - SYNTHESIS, STRUCTURAL-ANALYSIS OF ITS UNUSUAL DIMERIC GEOMETRY, AND STEREOCHEMICAL-BONDING IMPLICATIONS

The Fe2(CO)6(μ2-PPh2)2]2- dianion, which reacts with alkyl halides to yield (upon protonation) aldehydes in quantitative yield, was prepared as Na2[Fe2(CO)6(μ2-PPh2)2].5 THF from the reduction with sodium dispersion in THF of the corresponding neutral dimer (for which a cyclic voltammogram in acetonitrile solution exhibits a single two-electron reversible reduction wave). Suitable crystals for an X-ray diffraction investigation of this burgundy-red dianion were finally obtained as the [Na(2,2,2-crypt)]+ salt. Its centrosymmetrically required geometry containing a strictly planar Fe2P2 core is best described as being formally comprised of two distorted trigonal bipyramids which share a common axial-equatorial edge; this edge-bridged bi(trigonal-bipyramidal) architecture is uncommon in that thereare only a few other crystallographically known examples of a five-coordinate metal dimer without a metal-metal bond. A geometrical comparison of this dianion with its neutral diphenylphosphido-bridged parent reveals that a dramatic change in configuration has taken place upon reduction of the (Fe-Fe)-bonded Fe2(CO)6(μ2-PPh2)2 molecule due to cleavage of the electron-pair Fe-Fe bond. A complete flattening of the highly bent Fe2P2 core is found to occur which increases the Fe-Fe distance from a bonding value of 2.623 (2) Å in the parent molecule to a nonbonding value of 3.630 (3) Å in the dianion. This structural study of the dianion of a representative member of the well-known (metal-metal)-bonded Fe2(CO)6(μ2-X)2 series furnishes the first experimentally definitive evidence that electron occupation of the lowest unoccupied molecular orbital (i.e., LUMO) of the corresponding neutral parent completely ruptures the metal-metal bond, in agreement with previous molecular orbital calculations which showed that the LUMOs of these neutral dimers possess large antibonding dimetal orbital character. Its structural features are correlated with spectroscopic data for this class of dimers, and the resulting bonding and stereochemical implications are discussed. The finding that the metal-metal distance can increase by as much as 1.0 Å upon a two-electron reduction strikingly emphasizes the importance of the “net” metal-metal interactions in dictating the resultant geometry of a metal cluster. [Na(2,2,2-crypt)]2+[Fe2(CO)6(μ2-PPh2)2]2- crystallizes in a triclinic unit cell of Pl symmetry with lattice constants of a = 12.466 (6) Å, b = 16.861 (6) Å, c = 10.797 (4) Å, α = 127.92 (2)°, β = 84.04 (2)°, and γ = 101.83 (3)° pcalcd =1.37 g/cm3 for Z = 1. Least-squares refinement gave R1(F) = 7.9% and R2(F) = 7.8% for 2681 independent diffractometry data with I ≥ 2.0α(I). © 1979, American Chemical Society. All rights reserved.