Chalcogenides are a range of compounds that primarily contain oxygen, sulfur, selenium, tellurium or polonium and which may also occur in nature. The compounds may be in binary, ternary or quaternary form. Platinum group metal chalcogenides have attracted considerable attention in recent years due to their relevance in catalysis and materials science. Extensive application of palladium (∼ 27% of global production in year 2000) in the electronic industry in multilayer ceramic capacitors (MLCCs) and ohmic contacts has further accelerated research activity on platinum group metal chalcogenide materials.

The platinum group metals form several chalcogenides:

• binary,

• pseudo-binary for example, Ru_1−xOs_xS₂, Ni_xRu_1−xS₂, etc., and

• ternary, such as spinels: M′M₂E₄ (M′ = Cr, Mn, Fe, Co, Ni, Cu; M = Rh, Ir; E = S, Se, Te) and Tl₂Pt₄E₆ (E = S, Se, Te); MoRuS, etc. These differ in stoichiometry and structures. Several of the chalcogenides occur in nature as minerals, for instance, laurite (RuS₂), braggite (PdPt₃S₄), luberoite (Pt₅Se₄), Pd₆AgTe₄, etc. Although there is an extensive literature on the synthesis and structural aspects of bulk platinum group metal chalcogenides, only recently has there been research into their catalytic and electronic properties, and into the preparation of nanoparticles and thin films.

This review intends to cover these emerging aspects of platinum group metal chalcogenides and will consider first a selection of the structures adopted, followed by their catalytic and electronic uses.

Ruthenium and Osmium Chalcogenides

Ruthenium and osmium dichalcogenides of composition ME₂ (M = Ru, Os; E = S, Se, Te) are usually prepared by heating stoichiometric quantities of the elements in evacuated sealed ampoules at elevated temperatures (∼ 700°C) (1–3). Single crystals, such as RuS₂, see Figure 1, are grown either from a tellurium flux (4) or by chemical vapour transport techniques using interhalogens as transporting agent (5). This latter technique, employing Cl₂/AlCl₃ as the transport agent, has been exploited, for example, to prepare a low temperature modification of α-RuTe₂, which crystallises in the marcasite structure. The orthorhombic space group is Pnnm (No 58) (a = 5.2812(13), b = 6.3943(19), c = 4.0085(13) Å) (6). This low-temperature (marcasite) phase transforms into the pyrite structure at 620°C (7).

Fig. 1

Structure of RuS2 (pyrite structure) (77). The black circles are Ru and the open circles are S View this figure

Extensive structural studies both by powder (8–12) and single crystal (13, 14) X-ray measurements on ME₂ show that these compounds adopt a pyrite (FeS₂) structure, and crystallise in a cubic system with space group Pa3, see Table I. The chalcogenide ions in these structures are tetrahedrally surrounded by another anion (E⁻) and three metal ions. The metal ions occupy the tetrahedral holes formed by the anion sublattice, see Figure 1.

Magnetic susceptibility measurements on ME₂ indicate diamagnetic behaviour (15). Infrared and Raman spectra of pyrite-type ME₂ have been reported (16–19). A comparison of the Raman and IR frequencies shows that the corresponding metal-chalcogen and chalcogen-chalcogen bonds have nearly the same strengths (at least for RuS₂ and RuSe₂) (17), but the metal-chalcogen bond strength increases in the order Ru < Os (18). Of the five expected absorptions for ME₂, the MTe₂ compounds display only one phonon peak in the Raman spectra which indicates the metallic behaviour of the tellurides (19). Studies of their optical absorption (15, 20–22), electrical resistivity (15, 20, 23–25) and Hall effect measurements (23–26) have shown that ME₂ are indirect band n-type semiconductors.

Rhodium and Iridium Chalcogenides

Rhodium and iridium form a variety of chalcogenides differing in stoichiometry and structural patterns, see Table II. At least four different families of compounds can essentially be identified:

• ME₂ (RhS₂, RhSe₂, RhTe₂, IrS₂, IrSe₂, IrTe₂)

• M₂E₃ (Rh₂S₃, Rh₂Se₃, Ir₂S₃)

• Rh₃E₄ (E = S, Se, Te), and

• M₃E₈ (Rh₃Se₈, Rh₃Te₈, Ir₃Te₈).

Additionally, other rhodium chalcogenide stoichiometries have been isolated and these include Rh₁₇S₁₅, RhSe_2+x and RhTe, Rh₃Te₂.

