Localized molecular orbital studies of transition metal complexes. II: Simple π-accepting ligands

Localized molecular orbitals (LMOs) for several octahedral complexes are presented. Wavefunctions are calculated within the PRDDO approximations and localized by the Boys criterion. Complexes of general formula (NH₃)_x(CO)_6-xM, M = Cr⁰ or Mn⁺ and x = 1, 2, or 3 illustrate the general trends for carbonyl complexes. Weak to moderate π-bonding results in three equivalent inner shell LMOs dominantly of metal 3s, 3p and 3d character but highly delocalized to the carbonyls. These three LMOs flank the M-CO bond axis. Other π back-bonding situations result in metal-ligand double bonds which are nonequivalent and have σ-π separability [(NH₃)₅(py)Mn⁺] and also equivalent double bonds [(NH₃)₅(NO)Cr⁺].     

Molecular orbital calculations on transition metal complexes

INDO SCF MO calculations are reported for the complexes (C₅H₅)M(C₇H₇) (M = Ti, V and Cr), and for the corresponding V and Cr cations. The results correctly predict¹ A ₁ ground states for the V⁺ and the neutral Ti and Cr species, and for the neutral V and Cr⁺ complexes confirm the² A ₁ ground levels. The formally metald-levels followed theH ^core sequencee ₂<a ₁<e ₁, and the most important interactions were those between the metale ₂ level and the ligand C₇H₇ -orbitals, and between the metale ₁ level and the ligand C₅H₅ -orbitals. Calculations also satisfactorily reproduced other experimental quantities, and the results indicate that thee ₂ ligand interaction becomes more important, and thee ₁ ligand interaction less important, with increasing size of the ligand ring.     

Molecular orbital calculations on transition metal complexes part VII

INDO SCF Molecular Orbital Calculations have been made for manganocene (MnCp₂), its molecular cation (MnCp ₂ ⁺ ) and the ferricenium ion FeCp ₂ ⁺ . The computations yield a high spin ⁶ A _1g (e _2g ² a _1g ¹ e _1g ² ) ground state for MnCp₂, contrary to that which was assumed to interpret the photoelectron spectrum. The lowest energy ionisations from the ⁶ A _1g state have been obtained by differences in total energies of the ion and the neutral molecule. These are in very good agreement with the experimental values. The use of the eigen-values of the neutral molecule to obtain ionisation energies (Koopmans' Theorem) is shown to be inappropriate due to the considerable electronic redistribution which accompanies the formation of the molecular ion from the neutral molecule. The main factor which influences this charge rearrangement is found to be the covalency within the e _1g (d _xz , d _yz ) orbitals.     

Molecular orbital calculations on transition metal complexes. Part IX

Potential energy curves for the ground and some low energy excited states of a number of complexes with a 3d ⁵ electronic configuration have been computed from INDO type SCF MO calculations. The results agree extremely well with the known ground states of the complex ions MnF ₆ ^4? , FeF ₆ ^3? , CoF ₆ ^2? , and Fe(CN) ₆ ^3? , in particular the crossover from high to low spin being obtained for changes in both central metal ion oxidation state and ligand. The calculated contraction in metal ligand distance on passing from the high spin to the low spin state is ～ 0.05 Å for each complex in very good agreement with the value indicated by pressure dependent magnetic measurements. Computed electronic transition energies involving bothd-d type and charge-transfer excitations compare favourably with observed spectroscopic values.     

Molecular orbital calculations on transition metal complexes : XIX. π-cyclopentadienyl-π-cyclopropenylnickel

INDO SCF molecular orbital calculations for π-cyclopentadienyl-π-cyclopropenylnickel indicate a formally d¹⁰ configuration for the metal. Calculations of the ionisation energies show that electron loss should take place first from the occupied closely grouped set of dominantly d-orbitals, and then from a mainly π-cyclopentadienyl e orbital, this being the highest occupied ligand level. This latter level shows however only a slight mixing with the metal d-orbitals, resulting in a small ligand→metal electron donation; the dominant interaction is that between the higher lying π-cyclopropenyl e level and the metal 3d_xz and 3d_yz orbitals which leads to a substantial metal→ligand charge donation. The behaviour of the π-cyclopropenyl ligand is discussed using the calculated charge distributions.     

