[PTIN₆] OCTAHEDRA IN LOW VALENT INDIUMFLUORIDES AND INDIUMOXIDES - A NEW CLASS OF HIGHLY IONIC COMPOUNDS CONTAINING MAIN GROUP ELEMENT CLUSTERS

Jürgen Köhler

Max-Planck-Institut FKF, Heisenbergstr. 1, D-70569 Stuttgart,
e-mail: GA-DRJ@t-online.de

Keywords: In-fluorides, In-oxides, mixed valency, clusters, preparation, band structure.

The search for new compounds containing low valent metal atoms seems especially worthwhile since unusual chemical and physical properties are mostly connected to the non-closed-shell configurations of individual elements. This applies especially to fluorides or oxides, as the highly ionic character of these compounds in combination with the kind of chemical bonding of the excess valence electrons (delocalised, lone pairs, metal clusters...) leads to interesting properties. However, low valent oxides and fluorides are often difficult to obtain because of their tendency to disproportionate. The chemistry of In serves as a good example. Any efforts to prepare InO or In₂O have failed so far. With the exception of InBF₄ [1] fluorides containing In in an oxidation state less than +3 are also up to now unknown, in spite of some efforts for its syntheses which date back into the thirties [2].

The new mixed valent fluorides PtIn₇F₁₃ = [PtIn₆]¹⁰⁺In³⁺F^-₁₃ and Pt₃In₂₂F₄₀ = ([PtIn₆]¹⁰⁺)₃In³⁺₃In⁺F^-₄₀ have been obtained by the reduction of InF₃ by In in the presence of Pt. Characteristic building units in both structures are [PtIn₆]¹⁰⁺ octahedra, which are surrounded by 24 F to form a [PtIn₆]F₂₄ cluster, s. Fig. 1 a). The Pt-In distances are very short (d_Pt-In = 254-256 pm) compared to the corresponding distances in intermetallic phases like Pt₃In₇, in which Pt is square antiprismatically coordinated by 8 In (d_Pt-In = 273-277 pm). The [PtIn₆]F₂₄ clusters are connected through [InF₆]^3- octahedra to a three-dimensional network.

a) [PtIn6]10+ octahedron together with the 24 surrounding F	b) [InF15]14- polyhedron in Pt3In22F40.

Fig. 1

Besides [PtIn₆]¹⁰⁺ and [InF₆]^3- octahedra Pt₃In₂₂F₄₀ contains additional In⁺ embedded into a cage of 15 F with In-F distances of 280-360 pm, s. Fig. 1 b). Such a high coordination number is normally only found in intermetallic phases and has never been observed in an ionic compound. Obviously In⁺ is an extremely large ion because of the extension of the lone pair of electrons. According to Extended Hückel calculations this lone pair has nearly pure 5 s character and exhibits no stereochemical activity despite the rather asymmetric surrounding.

There are three possibilities for the charge distribution within the [PtIn₆]¹⁰⁺ octahedra resulting in integer oxidation states for the In and Pt atoms, which are all in agreement with an octahedral surrounding for Pt with respect to the approach of the ligand field theory. In all three cases Pt achieves a stable 18 electron configuration assuming that In⁺ is a 2-electron s-donor and In²⁺ is a 1-electron s-donor, respectively.

[Pt⁴⁺In⁺₆]¹⁰⁺ [Pt^2-In²⁺₆]¹⁰⁺ [Pt^8-In³⁺₆]¹⁰⁺

According to bond order summations, calculations of the Madelung part of lattice energy and also Mulliken population analysis the most reasonable oxidation states seem to be +2 for the In atoms and -2 for the Pt atoms, which are then isoelectronic to the well-known Au^-, as it is found in CsAu. However, semi-empirical quantum mechanical calculations show, that considerations in terms of an ionic limit are too simplified and that within the PtIn₆ octahedra both Pt-In interactions and In-In interactions are present, see Fig. 2. In contrast to the well-known complexes [Pt(NH₃)₆]⁴⁺ or [PtCl₆]^2-, the HOMO of a [PtIn₆]¹⁰⁺ cluster is a t_1u state and the calculated band gap D_o is determined by the difference in energy between this state and e_g* (LUMO). This band gap D_o is, as expected, slightly bigger than the optical gap determined by UV absorption spectroscopy of approximately 3.3 eV.

The replacement of In by Ga and F by O has recently led to the discovery of the new mixed valent compounds Pt₂In₁₄Ga₃O₈F₁₅ and PtIn₆(GaO₄)₂, which contain GaO₄ tetrahedra and PtIn₆ octahedra with d_Pt-In ~ 255 pm. Both, the oxidfluoride and the oxid, are stable in air, against water and non-oxidising acids. PtIn₆(GaO₄)₂ is isotypic to Co₉S₈ (or the mineral Pentlandit (Fe,Ni)₉S₈) which according to [CoS₆](SCo₄)₂ consists of CoS₆ octahedra and SCo₄ tetrahedra, see Fig. 3. There are indications for the existence of corresponding silicates and aluminates containing main group element clusters centered by a transition metal atom.

Fig. 3 Projection of the crystal structure of PtIn₆(GaO₄)₂. Dashed lines represent the cubic unit cell. PtIn₆ octahedra and GaO₄ tetrahedra are graphically emphasized.

1. H. Fitz, B. G. Müller, Z.Anorg. Allg. Chem. 623 (1997) 579.
2. O.Hannebohn, W.Klemm, Z.Anorg.Allg.Chem. 229 (1936) 22.