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:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

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 titanium hydride powder and carbon at 1773 K, and the phase diagram of the system TiC–WC–TaC has been investigated.

The high-temperature reaction between metal powders and SiCl4 affords the ternary silicides TiCo4Si3 and TaCo4Si3 as single crystals, and TiFeSi3 and TiNi4Si3 as fine powders. These compounds are structural analogues of hexagonal NbCo4Si3 with a ≈ 17 and c ≈ 8 Å. Lattice dimensions of low concentration metalloid stabilized Ti5Si3 have been reported.

Nitrides, Borides, Hydrides, etc. Titanium nitrides have been prepared from: TiH2 and nitrogen or ammonia; TiO2, carbon, and nitrogen; and from Ti metal and nitrogen atoms formed in a plasma jet. Material prepared by this last method had a homogeneity range of 34.5 — 50.0 atom %N, and exhibited superconductivity. The lattice parameters of a non-stoicheiometric titanium nitride have been determined. An examination of the properties of titanium carbonitrides prepared from TiCl4 and amines showed that the best deposits for use as hard coatings were obtained from triethylamine.

Titanium borides have been reviewed; conditions suitable for the growth of boride single crystals from solution have been reported. The electronic structure of titanium diboride is the subject of a conference report. The reduction of gaseous BCl and TiCl4 with hydrogen on a boron nitride substrate in the temperature range 1323 — 1523 K, yields the so-called I-tetragonal (B12)4B2Ti1.87. The (B12)4 system is composed of icosahedra which are arranged in a flat tetrahedron. The titanium lies at the centre of this with an environment of a 14-corner polyhedron. The single boron atoms have a distorted tetrahedral environment with respect to the boron atoms of the icosahedra.

A method for the preparation of titanium hydride has been patented. Hydrogen atom recombination coefficients for the system H–TiHn (n = 1.43 — 2.0) have been rationalized in terms of hydrogen atom distribution between tetrahedral and octahedral sites in the solid hydride; the mobility of the hydrogen in these hydrides has been confirmed by an independent n.m.r. study. Decomposition of the hydrides in sulphuric acid results in the liberation of hydrogen and the formation of Ti ions; the maximum dissolution rate occurs at a Ti: H ratio of 1:0.11.

Titanium(II). — A brown, air-sensitive complex formulated as Cp(1-methylallyl) (butadiene)titanium has been isolated from the reaction of 2-methylallylmagnesium chloride and CpTiCl3 or CpTiCl2.

Co-ordination of the iodate ion in M2[Ti(IO3)6]nH2O (M = Li, K, Rb, Cs, or NH4; n = 0 or 2) is indicated by the ion's i.r. spectrum, which shows three bands characteristic of the IO3 group distorted to C or C. symmetry. Phase transitions from the Ni-As type of the Mn–P type occur at an intermediate composition in solid solutions of titanium(II) sulphide and vanadium(II) sulphide.

Titanium(III). — Halides. MO calculations have been performed on the TiF3-6 ion. The inclusion of the 2s AO of fluorine has a significant effect both on the metal charge and on the relative energies of the MOs because of the large value of the 2s–3d overlap integral; the value of Δ for the F- ion is higher than that reported previously. Conditions for the preparation of pure NH4TiF4 have been described; this material is formed in the thermal decomposition of (NH4)3 TiF6.

The strongly temperature-dependent magnetic susceptibility of β-TiCl3 has been attributed to Ti3+ ions at the ends of chains in the surface of the crystal. Only very low yields of amines were obtained from the systems: TiCl3,3THF–Mg–N2 or TiCl3, 4PriOH–Mg–N2 and ketones or alcohols. The principal reactions of the TiCl3,3THF system were hydrogenation of the ketone and dehydrogenation of the alcohol. Three subsystems have been identified in the KCl–MnCl2–TiCl3 systems; they are: [FORMULA NOT REPRODUCIBLE IN ASCII], and MnCl2–TiCl3–K3TiCl6. The absorption of ammonia on β-TiCl3 has been investigated.

O-Donor Ligands. An increase in the intensity of the e.p.r. signal of an aqueous solution of TiCl3 with increasing pH was interpreted in terms of the occurrence of the titanium as Ti3+aq at pH 1, and as TiOH2+ at pH3. A low-field signal (1 500 G) in the spectrum at 77 K indicates the existence of a dimeric species. The intensity of the low-field signal increased with pH, and its increase paralleled that of the initial room temperature signal. Both the field position and lineshape of the signal remain unchanged between pH 1.6 and 3.4. The structure (1) was used as a basis for the computer simulation of the e.p.r. spectrum, and the parameters r = 3.15 ± 0.25 Å, g = 1.97 and g = 1.88 were derived. A species of lower symmetry appears to be formed in solutions which contain substantial amounts of ethylene glycol. A structure (2) with a single OH bridge has been suggested for this species. Aqua-halide complexes are formed in solutions of Ti3+aq ion and Cl-, Br-, or I- ions. The stabilities of these complexes decrease in the order Cl- > Br- > I-. The magnetic properties of hexaureatitanium(III) salts have been determined down to 4 K. The values of k, the orbital reduction factor, and λ, the spin–orbit coupling constant, obtained from this work are in accord with the results from studies at higher temperatures; but Δ, the splitting of the 2T2g ground term, shows a significant temperature dependence. Other magnetic and e.p.r. studies have been concerned with spin-coupling in complexes of the type [CpTi(O2CR)], CpTi(O2CR)2, and [CpTi(O2CR)]2. Hyperexchange via the π-electron systems of the carboxylate bridges in [CpTi(O2CR)]2 (R = CF3, Ph, m- FC6H4, CCl3, 2-carboxytetrahydrofuran, and 2-carboxytetrahydrothiophen) is proposed to account for the pronounced antiferromagnetism (J = 500 cm-1) of these complexes. The e.p.r. parameters, e.g., g[perpendicular to] = 1.93, g[parallel] = 1.99, D = 0.05 cm-1, are characteristic of dimers with tetragonal symmetry. Similar results have been obtained for the complexes [FORMULA NOT REPRODUCIBLE IN ASCII]. Thus it appears that the titanium complexes are analogous to cyclopentadienylvanadium(III) carboxylate complexes. The electrical and magnetic properties of linear chain complexes derived from biscyclopentadienyltitanium(III) including the carboxylates have been reviewed. Titanium(III) chelates of dialkylphosphoric acids (OR)2(OH)PO (R = Et, Pr, allyl, Bu, or 2-ethylhexyl) have been prepared by the reaction of TiCl3 with the esters (OR)3PO.

Titanium(III) forms stable ternary complexes with tetrabromopyrocatechol or tribromopyrogallol and antipyrine. The observation that TiIV also forms stable ternary complexes with these reagents suggests that there is no foundation in the proposal that reduction of TiIV occurs during its extraction from aqueous solution.

Formation constants of TiIII–mandelic acid complexes have been reported as log K1 = 3.25, log K2 = 3.07, and log K3 = 2.90 (I = 0.1 mol l-1, NaClO4). A solution of the tris complex exhibits a single maximum in its visible spectrum from which a value of Δ = 28 985 cm-1 was obtained for the mandelate anion. Complex formation between TiIII and citric acid has been investigated by n.m.r. relaxation methods.