Richard F. Fenske

Molecular orbital study of bonding and conformations in dinuclear cyclopentadienyldicarbonyl complexes of manganese and chromium containing germanium, sulfide, dinitrogen or phosphinidene bridges

Abstract We carried out nonempirical molecular orbital calculations on syn , gauche and anti conformations of four dinuclear complexes of Mn and Cr with various bridging ligands: [CpM(CO) 2 ] 2 (μ 2 -L) where M is Mn, L is C (as a model for Ge), N 2 or PPh; and M is Cr, L is S. Contrary to a recent claim, [CpMn(CO) 2 ] 2 Ge does not seem to contain Mn Ge double bonds and is not analogous to allenes. The Mn Ge bonds are partially triple so that internal rotation about the Mnz.sbnd;Ge Mn axis is facile, in accord with the infrared spectra. Bonding in [CpCr(CO) 2 ] 2 S and in [CpMn(CO) 2 ] 2 N 2 is very similar to that in the Ge-containing complex. We explain the observed nonrigidity of the Cr complex. Reported infrared data lead to mutually inconsistent conclusions about conformation of the N 2 -containing complex in solution. On the basis of calculations, we suggest that these complex molecules are not centrosymmetric in solution. The N 2 -bridged dinuclear molecules are better viewed as containing an N 2 molecule rather than two N atoms. The π-antibonding orbitals of N 2 are crucial for its bonding to metals; the filled π-bonding orbitals do not seem to donate electrons to the metal atoms. The calculations show substantial Mn P π-bonding in [CpMn(CO) 2 ] 2 PPh and this molecule is best viewed as a three-center, four-electron system. We critically examine several other qualitative and intuitive explanations of bonding in these and similar complexes and discuss conclusions and predictions based on such explanations.

The electronic structure of NiCO: A new prediction for the ground state

We have performed electronic structure calculations on the NiCO molecular fragment through the configuration interaction level. Our initial motivation was to compare the bonding in the low lying triplet states 3Δ, 3Σ+, and 3Π with that in the lowest closed shell singlet state 1Σ+. In both the σ and the π systems the Ni–CO bond is stronger in the 1Σ+ state than in the others. The σ‐bonding difference is a consequence of the occupation of the strongly antibonding Ni 4s‐like orbital in the triplet states, whereas in the 1Σ+ state it is unoccupied. The π‐bonding difference arises from the greater π‐backbonding ability of the more diffuse Ni 3d orbitals present in the 1Σ+ state. Most notably, though, we contradict previously published results by predicting a 1Σ+ ground state rather than a 3Δ ground state. A many configuration wave function is essential in accurately describing the π backbonding as well as producing the correct atomic energy level splittings. It is, for the most part, these two factors which pr...

Molecular orbital studies on cyclobutadienemetal complexes: the concept of metalloaromaticity

Nonempirical molecular orbital calculations have been performed on a variety of cyclobutadienemetal complexes. For C4H4Fe(CO),, a detailed analysis of the frontier orbitals indicates that the molecule is best described as a C4H4Fe fragment perturbed by the carbonyls rather than as an Fe(C0)3 moiety perturbed by the C4H4 ring. This description is more consistent with the photoelectron spectrum of C4H4Fe(C0)3 than the Hartree-Fock description of the molecule. The C4H4-Fe bond is highly covalent resulting in a delocalization of six electrons in metal-ring a orbitals, a phenomenon which shall be referred to as metalloaromaticity. These concepts are extended to C4H4Cr(C0)4 and C4H4Ni(CO),. The former species has been synthesized but the latter has not. Correlation of these facts with the calculations will be presented. Finally, a comparison will be made of C4H4 to C5H5 and C6H6 as ligands

Nonparameterized MO calculations of ligand-bridged M2(CO)8-(U2-X)2-type dimers containing metalmetal interactions: Evidence for dictation of stereochemistry by one-electron and two-electron metalmetal σ-type bonds☆

Nonparameterized MO calculations performed on the (edge-bridged)-bioctahedral metal dimers of the Dessy-characterized [Cr2(CO)8(μ2-PR2)2](n-2) series and of the [Mn2(CO)8(μ2-PR2)2]n series (n = 0, +1, +2) have revealed that the corresponding dimeric pairs with n = 0, +1, and +2 have two, one, and no electrons, respectively, in the antibonding 2b3u MO corresponding to a “net” no-electron metalmetal bond, a “net” one-electron metalmetal bond, and a two electron metalmetal bond. Of prime significance is that this 2b3u MO, which is the LUMO in both electron-pair (metalmetal)-bonded dimers (n = +2) and the HOMO in the corresponding dimers to which one or two electrons have been added, is found to be largely composed of in-plane antibonding σ★-type dimetal orbital character rather than either out-of-plane π★-type dimetal antibonding orbital character or bridging-ligand orbital character. These MO results are also shown to be completely compatible with the available spectral and X-ray data.

