Photochemical Reactions of Ligands in Transition-Metal Complexes

Three types of photochemical reaction of coordination compounds are well-known and have often been discussed: redox reactions, substitutions, and isomerizations. Less well known is a fourth type of reaction, which occurs fairly frequently: reaction of the ligands themselves. It is the purpose of this article to fill the corresponding gap in the photochemical literature and to summarize our current knowledge of such reactions and of their mechanisms and possible applications.     

Thermodynamic Evaluation of Solid-State Reactions in which a Volatile Product is Formed

Summary. The use of equilibrium thermodynamics in describing interfacial reactions between non-ionic inorganic solids is demonstrated using examples of high-temperature interactions in the Ti–Si–N and Mo–Si–N systems. In the case of a diffusion-controlled process, solid-state reactions can be interpreted with chemical potential (activity) diagrams. The role of volatile reaction products formed during interaction in developing the diffusion zone morphology is analysed. The interfacial phenomena in systems based on dense Si₃N₄ and non-nitride forming metals can be explained by assuming a nitrogen pressure build-up at the contact surface. This pressure determines the chemical potential of Si at the interface and, hence, the reaction products in the diffusion zone.     

Photochemistry of Transition-Metal Atoms: Reactions with Molecular Hydrogen and Methane in Low-Temperature Matrices

Reactions of metal atoms with molecular hydrogen and alkanes are prototypical of more complex systems involving metal-mediated reactions in solution, on supports, and on surfaces. The simplicity of metal-atom reactions makes them particularly attractive for fundamental experimental and theoretical studies. Following the first reports of H₂ and CH₄ activation by photoexcited metal atoms in cryogenic matrices, the behavior of a number of transition-metal atoms in their ground and selected electronically excited states has been examined with molecular hydrogen, methane, and ethane. A wealth of structural, electronic, reactivity, and selectivity information is emerging from these studies and it is now becoming possible to recognize the controlling factors at work in these fundamental chemical reactions. In this article, some experimental results for a number of metal-atom systems, in particular, those involving silver, copper, manganese, and iron atoms, are presented along with a discussion of the factors that are believed to be important in the understanding of the observed physical and chemical behavior.     

Influence of Ligands on the Activity and Specificity of Soluble Transition Metal Catalysts

The reaction of a covalently σ-bonded ligand (alkyl group, hydrogen) with a molecule (olefin, CO) attached to the transition metal by a coordinate bond is the essence of many catalytic processes. The influence of the other ligands in the complex on this reaction is discussed in the present article. Various ways in which the ligands can act, e.g. by electronic effects via the σ and π electron systems of the complex and by steric effects, are first described separately and then illustrated by examples.     

Some Recent Developments in the Chemistry of Transition Metal Oxocations

The range of stability of transition metal oxocations in aqueous solution is discussed. Species with a metal-oxygen bond of order higher than unity are particularly interesting. Bond orders can sometimes be determined from bond lengths and angles, but generally only by infrared spectroscopy. – The frequency range of the metal-oxygen stretching vibration, previously assumed as about 900–1100 cm^?1, must according to these investigations be extended at least to 780 cm^?1 if all multiple bonds, particularly those in trans-dioxo complexes, are to be included. Broad bands down to 650 cm^?1 are ascribed to metal-oxygen bridges. — The existence of cations such as ZrO²⁺ and VO, or of the “well-known” titanyl ion, TiO²⁺, has never been proved. In many cases polynuclear, oxygen-bridged oxometal species must be assumed.     

Energy-adjustedab initio pseudopotentials for the second and third row transition elements: Molecular test for M2 (M=Ag,Au) and MH (M=Ru,Os)

Summary Recently published nonrelativistic and quasirelativistic energy-adjustedab initio pseudopotentials representing the M^(Z–28)+ cores of the second row transition metal atoms and the M^(Z–60)+ cores of the third row transition metal atoms have been tested in SCF, CI(SD) and CEPA1 calculations of the spectroscopic constants (R _e ,D _e , and _e ) of the ground states of the neutral and singly charged silver and gold dimers, and in state averaged CASSCF and multi-reference CI(SD) calculations of the spectroscopic constants (R _e ,D _e , _e , _e , /R). Comparison is made with experimental and reliable theoretical data where available; in the case of the hydrides, additional calculations with pseudopotentials published by other groups have been made for comparison.     

