Metal halide complex

Metal halide complex

Metal halide complexes are coordination compounds containing metal ions with only halides as a ligands. These complexes are common for many metals and some are important technologically. They have the general stoichiometry [MXn]z-[1]

Contents

Stoichiometry and structure

Metal halides are known with several stoichiometries, but the main ones are the hexahalometallates and the tetrahalometallates. The hexahalides adopt octahedral coordination geometry whereas the tetrahalides are usually tetrahedral. Square planar tetrahalides are known as are examples with 2- and 3-coordination. Some members of this series are listed in the table below.

Complex colour electron config. geometry
[TiCl4 colourless (t2g)0 tetrahedral
[Ti2Cl10]2- colourless (t2g)3 bioctahedral
[TiCl6]2- yellow (t2g)0 octahedral
[CrCl6]3-  ?? (t2g)3 octahedal
[MnCl4]2- pale pink (eg)2(t2g)3 tetrahedral
[FeCl4]2- colourless (eg)3(t2g)3 tetrahedral
[CoCl4]2- blue (eg)4(t2g)3 tetrahedral
[NiCl4]2- blue (eg)4(t2g)4 tetrahedral
[CuCl4]2- blue (eg)4(t2g)5 tetrahedral
[PdCl4]2- brown d8 square planar
[PtCl4]2- pink d8 square planar

Preparation

These complexes are generally prepared by treating the binary halide complexes with additional halide. The tetrahedral tetrahalides of the first-row transition metals are prepared using quaternary ammonium chloride:[2][3]

MCl2 + 2 Et4NCl → (Et4N)2MCl4 (M = Mn, Fe, Co, Ni, Cu)

Properties

The charge-neutral complexes of Ti(IV) and V(IV) are volatile and readily distilled. Otherwise the halide complexes are anionic. For the first row metals, they are all high spin when possible and for the second and third row metals they are low spin. Only [CrCl6]3- is exchange inert.

Applications

The hexahalides of the platinum metals are intermediates in the separation of these elements. Chloroplatinic acid ( H2PtCl6) is an important catalyst for hydrosilylation. The volatility of the tetrachloride and tetraiodide complexes of Ti(IV) is exploited in the purification of titanium by the Kroll and Van Arkel Processes, respectively.

References

  1. ^ Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
  2. ^ Gill, N. S. and Taylor, F. B. (1967). "Tetrahalo Complexes of Dipositive Metals in the First Transition Series". Inorganic Syntheses 9: 136–142. doi:10.1002/9780470132401.ch37. 
  3. ^ G. D. Stucky, J. B. (1967). "The Crystal and Molecular Structure of Tetraethylammonium Tetrachloronickelate(II)". Acta Crystallographica 23 (6): 1064–1070. doi:10.1107/S0365110X67004268. 

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