Calculation of transition metal compounds using an extension of the CNDO formalism

An extended CNDO formalism for the treatment of large transition metal cluster systems is presented. After a detailed discussion of parametrization it is applied to a family of compounds, namely to Co(CO) ₄ ^– , Co₂(CO)₈, Co₄(CO)₁₂, Mn₂(CO)₁₀.The results can be interpreted in the light of simple electron counting rules and additionally allow detailed insight in bonding capabilities of large metal cluster systems.Bridged and unbridged clusters are compared and the results are extrapolated to surface systems. In the case of Co₄(CO)₁₂ two possible stereoisomers of symmetryC _3v andT _d are discussed.     

Calculation of transition metal compounds using an extension of the CNDO-formalism

A recently developed extension of the CNDO-method is used to study the electronic structure of a number of binuclear transition metal carbonyls and carboxylates with fourfold or quasi-fourfold symmetry. The results are compared to those available from nonempirical calculations. Special attention is paid to the nature of the metal-metal bond. Connections with qualitative MO-considerations allow a fairly general discussion of metal-metal bonding in binuclear transition metal complexes with basic fourfold symmetry. A few, up to now unknown, but possibly existing, complexes are considered.     

An extension of the CNDO/2 formalism for the study of transition metal complexes

The electronic structure and the conformational analysis of some Ni(PX₃)₄ (X = F, OCH₃, Cl, CH₃, H) complexes are investigated within the framework of the previously described extended CNDO/2 method in order to interpret their magneto-optical behaviour and to propose a suitable scheme for the electronic content of a metal-ligand ( + ) dative bond. Chatt and Wilkinson's pattern, used up to now, is criticized and a new scheme is proposed which — taking accurately the role of electronegativity into account and abiding by Pauling's principle of electroneutrality — fits well the sets of available physico-chemical data and allows us to understand whyab initio bonds overlap populations as well as IR force constants are low for any metal-ligand ( + ) dative bond.     

An extension of the CNDO/2 formalism for the study of transition metal complexes

A previously described extended CNDO/2 method is used for investigating the energy level distribution and electronic structure of trifluorophosphine metal complexes: Cr(PF₃)₆, Fe(PF₃)₅, Ni(PF₃)₄. The results are compared with the few experimental data which are known for these complexes. The metal-phosphorus bonds show large (P M) and (M P) charge transfers but small total charge transfers (M P) which induce on the metal in any case a small positive charge.Such ( + ) coordination bonds seem to be generally characterized by small bond overlap populations (or small Wiberg indices).     

Improved CNDO/S calculation of electronic spectra of organic compounds. I. New CNDO/S calculation by using an improved method of one-center electron repulsion integral

The NM-gamma CNDO/S program previously developed by our group was modified by the introduction of a new one-center electron repulsion integral gamma(AA)(new) approximation, namely, the gamma(AA)(new)-CNDO/S method. The value of this gamma(AA)(new) was evaluated according to the product values of the coefficient C with the gamma(AA) value proposed in our previous paper. This method using a new gamma(AA) was also found to improve the two-center electron repulsion integral gamma(AB) value with respect to the chemical softness proposed by Nishimoto and co-workers, together with the difference between HOMO and LUMO orbital energies. The results calculated by the present improved gamma(AA)(new)-CNDO/S method demonstrated that not only the calculated absorption maxima wavelengths and ionization potentials, but also the order and the assignment of orbitals coincided very well with those based on the results of experiments investigating a variety of polyenes, cyanynes, and polycyclic aromatic hydrocarbons.     

Effect of transition metal oxide additives on the activity of an Ag/SiO2 catalyst in carbon monoxide oxidation

The catalysts of silver supported on mesoporous silica modified with Co₃O₄, CeO₂, and ZrO₂ were prepared by an impregnation method; characterized by X-ray diffraction analysis, temperature-programmed reduction, and low-temperature nitrogen adsorption; and studied in a model reaction of CO oxidation. It was found that the Ag/SiO₂ system exhibited high activity in the reaction of CO oxidation, and the addition of transition metal oxides led to reduction of the temperature of 50% CO conversion by 40°C. The modification of Ag/SiO₂ with cerium dioxide was found most effective because of the interaction of silver particles and CeO₂ on the surface of silica gel.     

Calculation of the valence shell correlation effects on ionization potentials. A test on N2, C2N2 and H2NN

Two schemes for the definition of the valence space from an extended basis are proposed and tested for the calculation of ionic states in N₂, C₂N₂ and H₂NN. Good results are obtained with a projection scheme, leading to a 2h-1v model reproducing the essential features of the full basis 2h-1p results. Notably excellent agreement with experiment is obtained for the main ionization potentials in N₂ and C₂N₂. Unsatisfactory results are instead given by a simple extension of the minimal basis set SCF space, indicating the importance of employing accurate occupied orbitals as a starting point.     