Rhodium and iridium chalcogenides are usually prepared by sintering (at 650–1100°C) pressed stoi-chiometric mixtures of the constituent elements in evacuated sealed ampoules (27–35). Single crystals, for example, Rh₂S₃, are prepared by chemical transport techniques (for example, using bromine as a transport agent) (29) and tellurium flux techniques (for example, for Ir₃Te₈) (36).

Rhodium and iridium chalcogenides are usually diamagnetic, with a few exceptions, such as Rh₃S₄, which shows temperature-independent paramagnetism (35). The compounds display metallic to semiconducting behaviour (28, 33, 35–40).

The structures of these compounds have been established by powder as well as by single crystal X-ray diffraction methods. The compounds show diverse structural preferences, such as pyrite-type, CdI₂-type or NiAs structures. For instance, MTe compounds (M = Rh or Ir) exist in a NiAs structure while ME₂ compounds adopt pyrite- and CdI₂-type structures, see Figure 2 (28, 31, 32, 34, 40–42). Although RhS₂ has been reported, it seems that the pure compound does not exist as attempts to prepare it have resulted in the formation of Rh₂S₃, RhS₃ and other phases (28). RhTe₂ adopts the pyrite structure, see Figure 2, but does not exist as a stoichiometric compound above 550°C (41).

Fig. 2

Structures of IrTe2 (CdI2 system) (above) and RhTe2 (pyrite system) (right) (41) View this figure

IrTe₂

IrTe₂ shows polymorphism with three phases:

• 3D polymeric 2D-derived CdI₂-type (h-IrTe₂ because of its hexagonal cell)

• pyrite-type (c-IrTe₂ because it is cubic), and

• monoclinic (m-IrTe₂) (28, 41).

These three phases and four hypothetical phases (ramsdellite-type, pyrolusite-type, IrS₂-type and marcasite-type) have been analysed by extended Huckel tight-binding electronic band structure calculations (34, 43). Monoclinic IrTe₂ (m-IrTe₂) shows structural features of both CdI₂ and pyrite-type IrTe₂ (c-IrTe₂) phases (34). The high pressure behaviour of h-IrTe₂ was studied at up to 32 GPa at room temperature and two new forms were obtained (44). The first structural transition took place at ∼ 5 GPa and led to the monoclinic form (m-IrTe₂). The second transition occurred at 20 GPa and gave rise to the cubic pyrite phase (c-IrTe₂) (44).

IrSe₂

The structure of IrSe₂ is similar to the marcasite system (42), as are the structures of IrS₂ and the low-temperature modification of RhSe₂ (a = 20.91 (3), b = 5.951 (6), c = 3.709 (4) Å) (28). Each iridium atom of IrSe₂ is located at the centre of a distorted octahedron with three Se neighbours at a distance of 2.44 Å and three others at 2.52 Å. Half of the Se atoms are tetrahedrally surrounded by three iridium atoms and one selenium atom, the Se–Se distance being 2.57 Å. The other half of the Se atoms have a similar neighbourhood, but adjacent Se atoms are spaced at 3.27 Å (42).

M₂E₃

The structures of M₂E₃ (Rh₂S₃, Rh₂Se₃, Ir₂S₃) are isomorphous (29) and the metal atoms adopt an octahedral configuration. Every octahedron shares a common face with another octahedron to form octahedron pairs. These pairs are arranged in layers of stacking sequence ABABA…. Four metal atoms surround each chalcogen atom at the vertices of a distorted tetrahedron.

M₃E₈

The compounds M₃E₈ (Rh₃Se₈, Rh₃Te₈, Ir₃Te₈) exist in a pyrite-type structure (41, 45–47) and crystallise with rhombohedral symmetry. The structure is a three-dimensional network made of interlinked E₂ pairs, see Figure 3.

Fig. 3

Structure of Rh3Te8 (41) View this figure

Rh₃E₄

Rh₃E₄ (E = S, Te) adopts the NiAs structure and crystallises in a monoclinic system (35). The structure of Rh₃S₄ consists of edge sharing RhS₆ octahedrons which are connected by S₂ pairs (S–S = 2.20 Å). The molecule contains Rh₆ cluster rings in a chair conformation with the Rh–Rh single bond length of 2.70 Å. Both fragments are linked by common S atoms (35). Structures of Rh₁₇S₁₅ (48) and Rh₃Te₂ (49) have also been established by single crystal X-ray structural analysis.