Delocalization of the unpaired spin density in some niobocene complexes with σ-donor, π-acceptors

Extended Hückel calculations and qualitative MO have been used to analyze the EPR data relevant to the localization of the unpaired spin density in several paramagnetic niobocene complexes with σ-donor π-acceptors, acetylene, aldehyde, ketene, ketenimine, and carbon disulfide.     

Notes on Kuhn's ω″ molecular orbital method

(1) It is shown that, contrary to statements sometimes made, it is perfectly possible to obtain consistent sets of charges in Kuhn's ω″ molecular-orbital method, whatever the values of the parameters ω, ω′, ω″.

(2) Particular applications are made to the benzyl cation, and to the non-alternants fulvene, heptafulvene and azulene. It appears that including the ω-terms decreases the magnitude of the largest of the net atomic charges as calculated by the simple Hückel method, and results in an overall smoothing-out process. Further inclusion of the ω′-terms continues this smoothing-out process, but inclusion of the ω″-terms may sometimes slightly reverse this process.

(3) The charge distributions obtained for fulvene and azulene in (1) and (2) lead to molecular dipole moments which are still much too large.

In view of (3) it is not clear that the additional work involved in the inclusion of ω′ and ω″ is justified by the greater accuracy thus obtained.     

Molecular structure and electronic spectrum of ellagic acid: Semiempirical molecular orbital calculations

The molecular structure of ellagic acid has been optimized by using PM3 semiempirical MO method. Ellagic acid has been calculated to be planar with the molecular symmetry of C_2h. The lactone carbonyl groups are not tilted from the molecular plane. CNDO/S MO method has been used to interpret the experimental uv-vis spectroscopic data. The results of the PM3 and CNDO/S calculations have been in good agreement with the experimental data.     

Molecular orbital calculations on transition metal complexes : XX. The π-cyclopentadienyl-π-benzene complexes of chromium and manganese

INDO SCF MO calculations are reported for the complexes (C₅H₅)M(C₆H₆) (M = Cr and Mn) and for the corresponding cations. A ²A₁ ground state is correctly predicted for the neutral Cr system, and other experimental quantities are satisfactorily reproduced. The dominantly metal d-levels yield the H^core sequence e₂ < a₁ < _e1 and the principal interactions were found to be those between the metal e₁ level and the π-orbitals of the C₅H₅ ligand, and between the metal e₂ level and the ligand C₆H₆ π-orbitals. From the state energies of the formally 3d⁵ species estimates were made of the one-electron 3d splittings in the ligand field model, and comparisons made with similar results for M(C₅H₅)₂, M(C₆H₆)₂, and (C₅H₅)M(C₇H₇) systems: these indicated the general validity of the bonding scheme in which the extent of the metal—ligand interaction via the e₁ and e₂ 3d levels varied systematically with the sizes of the ligand rings.     

Mass spectrometry of charge-transfer complexes of 7,7′,8,8′-tetracyanoquinodimethane-π-donor systems

Electron impact (El) ionization and positive and negative liquid secondary ion mass Spectrometry (pLSIMS and nLSIMS) of eight charge-transfer complexes of 7,7′,8,8′-tetracyanoquinodimethane (TCNQ)-aromatic systems and of the individual π-donors and -acceptors was examined. The El spectra exhibited the molecular ions of both the donor and the acceptor of each complex. The molecular ion of the π-donor was observed in pLSIMS using m-nitrobenzyl alcohol (NBA) if its oxidation potential is lower than 1.2 V, but when the oxidation potential is higher than 1.9 V, no molecular ion was detected. On the other hand, nLSIMS exhibited the molecular ion of TCNQ in all cases. Participation of an excited state of NBA in the ionization process is suggested.     

