Oxyhalides of transition metals

Palladium complexes with an heterocyclic N-containing ligand have been used in the carbonylation of aromatic nitro compounds with cocatalysts such as cyclopentadienyl halides or oxyhalides of transition metals belonging to group V e.g. CP2VCI) [168], a cyclopentadienyl halide or oxyhalide together with phosphorus oxytrichloride [169], or a 1,3-diketone derivative of vanadium [170], even also in the presenee of phosphorus oxychloride [171]. 

George, J. W., Halides and Oxyhalides of the Elements of Groups Vb and Vlb George, Philip and McClure, Donald S., The Effect of Inner Orbital Splitting on the Thermodynamic Properties of Transition Metal Compounds, and 2 33... 

In reporting a Ziegler-Natta catalyst, the kind of transition metal compound should not be omitted. Group 4-8 transition metal compounds, such as halides, oxyhalides, alkoxides, acetylacetonates, etc., have been used as catalyst precursors with activators such as alkyl derivatives or hydrides of group 1-4 metals. Titanium chlorides and triethylaluminium are most commonly applied for the preparation of heterogeneous catalysts in an aliphatic hydrocarbon medium. Also, vanadium oxychloride or acetylacetonate and dialkyaluminium chloride are often used for the preparation of homogeneous catalysts in an aliphatic hydrocarbon or an aromatic hydrocarbon medium. 

We include in this chapter a short section on oxyhalide ions of transition metals. 

Systematic features in the structural chemistry of the uranium halides, oxyhalides and related transition metal and lanthanide halides, J. C. Taylor, Coord. Chem. Rev., 1976, 20,197-273 (205). 

As mentioned in the previous section, transition metals in high oxidation states exhibit behavior that is similar to that of some nonmetals. Vanadium(V) does this with the formation of oxyhalides having the formulas VOX3 and V02X. 

Chemical transport reactions involving the element and/or an elemental subhalide and a transition metal halide/oxyhalide. The components are heated in evacuated sealed glass tubes at 100-300 °C for a period of days to weeks. Applying a small temperature gradient (10-20 °C) leads to gas phase transport and crystallization at the cooler end of the ampoule [19]. 

Several types of Werner complexes have been investigated over the last few years by TDDFT methods. They include metal oxide, metal halide, metal oxyhalide compounds, and transition metal complexes with bidentate ligands such as ethylenediamine and acetylacetonato. 

Ziegler-Natta Catalysts (Heterogeneous). These systems consist of a combination of a transition metal compound from groups IV to VIII and an organometallic compound of a group I—III metal.23 The transition metal compound is called the catalyst and the organometallic compound the cocatalyst. Typically the catalyst is a halide or oxyhalide of titanium, chromium, vanadium, zirconium, or molybdenum. The cocatalyst is often an alkyl, aryl, or halide of aluminum, lithium, zinc, tin, cadmium, magnesium, or beryllium.24 One of the most important catalyst systems is the titanium trihalides or tetra-halides combined with a trialkylaluminum compound. 

Graphite is an example of an extended solid with a band gap equal to 0.0 eV at 0 K. Hence it can readily accept electrons into its vacant conduction band or relinquish electrons from its full valence band. Other hosts used in intercalation reactions, such as transition metal dichalcogendies and transition metal oxyhalides, tend to prefer acting as oxidizing agents only given the high formal oxidation state of the metal ion. 

The transition metal oxyhalides occur with four main structure types (FeOCl, AlOCl, SmSI, and PbFCl) but only the FeOCl structure undergoes topotactic redox reactions. A layer of the orthorhombic FeOCl structure is shown in Figure 21. The structure is characterized by double sheets of distorted octahedra that share edges. Each iron atom is coordinated to four oxide anions and two chloride ions, with the two chloride ions in cis octahedral positions on the outside of the layer. The arrangement of octahedra within a layer is similar to that found in y-FeOOH and y-AlOOH. 

Finite ions and neutral molecules containing 0 directly bonded to a metal atom are formed by many transition metals. They include molecular oxyhalides and oxyhalide ions, which are included in Chapter 10, vanadyl and uranyl compounds, and numerous oxo-molecules and ions of metals such as Mo, Re, Os, etc. We give here a few examples of complexes of which the structures have been determined. The octahedral ion in K2 [0s 02(0H)4] has the same (trans) configuration as the anion in K2(0s02Cl4). Rhenium forms many octahedral ions (Re X40L) where X is halogen and L is H2O, CH3CN, etc. 

Current interest in metal cluster compounds has arisen from the demonstration that metal-metal bonds play a key role in determining the chemistry of large classes of compounds, in particular, those with heavy metal atoms in low valent states. The occurrence of metal-metal bonding in transition metal complexes has been surveyed 21, 26, 59, 271, 275), and the criteria for metal-metal bonding and the factors contributing to the stability of such bonds have been discussed. Schafer and Schnering Sll) and more recently Keppert and Vrieze 229) have reviewed the lower halide, oxide, and oxyhalide clusters of the heavier transition metals. Cotton 102) has considered the transition metal clusters in terms of structural types, and a similar approach has been adopted in a review of molecular polyhedra of high coordination number 309). 

A number of unique difficulties pertain to oxidation states of metal ions encountered in molten salt solutions. For example, for first-row transition metals, the highest oxidation state prevailing is often +3, as in the case of Fe and Cr. Frequently, for chlorides in particular, the +3 state compounds are volatile at suitable operating temperatures and, hence, their solutions are thermally unstable.Other problems encountered include rapidly dispropor-tionating states, the formation of oxyhalides, and precipitation of complexes by reaction with the melt. While redox reactions per se involve very fast charge transfer steps, these may occur at the extremes of the range of electrochemical stability, thus leading to concomitant solvent melt decomposition. Nevertheless, suitable processes such as Fe /Fe on vitreous carbon in chloride melts can be employed to determine the effective electrochemical areas of electrodes where diffusion coefficients are accurately known. ... 

The transition-metal oxyhalide FeOCl was the first investigated host (38). In a typical synthesis, a mixture of FeOCl and monomer (neat or in acetonitrile solution) is stirred at 40-50°C for several days. The violet solid turns brown-black gradually with a shiny metallic luster and, few days later, it is recovered by filtration, washed, and dried. The success of the redox intercalative polymerization is strongly dependent on the first anodic potential ( pa) of the heterocyclic monomers with respect toward the reduction potential of the inorganic phase. Although the precise redox potential of FeOCl under the reaction conditions is not known, it may be inferred from RIP experiments with different monomers that this value is comprised between 1.32 and 1.86 V versus SCE (40). These lower and upper bounds indeed correspond to the of the most oxidizible monomers which are readily... 

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