The Early Transition Metals

BY F. L. BOWDEN, K. GRUNDY, AND B. E. REICHERT

PART I: Titanium, Zirconium, Hafnium, Vanadium, Niobium and Tantalum

by F. L. Bowden

1 Titanium

Introduction. — Various aspects of titanium chemistry are described in books published this year. Reviews have dealt with sandwich compounds of titanium and with its general, industrial, geological, and structural chemistry. The annual survey for 1972 of the organometallic compounds of titanium has appeared. Further interest in a possible biological role for titanium has been generated by the detection of up to 2 percent of the element in the red (Ahnfeltia plicata), brown (Laminaria japonica), and green (Ulva tenestrata) seaweeds. The titanium is thought to be present in combination with substances similar to 4'-phosphopantetheine or its derivatives.

Titanium atoms formed by evaporation from the metal using an electron beam technique have been co-condensed with benzene, toluene, and mesitylene to give the red di-η-arene-titanium compounds Ar2Ti, (Ar = C6H6, MeC6H5, and Me3C6H3). These compounds decompose into their constituents on heating. The titanium atoms produced in this way showed no catalytic activity in the oligomerization of butadiene, but catalytically active species resulted from the treatment of the di-η-arene compounds with organochloroaluminium reagents, or from condensates of titanium, aluminium, and organic halide vapours.

Continuing studies of charge distribution in sandwich compounds of low-valent titanium have revealed that CpTiC8H8 is more difficult to metallate than CpTiC7H7, and that methyl substitution of the metallated product occurs predominantly in the Cp ring. These chemical indications of decreased negative charge in the C8H8 ligand are substantiated by photoelectron spectroscopy, which shows the +0.4e charge on the titanium in CpTiC8H8 to be almost equal to that on the Cp ring, –0.3 to –0.4e, thus leaving the C8H8 ring almost neutral; this contrasts with charge of –0.7 to –0.8e for the C7H7 ring in C7H7TiCp.

Binary Compounds and Related Systems. — Halides and Oxyhalides. MO calculations on the TiF molecule have been performed. The equilibrium:

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is characterized by ΔH°(298 K) = –40.6 kcal mol-1, ΔS°(298 K) = 36.4 e.u., and ΔCp = 4 cal deg-1 mol-1. Negative ion electron impact studies of TiX4 (X = Cl, Br, or I) have allowed the electron affinities of TiX3 to be estimated as: 0.6 ± 0.2 (Cl), 0.8 ± 0.3 (Br), and >0.9 (I) eV. A distortion in layer-structured TiCl3 below 217 K has been revealed by far-i.r. spectroscopy; it is thought to arise from the formation of pairs of covalently bonded Ti3+ ions.

Methods for the production and purification of TiCl4 have been reviewed, and one method has been patented.

The energies of the higher filled molecular orbitals and lower excited states of TiCl4 calculated by the all-valence electron self-consistent MO method, are in good agreement with values obtained from photoelectron spectroscopy. Other spectroscopic studies include the detection by mass spectrometry of dimers in TiCl4 vapour; the assignment of i.r. absorption bands at 135, 83, and 64 cm-1, to the v modes of TiX4-(X = Cl, Br, or I); and the examination of the effects of organic solvents on the resonance Raman spectrum of TiI4. Vapour pressure measurements on TiI4 yield enthalpy values for sublimation, evaporation, and formation, of: 18.00 ± 0.05, 15.12 ± 0.01, and 2.88 ± 0.55 kcal mol-1, respectively.

An attempt to prepare TiOF2 from 40% aqueous HF and TiO2 gave material whose i.r. spectrum indicated the presence of hydroxy-groups.

Chalcogenides. Ternary phases [FORMULA NOT REPRODUCIBLE IN ASCII], having exchangeable cations, can be obtained by the electrochemical reduction of the binary M2X2 compounds or by chemical reduction with powerful reducing agents. Oxidation of the ternary phases yields the binary chalcogenides. Cation exchangeability and the degree of solvation are directly and mostly reversibly coupled with the redox state of the solid phases. The reaction between Cu2S, Ti2S3 or TiSx, and an excess of sulphur at 973 K, affords the spinels [FORMULA NOT REPRODUCIBLE IN ASCII]. Single crystals were obtained using I2 as transport agent. The temperature dependence of the magnetic susceptibility of these spinels indicated the existence of solid solutions deficient in copper. Optical, electrical, and heat-capacity studies have been carried out on the solid solutions TixTa1-xS2, and TixNb1- xSe2. An atomic absorption method has been developed for the determination of titanium in ternary sulphides of the type Fe1- xTixS(x ≤ 0.3). Intercalation of lithium or sodium into layer-structured TiS2 produces phases in which the alkali-metal cations are mobile in the midst of van der Waals forces, according to the results of an n.m.r. study; this study also confirms the formulation of the phases as xA+TiSx-2 and suggests that the structural type of the intercalation compounds of layered chalcogenides may be related to the ionicity of the A — S and M — S bonds. Two phases CaxTiS2 (0.03 < x< 0.50) have been obtained from the reaction between calcium and TiS2 in liquid ammonia. The location of the nitrogen atoms mid-way between the TiS2 layers in TiS2,NH3 lends support to the inference from the observed anisotropy of the proton spin-lattice relaxation in solid TaS2,NH3 that the lone-pair orbitals lie parallel to the MS2 layers and not perpendicular as previously postulated. Cobalt occupies octahedral holes of the sulphur hexagonal close-packing in the empty metallic layers of the TiS2 structure. The sulphur octahedra are distorted and the occupancy ratio for octahedral holes varies from 0.21 to 0.41. The electrical, magnetic, and structural properties of the intercalation phases MxTiS2 (M = Fe, Co, or Ni) have been reported.

Carbides and Silicides. The preparation and structures of titanium carbide single crystals have been reviewed. High purity TiC has been obtained by heating together TiH2 and carbon in the presence of a fatty acid or fatty acid ester. Titanium carbides have been included in a study of solid-phase reactions of high-melting compounds with transition metals and graphite. TiO2 and CO2 are the final products of the oxidation of TiC in an excess of oxygen. The oxidation scale also contains Ti3O5, Ti2O3, and TiO. X-Ray diffraction has detected the presence of titanium oxycarbides at the interface between the TiC and the scale; this indicates the intermediacy of these compounds in the oxidation of the carbide. Substoicheiometric titanium carbides have been prepared from...