Semiempirical calculations of hyperpolarizabilities for extended π systems: Polyenes,polyynes, and polyphenyls

The dipole moments (μ), polarizabilities (α), hyperpolarizabilities (β), and second-order hyperpolarizabilities (γ) of polyenes, polyynes, and polyphenyls have been calculated by a finitefield method with the PM -3 parameterization of the semiempirical MNDO Hamiltonian at the optimum geometries. These results were compared to experimental values obtained from EFISH and THG measurements. The calculations reproduce the magnitudes of β and γ, as well as the effect of the substituents and the effect of bond alternation on β and γ. The coefficients of the power law, which describes the dependence of β and γ on the number of π centers, were calculated. For β, exponents of 1.5–2.2 and 0.03–0.04 were obtained for polyenes and polyynes, respectively, and for γ, exponents of 3.9–4.9, 2.9–3.3, and 2.5–2.7 were obtained for polyenes, polyynes, and polyphenyls, respectively. These results confirm the efficiency of enhancing γ by insertion of C = C double bonds into a chain. © 1992 John Wiley & Sons, Inc.     

Nonlinear optical properties of dicyanomethylene‐derived heteroaromatic dyes: Semiempirical molecular orbital calculations and experimental investigations

The effect of conformation (E/Z isomerism), nature (donor/acceptor) of substituents, and endgroups (indandione, pyrazolone, pyrazoledione) on the molecular hyperpolarizability β_vec of dicyanomethylene (hetero)aromatic dyes is investigated by means of semiempirical (AM1, ZINDO) molecular orbital calculations. Unless Z isomers are stabilized by intramolecular hydrogen bonding, generally E conformers have larger β_vec's. Replacement of one nitrile group of the dicyanomethylene moiety by p‐aminoaryl rather than p‐R‐arylamino (R=NMe₂, MeO, H, NO₂) is found to be advantageous. Increasing the acceptor strength of 29 by successively replacing the carbonyl with dicyanovinyl groups leads to a maximum of β_vec for the derivative with one rather than two C(CN)₂ groups. With respect to endgroups, the indandione moiety generally is the least active group. Solvent effects are treated within the framework of the self‐consistent reaction field approximation. In most cases gas‐phase tendencies are either parallel or even reinforced if solvent effects are taken into account. The calculated results are compared with electric field induced second harmonic generation (EFISH) measurements. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 79: 253–266, 2000     

Ab initio molecular orbital calculations on ion pair-water complexes of metal halides and oxides

Ab initio MO calculations are performed on a series of ion-molecular and ion pair-molecular complexes of H₂O + MX (MX = LiF, LiCl, NaCl, BeO and MgO) systems. BSSE-corrected stabilization energies, optimized geometrical parameters, internal force constants and harmonic vibrational frequencies have been evaluated for all the structures of interest. The trends observed in the geometrical parameters and other properties calculated for the mono-hydrated contact ion pair complexes parallel those computed for the complexes of the individual ions. The bifurcated structures are found to be saddle points with an imaginary frequency corresponding to the rocking mode of water molecules. The solventshared ion pair complexes have high interaction energies. Trends in the internal force constant and harmonic frequency values are discussed in terms of ion-molecular and ion-pair molecular interactions.     

Vibrational spectra of π-olefin-transition metal complexes : (π-maleic anhydride)iron tetracarbonyl

The infrared and Raman spectra of solutions and solid samples of (π-maleic anhydride)iron tetracarbonyl have been studied. An assignment of the modes is suggested and the ligand vibrations in the complex are compared the data for maleic and succinic anhydrides. The C=C stretching frequency maleic anhydride shifts from 1595 to 1352 cm⁻¹ after coordination with the The essential decrease of the IR intensities of out-of-plane CH modes is for the complex and explained by the lowering of the effective charge on olefinic protons due to back-donation from metal to ligand.     

Simple molecular orbital calculations on the electronic structure of iron-porphyrin complexes

The simple Hückel method was first applied to the electronic structure of the iron-porphyrin complexes by Pullman et al. [12]. In this paper, their work is extended to include (a) the effect of a dipole or a point charge placed at the sixth coordination position, and (b) the effect of a nitrogen atom placed at the fifth coordination position. A set of new parameter values is used, whose estimation is made by directing special attention to their dependence on the charge distribution among the atoms.The resulting charge distribution for ferro-porphyrin seems to be reasonable. The fact that the position of the Soret peak is insensitive to the sixth ligand can be understood from the resulting orbital energy levels.The difficulty of finding a reasonable charge distribution for ferri-porphyrin is discussed.