Siloxane Compounds of the Transition Metals

Heterosiloxanes of transition metals contain the characteristic grouping Si? O? X; in the compounds known so far, X may be Au, Zn, Cd, Hg, Ti, Zr, Hf, V, Nb, Ta, Cr, U, Re, Fe, Co, Ni, or Pt. Most of these compounds are obtained from silanols, metal silanolates, acyloxysilanes, disiloxanes, or alkoxysilanes, i.e. from compounds that already contain an Si? O bond. The transition metal may be introduced e.g. as a halide, as an organometailic compound, as an alkoxy compound, or as an oxide. Heterosiloxanes are also formed on reaction of many silicon compounds that do not contain oxygen with transition metal compounds containing oxygen. Some of these compounds, e.g. some titanosiloxanes, are very stable, whereas others decompose explosively. Oligomeric and polymeric heterosiloxanes exist in addition to the monomeric compounds.     

The Cadmium Oxide Route to Alkali Metal Oxometallates with Uncommon Structural Features

The aim of this research report is to give an introduction to a specific solid state reaction applied in the field of alkali metal rich transition metal compounds. The approach is an explorative investigation of the oxidation of transition metals with CdO in the presence of alkali metal oxides and / or compounds with complex anions such as Na₂SO₄. Thereby, a range of unusual compounds have been obtained and structurally characterized. In particular, low valences and uncommon coordination numbers (C.N.) of the late 3d transition metals, as well as interesting varieties in the anionic part of the structures are accessible. The presented examples were selected mainly in order of their inherent aspects concerning the reactivity, structural features or electronic structures.     

Metallocycle synthesis accelerated by high pressure

Reaction of cis-[FeH₂(dmpe)₂](1) with diphenylbutadiyne results in an insertion into both of the iron-hydride bonds to form an iron metallocycle. Spectroscopic and crystallographic data of [Fe(PhHCC₂CHPh)(dmpe)₂] (3) show 1,4-diphenylbutatriene is symmetrically bound to the metal via the central double bond. The reaction to form the metallocyclic complex is greatly accelerated by application of external pressure. A 41% yield of (3) is isolated after two days at atmospheric pressure or after approximately 75 min at 800MPa.     

Transition Metal Catalyzed Coupling Reactions under C−H Activation

C−C coupling by transition metal catalyzed C−H activation has developed into a diverse area of research. The applicable catalysts are manifold, and the variety of products obtained range from basic chemicals to pharmaceuticals and building blocks for carbon networks. One reaction, in which several C−C bonds are formed under C−H activation of a methyl group, is the conversion of ortho-iodoanisole according to Equation (1).     

On nd bonding in the transition metal trimers: comparison of Sc3 and Y3

CASSCF/CCI calculations are presented for the low-lying states of Y₃. Comparison of the wave functions for Y₃ and Sc₃ indicates substantial 4d-5p hybridization in Y₃, but little 3d-4p hybridization in Sc₃. The increased 4d-5p hybridization leads to stabilization of 4d bonding with respect to 4d bonding for equilateral triangle Y₃, and also leads to 4d-5p bonding for linear geometries. These effects lead to a different ordering of states for equilateral triangle geometries and a smaller excitation energy to the linear configuration for Y₃ as compared to Sc₃.     

Nitrido Complexes of Transition Metals

In the past decade new syntheses and numerous structural determinations have enlivened studies in the still relatively young field of nitrido-transition-metal complexes. Aside from the terminal function of the nitrido ligand M?N:, this group also occurs as linear μ₂-bridging ligand in symmetric and asymmetric coordination; examples are known with almost right-angled bridge function; and, finally, it also functions as μ₃-bridging ligand. Accordingly, the fresh impulses given to synthetic chemistry by nitrido complexes are also many-sided: such complexes are used, inter alia, for the preparation of phosphaniminato and thionitrosyl complexes as well as for the synthesis of metallaheterocycles of the type MN₃S₂ and MN₃P₂ with delocalized π-systems. In technetium chemistry complexes with terminal nitrido group are employed as radiopharmaceuticals, and, owing to the strong trans influence of the M?N: group, nitrido complexes of molybdenum are suitable as catalysts in olefin metathesis. Finally, nitrido complexes are also of wide interest in theoretical studies.     