Palladium and Platinum Chalcogenides

Palladium and platinum form a wide variety of chalcogenides. They are prepared by heating the required amounts of two elements in an evacuated sealed tube. The material thus obtained is made into powder and annealed at various temperatures (50), often for several days (as in the synthesis of PtS and PtS₂) (51, 52). The palladium-sulfur (53), palladium-selenium (50), palladium-tellurium (54), platinum-selenium (55) and platinum-tellurium (56, 57) systems have been investigated by differential thermal analysis and X-ray powder diffraction methods.

In the palladium-tellurium system at least eight binary phases (PdTe, PdTe₂, Pd₃Te₂ Pd₇Te₃, Pd₈Te₃, Pd₉Te₄, Pd₁₇Te₄ and Pd₂₀Te₇ have been identified and characterised by X-ray diffraction (54). The platinum-tellurium system on the other hand exhibits only four binary phases (PtTe, PtTe₂, Pt₂Te₃ and Pt₃Te₄) (57, 58). These compositions are constant and there is no appreciable compositional range. The Pt₂Te₃ is stable up to ∼ 675°C whereas Pt₃Te₄ melts above 1000°C. The band structures of some of these chalcogenides (PtS (58), PtS₂ (59, 60), PtSe₂ (60)) have been determined by first principle electronic structure calculations. Semiconducting behaviour for some of these compounds has been noted (58, 60–62).

The binary palladium and platinum chalcogenides show a higher diversity of structures than found for the Rh/Ir and Ru/Os compounds. Besides several other binary phases, see Table III, four general families have been isolated:

• ME (PdS, PdSe, PdTe, PtS, PtSe, PtTe)

• Pd₃E, (E = S, Se, Te)

• Pd₄E (E = S, Se, Te), and

• ME₂ (PdS₂, PdSe₂, PdTe₂, PtS₂, PtSe₂, PtTe₂).

Single crystal X-ray analysis of PdSe shows that there are three crystallographically unique Pd atoms, each site being in a slightly distorted square-planar environment. Each of the two crystallographically independent Se atoms is coordinated by a distorted tetrahedron of Pd atoms (63).

PdTe crystallises in the NiAs structure and can be modelled as a single h.c.p. lattice.

PdS₂ and PdSe₂ exist in a deformed pyrite-type structure (50), while the remaining ME₂ adopt a CdI₂ structure. PdTe₂ has a layered structure with the layers stacking along the (001) direction, see Figure 4. The Pd⁴⁺ cations are octahedrally coordinated. The layers are formed by octahedra sharing edges along the [100], [010] and [110] directions. The Pd–Te bond distance is 2.693 (2) Å (64).

Fig. 4

Structure of PdTe2 (64) View this figure

Although the single crystal X-ray structure of Pd₁₇Se₁₅ can be analysed in any of the three space groups, viz. Pm3m, P43m and P432, refinement based on the former space group gives the lowest standard errors (65). There are four crystallographically different palladium atoms see Figure 5. One of the palladium atoms has a regular octahedron of selenium atoms with Pd–Se distances of 2.58 Å. The remaining three palladium atoms are coordinated each with four selenium atoms either in a flattened tetrahedron (one Pd centre) with average Pd–Se distances of 2.48 Å or square plane (two Pd atoms) with Pd–Se distances of 2.53 and 2.44 Å. The square planar palladium atoms are also coordinated to palladium atoms with Pd…Pd distance of 2.78 Å.

Fig. 5

Structure of Pd17Se15 (65) View this figure

The Use of Platinum Group Metal Chalcogenides in Catalysis Hydrodesulfurisation

Several platinum group metal sulfides, particularly RuS₂, have been extensively employed as catalysts for hydrodesulfurisation (HDS) reactions (66–79). It has been shown that semiconducting transition metal sulfides, such as PdS, PtS, Rh₂S₃, Ir₂S₃, RuS₂, have higher catalytic activity than the metallic sulfides (66). They have been used, supported on γ-Al₂O₃ or carbon, or as bulk catalysts, for HDS of several thiophene derivatives, such as thiophene, 3-methylthiophene, benzothiophene, dibenzothiophene or 4,6-dimethyldibenzothiophene, see Equation (i)..

