The Early Transition Metals

BY F. L. BOWDEN AND C. D. GARNER

PART I: Titanium, Zirconium, Hafnium, Vanadium, Niobium, and Tantalumby F. L. Bowden

1 Titanium

Introduction. — There have been several reviews of the chemical technology of titanium and its compounds. The organometallic and structural chemistry of titanium reported during 1972 has been reviewed as have some aspects of its synthetic chemistry. According to ab initio calculations the Ti — C bond in TiCO and TiCO+ can be described as a CO non-bonding pair that is shifted slightly onto the Ti and a shift of Ti 3d π-orbitals back onto the CO.

Vibrational assignments have been made for bis(cyclo-octatetraene)titanium. The intensities of the skeletal vibrations indicate that the two C8H8 rings are more electrostatically bonded than in the corresponding Th and U compounds. X-Ray photoelectron spectra of (C5H5)M(C7H7) (M = Ti, V or Cr) show that the oxidation state of the metal increases in the sequence M = Cr < V < Ti, resulting in an increased electron density on the ligands in the same sequence. The highest negative charge is found on the C7H7 ring for the Ti compound, in accord with other results.

The magnetic behaviour of Cp2Ti+pic- has been discussed in terms of the ligand-field model.

The blue, diamagnetic mono- and di-methyl substituted complexes [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Ti have been prepared by the reaction of (h5-C5H4R)TiCl3 with PriMgBr in the presence of an excess of C7H7R. They are stable up to 300°C but are decomposed by air and water. Their mass spectra indicate substantial rearrangement involving the transfer of a CH or CCH fragment from the seven- to the five-membered ring leading to ions of dibenzenetitanium derivatives.

The full report of the crystal structure of Cp3Ti has appeared. Diphenylacetylene reacts with CpTi(CO)2 in the cold to give the yellow monoacetylene complex Cp2Ti(CO)PhCPh; the acetylene is readily displaced by CO and the complex is an active catalyst for the hydrogenation of olefins and acetylenes. Under more vigorous reaction conditions the metallocycle (1) is formed.

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X-Ray diffraction and chemical studies on [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2) and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3), and 13C n.m.r. studies of 'titanocene' (4) have finally established the structure of this unusual molecule. In (2) the titanium atoms are bridged by a µ-(h5-C5H4–C5H4), fulvalene, ligand and by H and H2AlEt2 ligands, whereas in (3) the bridging ligands include both a fulvalene ligand and an [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] ligand. The chemical relationships between 'titanocene', (2), and (3) (4) indicate strongly that 'titanocene' has the structure shown and this has been supported by C n.m.r. spectroscopy.

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'Titanocene' has been used to reduce azo-compounds, ketones, alkynes, and organic halides. Results of deuterium-labelling experiments indicate that the hydrogen in the reduced products is derived from the h5-C5H5 rings of the 'titanocene'. In view of the dihydride structure of (4) it seems more likely that deuterium labelling of titanocene has produced its deuterium analogue and that reduction occurs via the hydride hydrogens.

E.s.r. evidence has been presented to support the proposal that hydrogen and hydrogen-alkyl complexes are formed by treating Cp2M-AlEt complexes with H2. The involvement of titanocene and related species in the reduction of dinitrogen has been further investigated. Permethyltitanocene undergoes a rapid and reversible reaction with N2 forming C5Me5TiN2TiC5Me5. Solutions of this compound in toluene absorb more nitrogen at low temperatures with an accompanying colour change from dark blue to intense purple-blue. At –80°C the absorbed N2 is entirely retained, even under reduced pressure. The amount of nitrogen released when the low-temperature N2 complex reverts to(C5Me5)2 Ti at room temperature is consistent with the stoicheiometry (C5Me5)2TiN2. Furthermore, the H and C n.m.r. spectra of solutions of the complex below –62°C indicate that it exists in two forms. The 15N n.m.r. spectrum of (C5Me5)2Ti15N15N at –61°C showed two doublets, and a singlet at lower field; the equilibrium (5) [??] (6) was proposed. The doublet structure of the higher-field signals was attributed to 15N–15N coupling [J(15N–15N) = 7 [+ or -] 2 Hz] between the non-equivalent nuclei of (5). The i.r. spectrum provided further evidence for two forms of the complex and showed that the N [equivalent to] N bond orders are affected to similar extents by co-ordination in both complexes.

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Binary Compounds and Related Systems. — Halides and Oxyhalides. At 1100 — 1450°C TiCl4 reacts with metallic Ti to give lower Ti chlorides with a Cl :Ti ratio in the gas phase of 2.00 — 2.50. An increase in temperature does not affect the degree of reaction of TiCl4. Reduction of the partial pressure of TiCl4 vapour in the original mixture with Ar reduces the Cl:Ti ratio from 2.60 to 2.00. The fraction of Ti2+ ions in a NaCl melt at a concentration of Ti of 0.83 — 0.4 wt. % and at 950 — 1100°C is close to unity (0.80 — 0.97). The amount of Ti2+ increases with decreasing temperature. At the temperature of the electrolytic processes (850 — 900°Q this value is still lower, as indicated by the isotherms of equilibrium potentials of Ti relative to a Cl comparison electrode as a function of the amount of Ti dissolved in the melt. With an increase in temperature the equilibrium constant of the reaction

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decreases. For low concentrations of Ti and relatively low temperatures (800 — 900°C) it may be considered that all the Ti dissolved in the melt is in the form of Ti2+. Disproportionation of TiCl3 begins at 550°C in an inert atmosphere and at 660 — 80°C in a TiCl4 atmosphere. At 600°C, 95 — 96% TiCl3 is still present. The equilibrium diagram, electrical conductivity, and density of fused TiCl4 — TiBr4 mixtures indicate almost ideal solution behaviour without appreciable chemical interaction in either the liquid or solid phase. This is at variance with an earlier report. TiCl4 has been investigated as an n.m.r. shift reagent.

Titanium(m) oxychloride has been obtained as a hydrate from the hydrolysis of TiCl2–TiCl3 mixtures in air, and as the anhydrous material from the reaction between TiO2 and TiCl2 at 650...