Retention behavior of transition metals on a bifunctional ion-exchange column with oxalic acid as eluent

The common eluents used with a bifunctional ion-exchange column (IonPac CS5A) for separating transition metals are pyridine-2,6-dicarboxylic acid and oxalic acid (Ox). When Ox is used, cadmium and manganese co-elute. Although much research has been done to overcome the Cd²⁺–Mn²⁺ co-elution problem, the role of lithium hydroxide in separating the transition metals has received little attention. In this study, it is found that when the Ox concentration is higher than 35 mM, Cu²⁺ elutes after Pb²⁺ and Ox plays a predominant role in the retention behavior of the seven metals. When Ox concentration is lower than 35 mM especially when its concentration (25 mM) is half of the usually used standard concentration (50 mM), Cu²⁺ elutes before Pb²⁺, and at the same time, Mn²⁺and Cd²⁺ can also be baseline separated. Lithium hydroxide plays a predominant role in the separation of the metals separated by cation exchange. So, lithium hydroxide is used to adjust the pH of the eluent. The use of an isocratic elution (25 mM Ox/LiOH/2 mM Na₂SO₄, pH 3.88) allows the separation of seven metals (Cu²⁺, Pb²⁺, Co²⁺, Mn²⁺, Cd²⁺, Zn²⁺ and Ni²⁺) in a single run. The effects of inorganic modifiers such as NaNO₃, Na₂SO₄ and Na₄P₂O₇ on retention behavior of the metals are also investigated.     

Transition Structures of Hydrocarbon Pericyclic Reactions

Twenty-five years after the discovery of a vast class of organic reactions named “pericyclic reactions” by Woodward and Hoffmann, ab initio quantum mechanics provides a detailed analysis of the geometries, energies, and electronic characteristics of the transition structures of these reactions. Common features are found in all these reactions, and generalizations permit prediction of other transition-structure geometries and energies. At the same time, great diversity is observed—from strongly bonded, rigid, closed-shell entities to weakly interacting, flexible diradical structures.     

Quaternäre Chloride des zweiwertigen Europiums mit dreiwertigen Übergangsmetallen: Synthese und Kristallstruktur von Na6Eu3M4Cl24 (M = Ti,V, Cr)

Quaternary Chlorides of Divalent Europium and Trivalent Transition Metal Ions: Synthesis and Crystal Structure of Na₆Eu₃M₄Cl₂₄ (M = Ti, V, Cr) The reaction of EuCl₂, NaCl and MCl₃ (M = Ti, V, Cr) yields the chlorides Na₆Eu₃M₄Cl₂₄. According to X‐ray single crystal investigations, their crystal structure is a variant of the monoclinic cryolite‐type structure. One crystallographic site is occupied by Na1 and Eu simultaneously. For charge compensation the Na2 site is not fully occupied. In Na₆Eu₃Ti₄Cl₂₄ (P2₁/n, Z = 1/2, a = 663.8(1) pm, b = 718.3(1) pm, c = 953.3(2) pm, β = 91.55(2)°, R_all = 0.0314), Na₆Eu₃V₄Cl₂₄ (P2₁/n, Z = 1/2, a = 660.4(1) pm, b = 715.8(1) pm, c = 946.5(2) pm, β = 91.41(2)°, R_all = 0.0313) and Na₆Eu₃Cr₄Cl₂₄ (P2₁/n, Z = 1/2, a = 654.8(1) pm, b = 706.5(1) pm, c = 945.4(2) pm, β = 91.07(2)°, R_all = 0.0368) the ratio of Na1 : Eu amounts to 5 : 3. The colours of the compounds, orange yellow for M = Ti, orange red for M = V and dark red for M = Cr, indicate electronic interactions between Eu²⁺ and M³⁺.     

Cyclooligomerization with Transition Metal Catalysts

This report is concerned not only with the syntheses of new ring systems and the preparation of derivatives of known rings but also with new types of catalysts based upon, e. g., nickel, iron, molybdenum, manganese, and palladium. The broad applicability of the methylene insertion reaction as a method for identifying mono- and di-methyl-substituted eight-, ten-, and twelve-membered rings is also demonstrated. A further section is concerned with the possible mechanism of C? C bond formation in transition metal complexes; several earlier suggestions have been revised in the light of new experimental data. In addition, experimental results are discussed which indicate that the reactivity and selectivity of the complexes formed by the nickel-ligand catalyst and olefins or alkynes depend upon the structure of the ligands.