In RuS₂ the three coordinate surface Ru atoms, such as those found on the (111) surface, appear to provide active sites for HDS (77). The temperature programmed desorption profiles indicate that two different adsorbed species that have different relative concentrations are a factor for the degree of reduction caused by RuS₂. NMR results suggest that one of the species leads to the formation of SH groups while the other species has hydridic character (80). To study the effect of the surface Ru–S coordination number on the surface S–H and Ru–H species for thiophene adsorption on RuS₂, a topology study of the Laplacian of electron density of selected (100) and (111) surfaces was carried out (81, 82). Acidic Lewis and Brønsted sites are created in mild reducing conditions. The Lewis acidic sites play an important role in activating sulfur-containing molecules and subsequently in their transformations. Hydrogenation properties are related to Ru sites with a low S coordination (83).

Thiophene adsorption on stoichiometric and reduced (100) surfaces of RuS₂ has been studied using ab initio density functional molecular dynamics. On the stoichiometric RuS₂ surface, thiophene is adsorbed in a tilted η¹ position where the sulfur atom of the thiophene molecule forms a bond with the surface Ru atom similar to that in bulk RuS₂; but there is no activation of the molecule. The formation of sulfur vacancies on the surface creates a chemically active surface and the possibility for thiophene adsorption in the η² position when the thiophene molecule is activated (84, 85). Scattered-wave calculations on model catalyst clusters and catalyst-thiophene (or related compounds) systems have indicated that pπ bonding between the S atoms of the catalyst and the S and C atoms of thiophene is responsible for binding the thiophene molecule to the catalyst in the initial stages of the HDS process (86). Electronic and bonding properties of the RuS₂ and related thiophene adsorption systems have also been studied by discrete vibrational-X_α calculations (87).

The cleavage of the sp² carbon-heteroatom bond in the hydroprocessing of substituted benzenes, such as aniline, phenol, diphenylsulfide, chlorobenzene, over unsupported transition metal sulfides at 250°C and 70 bar H₂ pressure was studied. Hydrogenolysis of the sp² carbon-substituent bond results from attack by a soft nucleophile, such as an hydride ion, on the carbon bearing the substituent (88).

Hydrodenitrogenation

The activity of a carbon-supported metal sulfide catalyst in the hydrodenitrogenation (HDN) of quinoline increases in the order Ni < Pd < Pt (89). Platinum group metal chalcogenides have been employed in HDN reactions (89–93) and the activity is related to the acidic-basic properties of the active phase. The best catalyst appeared to have a good balance between the acidic-basic properties responsible for C–N and C–C bond cleavage and labile superficial S anions, which, in a reducing atmosphere, leads to a large number of active sites for hydrogenation reactions (93). Ruthenium and rhodium sulfides gave only a low conversion of quinoline to hydrocarbons (propylbenzene and propylcyclohexane) (89), while the hydrodenitrogenation of quinoline, decahydroquinoline, cyclohexylamine and o-propylamine over Rh and Ir sulfide catalysts was shown to lead to the formation of hydrocarbons (90, 94).

Hydrotreatment of naphtha, which contains mainly nitrogen (pyridines, anilines and quinolines), sulfur- and oxygen-containing heteroatom compounds (92), has been carried out on transition metal sulfides which were used for the removal of nitrogen compounds.

Hydrogenation Reactions

Platinum group metal chalcogenides have also found use as catalysts in hydrogenation reactions (95–102). Sulfides (PdS₂, Ir₂S₃, OsS₂ (95)) selenides (Ru₂Se₃ (96)) and tellurides of Rh, Pd and Pt have been used for the reduction of nitrobenzene to aniline in 95–99% yield. The catalysts are insensitive to sulfur poisoning and are active at low temperatures and low hydrogen pressure. The reductive alkylation of aniline and substituted anilines on Ru, Rh, Pd or Pt selenides/tellurides has also been carried out (96). For instance, aniline was converted to isopropylaniline in the presence of ruthenium selenide (Ru₂Se₃) catalyst, see Equation (ii) (96):

Palladium sulfide catalysts are active for the hydrogenation of thiophenes (thiophene, 2-methylthiophene and benzothiophene) to tetrahydrothiophenes (Equation (iii), (97–101).

In these reactions hydrogenation and hydrogenolysis often proceed simultaneously. Thus, the hydrogenation of thiophene yields thiolane and the hydrogenolysis products, butane and H₂S, which are formed during the decomposition of thiophene and thiolane (97). PdS supported on aluminosilicate showed higher activity (by 1 to 2 orders of magnitude) than Rh, Ru, Mo, W, Re, Co and Ni sulfides (101).