---

Zusammenfassung Die einfache Hückelsche Methode ist auf die Elektronenstruktur der Eisen-Porphyrin-Komplexe zuerst von Pullman et al. [12] angewandt worden. In der folgenden Arbeit wird ihr Verfahren auf (a) die Wirkung eines Dipols oder einer Punktladung an der sechsten Koordinationsstelle und (b) die eines Stickstoffatoms an der fünften erweitert. Ein Satz neuer Parameterwerte wird verwandt, bei deren Bestimmung besonders auf ihre Abhängigkeit von der Ladungsverteilung geachtet wird.Die erhaltene Ladungsverteilung für Ferroporphyrin erscheint vernünftig. Die Unempfindlichkeit der Lage der Soret-Bande gegen den sechsten Liganden ist aus den erhaltenen Energieniveaus zu verstehen.Die Schwierigkeit, eine vernünftige Ladungsverteilung für Ferriporphyrin zu finden, wird diskutiert.

---

Résumé La simple méthode de Hückel a été appliquée à la structure électronique des complexes fer-porphyrine pour la première fois par Pullman et al. [12]. Dans l'article suivant, leur travail est étendu afin d'inclure (a) l'effet d'un dipôle ou d'une charge ponctuelle sur la sixième position coordinative, et (b) l'effet d'un atome de nitrogène sur la cinquième position. Un jeu de nouvelles valeurs des paramètres est usé qu'on détermine en tenant compte spécialement de leur dépendance de la distribution des charges atomiques.La distribution de charge obtenue pour la ferroporphyrine semble être raisonnable. Le fait que la position de la bande Soret est insensitive contre le sixième ligand, peut être compris à l'aide des énergies des orbitales calculées.La difficulté de trouver une distribution de charge raisonnable pour la ferriporphyrine est discutée.

---

---

The research reported in this paper was sponsored in part by the King Gustaf VI Adolf's 70-Years Fund for Swedish Culture, Knut and Alice Wallenberg's Foundation, the Swedish Natural Science Research Council, and in part by the Aeronautical Research Laboratory, OAR, through the European Office, Aerospace Research, United States Air Force.

---

On leave from Department of Physics, Faculty of Science, University of Tokyo, Tokyo, Japan.     

Bifunctional even-electron ions: 1—Fragmentation behaviour of ω-methoxy-, ω-hydroxy- and ω-chloro-oxonium ions

Bifunctional oxonium ions—generated from tertiary aliphatic alcohols containing an additional hydroxy, methoxy or chloro group at the end of an alkyl side-chain—do not markedly exhibit fragmentations typical of ordinary oxonium ions, but show as the main reactions those caused by functional group interaction, through-space interaction being the dominant factor. The main primary fragmentation is loss of the additional functional group X as HX, followed by loss of the side-chain originally separating the two functional groups, leading to carbonyl cations. This typical reaction sequence is initiated by proton migration from the oxonium moiety to the additional functional group. The reaction behaviour of the bifunctional ions is discussed. The lowest homologues show specific deviations from the general fragmentation behaviour.     

On the electronic structure of transition metal complexes. I. An INDO-MO investigation of some octahedral complex ions

Molecular-orbital calculations within the INDO framework are reported for the hexafluoro, hexachloro, and hexaaquo metallate (II) ions of the first transition series. The parameterization scheme and choice of orbital exponents have been detailed. The results indicate that the method in its present form gives fairly satisfactory predictions of molecular properties.     

Ab initio molecular orbital calculations on the water-carbon dioxide system: Molecular complexes

An ab initio LCAO MO SCF calculation has been carried out on three different complexes between water and carbon dioxide. It has been found that a planar complex in which the carbon atom is bound to the oxygen atom in the water is the most stable, with a calculated energy of formation of ?25.7 kJ/mol.