Ferrocene molecular orbital calculations

Continuing advances in the theoretical treatment of metallocenes and substituted derivatives can be anticipated. Despite the historical difficulties with molecular orbital calculations on ferrocene, the problems are now recognized to stem from failure to account for electron correlation effects. New approaches to addressing this issue, coupled with inevitable increases in computing power, should make more metallocenes with substituted groups systems amenable to accurate calculation. More realistic predictions of donor ability, and thus better estimates of their effects in nonlinear optical (NLO) systems, will be possible. 

The interest in the rotation of cyclopentadienyl rings (or aromatic organic rings in general) attached to metals (15) originated with the discovery of ferrocene itself (3b). Many methods have been employed to study the phenomenon, among them solid-state and solution NMR (187), dipole moment measurements (188), electron and X-ray diffraction techniques (189,190), mechanical spectroscopy (117), and last but not least molecular orbital calculations (191). 

A number of approximate molecular orbital calculations on ferrocene were reviewed by Cotton and Wilkinson in 1959 and the result of an ab-initio calculation was published in 1972 The latter work also contains references to approximate molecular orbital calculations published between 1959 and 1972. It seems that the particular stability of ferrocene is due to its closed-shell structure all occupied molecular orbitals are bonding between the metal and the ligands, the cyclopentadienyl rings function as five-electron ligands, and the iron atom attains an inert gas configuration of 18 electrons in the valence shell. 

Application of pressure to ferrocene decreases both the chemical isomer shift and the quadrupole splitting [36]. Molecular-orbital calculations give predictions approximately in agreement with experiment. 

Generally, the atomic, outer, and interatomic sphere radii are chosen so as to minimize the volume between the spheres. Other physical considerations, such as empirical atomic or ionic radii, can also influence the initial choice of sphere sizes. It has also been recently shown that overlapping spheres may be used since the various expansion theorems are still valid (192). This technique has been used to calculate the molecular orbitals of Zeise s salt and ferrocene discussed later in this review (191, 193). 

The extended Hiickel method, which is a semiempirical quantum chemistry method, is often used as a preliminary step in the DFT study of molecular orbital analysis. The acetylide-bridged organometallic dinuclear complexes 5.2 were studied by Halet et al. using the extended Hiickel method for qualitative analysis and DFT for additional electronic properties [97], The extended Hiickel analysis concluded that the main contribution of the Pt-C bond arises from ct type interactions while the n back-donation is very weak. The DFT/BP86 calculation gives a 2.371 eV HOMO-LUMO gap. The electronic communication parameter Hdb between the bis-ferrocene compound linked with platinum acetylide (5.3) was calculated to be 0.022 eV, compared with 0.025 eV obtained experimentally by Rapenne and coworkers using DFT and the extended Hiickel method [98],... 

Despite the huge increase in computational effort, this direct symmetry-adapted LCAO method was used to study ozone [22], tetrahedral Ni4 [23], and D5h-symmetric ferrocene (Fe(C5H5)2) [24] using molecular orbital (MO) contraction coefficients in the linear-combination-of-Gaussian-type orbital (LCGTO) computer code of [25]. Obviously, symmetry-adapted calculations are important enough to pay an order-TV computational price. The reasons are first, and foremost, that the calculations converge, and second that the wavefunction and one-electron orbitals can be used to address experiment, which typically must first determine the symmetry of the molecule. 

F. Fenske. We demonstrate for transition metal complexes that the non-empirical Fenske-Hall (FH) approach provides qualitative results that are quite similar to the more rigorous treatment given by density functional theory (DFT) and are quite different from Hartree-Fock-Roothaan (HFR) calculations which have no electron correlation. For example, the highest occupied molecular orbital of ferrocene is metal based for both DFT and FH while it is ligand (cyclopentadienyl) based for HFR. In the doublet (S = 1/2) cluster, Cp2Ni2(pi-S)2(MnCO)3, the unpaired electron is delocalized over the complex in agreement with the DFT and FH results, but localized on Mn in the HFR calculation. A brief description of the theory of FH calculations is used to rationalize the origin of its similarity to DFT. 

A molecular orbital analysis by using DFT calculations was performed for (NN )M1(THF)2 (Fig. 9). From the point of view of reduction chemistry, the LUMO of (NN )M1(THF92 is of particular interest because it may indicate whether the ferrocene backbone is involved in the reduction process. Somewhat surprisingly, the LUMO of various metal iodides was not the same. 

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