The LCAO representation of Xα‐SW molecular orbitals

The Xα‐SW molecular orbital method generates one‐electron orbitals which are numerical, and hence interpretation of the bonding characteristics of these orbitals must be inferred from electron density maps. In this paper a method is presented for the projection of LCAO MO’s out of the Xα‐SW orbitals, based upon maximization of the total overlap. Lagrange multipliers in the method indicate how well the individual projected orbitals overlap the original orbitals. Numerical quadrature over direct space is used to calculate some of the required matrix elements. Results are presented for an extended Slater orbital projection applied to N2.

The He(I) photoelectron spectra and valence electronic structures of η5-C5H5Mn(CO)2N2 and η5-C5H5Mn(CO)2NH3

Abstract The He(I) photoelectron spectra of η 5 -C 5 H 5 Mn(CO) 2 N 2 and η 5 -C 5 H 5 Mn(CO) 2 NH 3 have been obtained. The general features of these spectra resemble those of the parent carbonyl complex, η 5 -C 5 H 5 Mn(CO) 3 . The major differences appear in the ionizations associated predominantly with the metal d levels, where shifts in ionization energies and loss of degeneracy reflect the differences in bonding of the nitrogen ligands and a carbonyl ligand with the metal center. Non-empirical molecular orbital calculations were used as an aid for the interpretation of these binding energy shifts. In the case of N 2 bound to the metal, the shifts in ionization potentials are predicted with extreme accuracy by the shifts in eigenvalues of the calculations. Thus the electronic structure of transition metal dinitrogen complexes, as compared to carbonyl complexes, is accurately described. The quantitative prediction of binding energy shifts for the amine complex are less satisfactory, although the qualitative behavior is reproduced quite well.

The application of Hartree–Fock and configuration interaction calculations to transition metal nitrosyls

This is a study of the Hartree–Fock single configuration wave function as a reasonable description of the electronic structure of molecules or molecular fragments containing transition metals. For the linear species MnNO and CoNO, intentional reduction of the electronic symmetry from C∞v to C2v resulted in an asymmetric charge distribution which was physically meaningless but lower in calculated total energy than the proper Hartree–Fock result. Configuration interaction was required to restore the cylindrically symmetric charge density. An analysis of the origin of this observed saddle point in the Hartree–Fock energy surface suggests such situations will tend to arise when the lower virtual orbitals are in energetic proximity to the higher occupied orbitals and when the virtual orbital or orbitals delocalize the electron density within the species. The implications of the effects observed in the triatomic species are examined with respect to the known molecules Mn(CO)4(NO) and Co(CO)3(NO). In these latte...

The transferability of molecular fragment canonical orbitals

The ability of certain canonical orbitals of isolated molecular fragments to transfer largely unchanged to the molecular environment is examined. The separation of fragment canonical orbitals from the total molecular electronic problem is compared with the more familiar separations of atomic core orbitals and fragment localized orbitals. The specific example of the carbonyl functional group in formaldehyde is examined in detail. These studies lead to a new concept of valence electron only calculations in which the molecular valence electrons are assumed to move in an effective field provided by frozen molecular fragment canonical cores. In addition, for the case of assumed fragment canonical orbital transfer, perturbation theory analysis is found to be an efficient method of assessing the quality of the approximate wavefunction, thus eliminating much of the uncertainty as new systems are studied. The methods developed in the course of these studies offer certain practical advantages for the construction of approximate wavefunctions for large molecules. The details of application of these concepts to existing molecular orbtial methods are also presented.

Theoretical calculations of carbon‐13 NMR chemical shifts via the Xα‐scattered wave method

The Xα‐SW molecular‐orbital method is applied to the calculation of carbon‐13 NMR nuclear shielding constants in a series of small organic molecules. The application of perturbation theory within the Xα framework is discussed in order to place the equations used in the calculation of the shielding constants on firm theoretical ground. Use is made of a highly accurate mixed analytic‐numerical method of integration in calculating the necessary matrix elements of 1/r, L/r3, and L. The results for both overlapping and nonoverlapping spheres calculations are shown to reproduce trends in experimental shielding constants remarkably well, but not the correct magnitudes. Calculated shielding constants are shown to be strongly dependent upon the carbon atomic‐sphere radius, and the origins of this dependence are discussed in terms of discontinuities in the muffin–tin potential. An established method of choosing sphere radii based upon an initial approximation to the molecular charge density yields results which cor...

Approximate molecular orbital calculations on carbene ligands and complexes: the ability of the carbene ligands to act as σ donors and π acceptors

Abstract Non-parameterized molecular orbital calculations have been performed on a series of carbene ligands, C(X)Y, and carbene complexes, (CO)5CrC(X)Y. In accord with previously obtained experimental data, methoxycarbenes were found to be better π acceptors than aminocarbenes. All of the carbene ligands were found to accept less charge from the chromium than the carbonyl ligand. While the eigenvalues of the highest occupied and lowest unoccupied molecular orbitals were found to be important factors in determining the σ donating and π accepting abilities of the ligands, other factors (such as the spatial localizations and degeneracies of the orbitals) were also found to affect these abilities. An explanation is given for the absence of a significant trans influence in most carbene complexes.