Iron Pentacarbonyl:  Are the Axial or the Equatorial Iron−Carbon Bonds Longer in the Gaseous Molecule?

Abstract

The structure of iron pentacarbonyl, Fe(CO)(5), was reinvestigated by gas-phase electron diffraction using an experimental rotational constant available from the literature as a constraint on the structural parameters. The study utilized a B3LYP/6-311+G(d) ab initio quadratic force field, scaled to fit observed infrared wavenumbers, from which were calculated corrections for the effects of vibrational averaging on distances and certain other quantities useful for the structural analysis. The results confirm that the equatorial Fe-C bonds are longer than the axial ones, an important difference with the structure in the crystal where the equatorial Fe-C bonds are the shorter. Some distance (r(g)/A) and vibrational amplitude (l(alpha)/A) parameter values with estimated 2sigma uncertainties based on assumption of D(3h) symmetry are [r(Fe-C)] = 1.829(2), r(Fe-C)(eq) - r(Fe-C)(ax) = 0.032(20), [r(C=O)] = 1.146(2), r(C=O)(eq) - r(C=O)(ax) = 0.006(27), r(Fe-C)(ax) = 1.810(16), r(Fe-C)(eq) = 1.842(11), r(C=O)(ax) = 1.142(23), r(C=O)(eq) = 1.149(16), l(Fe-C)(ax) = l(Fe-C)(eq) = 0.047(5), and l(C=O)(ax) = l(C=O)(eq) = 0.036(3).

Solvent induced structural dynamics of ruthenium pentacarbonyl in benzene

“…Isolated M(CO) 5 compounds have 18 normal vibrational modes, but only the m 10 and m 6 carbonyl stretching modes, which represent the asymmetric in-plane equatorial carbonyl stretch and asymmetric axial carbonyl stretch, respectively, are IR active in the region near 2000 cm À1 [32][33][34][35][36][37][38][39][40]. However, when IPC forms a complex with a single solvent molecule, the totally symmetric m 1 vibrational mode becomes IR active due to the induced break in D 3h symmetry.…”

Convergence properties of the Harris density functional and the self-consistent atom fragment approximation

“…51 More recently, in vaporphase, Fe͑CO͒ 5 has seen use as an ingredient in the production of carbon nanotubes. 52 Consequently, the structure of this molecule has been examined both theoretically 33,35,53 and experimentally, 54 and results are compared with findings from this study in Table V. From these studies, a minor discrepancy 53 occurs between experiment and theory over the relative sizes of the planar and axial Fe-C distances.…”

A quantum chemical calculation on Fe(CO)5 revealing the operation of the Dewar–Chatt–Duncanson model

“…Experimental FeACO bond lengths are rather well reproduced by the CCSD calculation ( Table 1). [46] In the solid state, however, axial and equatorial FeACO bond lengths were found to be 1.811(2) Å and 1.803(2) Å , respectively. This conclusion, however, is at variance with the gas phase electron diffraction study performed by Hedberg and coworkers.…”

“…[40][41][42][43] Computed and experimental FeACO bond distances [44] are recorded in Table 1. [46] The authors determined bond lengths of 1.810 Å and 1.842 Å for the axial and equatorial FeACO bonds, respectively. The axial FeACO bonds in Fe(CO) 5 should be slightly longer than the equatorial FeACO bonds.…”