The valence shell spectra of acetylene and hydrogen excited by Zr Mζ x-rays. An example of the advantages provided by an ultra-soft x-ray source

Excitation of the valence shell molecular orbital spectrum of acetylene with Zr M_ζ (151.4 eV) radiation shows clearly all four expected bands 1π_u (11.4 eV), 3σ_g (16.8 eV), 2σ_u (18.8 eV) and 2σ_g (23.5 eV) with approximately equal intensity in contrast to He or Mg K_α excited spectra. A spectrum of molecular hydrogen can also be obtained with this radiation.     

Promoting nontransition metal alkylation with organic halides in the presence of binary systems based on an organometallic compound and a transition metal compound: V. Effect of the nature of the transition metal compound in the binary system on the rate of a steady-state process

The example of alkylation of commercial zinc and cadmium powders with organic halides in the presence of binary systems comprising an organometallic and a transition metal compounds was used to show that the nature of the transition metal compound in the binary system strongly affects the rate of a steady-state process. Therewith, the significant factors are both the nature of the transition metal and the ligand composition. It was found that the activity of the binary systems correlates with the activity of the transition metal compound in the transmetalation reaction with the organometallic component of the binary system (reduction of the transition metal compound).     

Electronic effects in transition metal porphyrins. 10. Effect of ortho substituents on the temperature dependence of the NMR spectra of a series of spin-admixed perchloratoiron(III) tetrakis(2,6- or 2,4,6-phenyl substituted)porphyrinates

The perchloratoiron(III) complexes of a series of 2,6-disubstituted tetraphenylporphyrin ligands, where the 2,6-phenyl substituents were -H, -F, -Cl, -Br, or -OMe, as well as two 2,4,6-phenyl-substituted complexes, where the substituents were -Me and -OMe, have been investigated as a function of temperature by 1H NMR spectroscopy. Curvature in the 1/T dependence was evident in most cases. Forced linear extrapolation of the temperature dependence observed over the range of the study yielded Curie plots that include negative slopes with very large positive 1/T intercepts (Cl approximately Br > Me > H) to negative slope with near zero intercept (tri-OMe) to positive slope with very large negative intercept (F, di-OMe). The NMR results were combined with EPR spectroscopic data and curve-fitting procedures based on an expanded Curie law to arrive at a consistent overview of the variety of temperature-dependence behaviors observed. This overview relies upon the premise that, in addition to the ground state observed by EPR spectroscopy, one (or more) thermally accessible excited state(s) are populated to varying degrees over the temperature range of the NMR measurements. If only one excited state is considered, the analysis is consistent with the ground state being a largely intermediate-spin state (S = 3/2) for the majority of the complexes but a largely high-spin state (S = 5/2) for ((2,6-F2)4TPP)FeOClO3 and ((2,6-(OMe)2)4TPP)FeOClO3.     

Structure and vibrational spectra of tungstovanadate compounds containing an anion of the Keggin type. The structure of the salt (C2N2H10)2[VW11V1O40]·6H2O

Institute of Inorganic Chemistry, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 3, pp. 82–87, May–June, 1989.     

Transition metal nitrosyl compounds. Part XIV(1). The oxidation of carbon monoxide by nitric oxide using [M(NO)2(PPh3)2]n+ (M=Fe,Ru or Os; n=0. M=Co,Rh or Ir; n=1)

Summary Carbon monoxide reacts with the cationic dinitrosyls [M(NO)₂(PPh₃)₂]⁺ (M = Rh, Ir) under ambient conditions to produce CO₂, N₂O and the tricarbonyl cations, (M(CO)₃(PPh₃)₂]⁺. The cationic tricarbonyls are reconverted into the dinitrosyl reactants on treatment with NO atca. 80°. The Ru(NO)₂(PPh₃)₂ and Os(NO)₂(PPh₃)₂ complexes react similarly with CO but under more vigorous conditions whereas the corresponding dinitrosyls of cobalt and iron do not undergo this reaction under similar conditions. A pentacoordinate dinitrosyl intermediate [M(NO)₂(CO)(PPh₃)₂]ⁿ⁺ is proposed and a mechanism for the catalytic oxidation of CO by NO is presented. Studies of Pt(N₂O₂)PPh₃)₂ establish that a dinitrogcn dioxide intermediate, produced by the coupling of two nitrosyl ligands, is reasonable.     

Photoinduced reactions of organic compounds with transition metal complexes. Communication 1. First example of substitution in the aromatic nucleus in photoexcitation of an electrophilic reagent: formation of σ-aryl complexes of Pt(IV) in the reaction of PtCl 6 2−2 with arenes

Conclusions The proposed mechanism of the photoinduced reaction of the PtCl ₆ ^2– ion with an arene in acetic or aqueous trifluoroacetic acid, completed by the formation of a platinum (IV) -aryl complex, includes a stage of electron transfer from the arene to platinum(IV) and the formation of an ion radical pair which is then transformed into a Wheland complex.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2681–2687, December, 1984.