Pyridine hydrogenation to piperidine has been investigated using ruthenium sulfide supported on Y-zeolite or alumina catalysts (103).

RuS₂ supported on a dealuminated KY-zeolite showed very high activity (roughly 300 times that of an industrial NiMo/Al₂O₃ hydrotreating catalyst) for the hydrogenation of naphthalene to tetralin (104).

Both palladium sulfides and platinum sulfides have been employed for hydrogenation reactions, such as the hydrogenation of a gasoline pyrolysis residue over Pd sulfide/Al₂O₃ (105), and the hydrogenation of naphthalene to tetralin over carbon-supported Pd or Pt sulfide (78).

The hydrogenation of diethyldisulfide gives ethanethiol with selectivity > 94% at atmospheric pressure in the presence of supported transition metal sulfide catalysts (102). Bimetallic catalysts were less active than monometallic compounds in this reaction. Both bulk and SiO₂ supported palladium sulfides have been employed for the production of methanol from the hydrogenation of CO (106, 107).

Isomerisation and Acetoxylation Reactions

Solid bed catalysts containing Pd/Se or Pd/Te on a SiO₂ carrier have been employed for the isomerisation of alkenes, particularly 3-butene-1-ol (108). Besides isomerisation, Pd/Te or Pt/Te supported on SiO₂ have been used to prepare unsaturated glycol diester compounds by treating a conjugated diene (such as butadiene) with a carboxylic acid (such as acetic acid) in the presence of oxygen (109). This process, see Equation (iv), has been industrialised by Mitsubishi Kasai Corp. for the production of 1,4-butanediol from 1,4-butadiene (110). The production of unsaturated glycol diesters (such as 1,4-diacetoxy-2-butene and butanediols) comprises reacting a conjugated diene with the carboxylic acid and O₂ in the presence of a solid Rh-Te catalyst (111). A Rh₂Te catalyst (∼ 3%) on activated charcoal has been used for the acetoxylation in AcOH of 1,3-cyclopentadiene to diacetoxycyclopentane (112).

Dehydrogenation Reactions

Platinum group metal chalcogenides have found use in dehydrogenation reactions. The dehydrogenation of tetrahydrothiophene over RuS₂ yields thiophene in 92% yield (Equation (iii)) (113). The active sites for the hydrogenation and hydrodesulfurisation are anionic vacancies in the sulfide catalysts (RuS₂). The higher lability of S₂²⁻anions relative to S²⁻ anions in a reducing atmosphere explains the higher activity of RuS₂ (93). The catalytic dehydrogenative polycondensation of 1,2,3,4-tetrahydroquinoline using transition metal sulfides (PdS, PtS, RuS, RhS₂) provides a direct route to the synthesis of unsubstituted quinoline oligomers. RuS gives a maximum yield of 97% (114, 115).

The photocatalytic decomposition of H₂O into H₂ and O₂ takes place over RuS₂ powder as well as over RuS₂ supported on various substrates (such as SiO₂, zeolites) under UV irradiation (116–118).

Platinum Group Metal Chalcogenides as Semiconductors and in the Electronic Industry

Platinum group metals (particularly Pd and Pt) are used for low resistance ohmic contacts in semiconducting electronic devices. For device reliability thermodynamically stable contacts are very important. Therefore reactions occurring at the interface between the metal contacts and the II-VI semiconductors have been extensively studied in recent years (119–133). For instance, it has been found that at the interface of Pt/CdTe diffusion couples a nonplanar reaction layer of the intermetallics CdPt and PtTe is formed (121).

The crystallographic microstructure and electrical characteristics of platinum group metals (mainly Pd) and Ni or Au ohmic contacts on ZnSe and on p-type (001) ZnTe layers have been investigated as a function of annealing temperature. The specific contact resistance of these contacts depends strongly on annealing temperature and the palladium layer thickness (122).

In Pd/ZnSe, palladium forms a ternary epitaxial phase Pd_5+xZnSe at 200°C which is stable up to 450°C while platinum begins to form Pt₅Se₄ at 575°C at the Pt/ZnSe interface (119).

PdS and PtS have been employed as light image receiving materials with silver halides (134, 135). Photographic film containing 6–10 mol m⁻² PdS coating gives dense black images with high contrast (136). In optical disc recording films PdTe₂ is one of the active components (137). Palladium sulfide has also been used for lithographic films (138, 139) and lithographic plates with high resolution (140, 141). Semiconducting films of metal sulfide polymer composites were obtained when organosols of PdS in DMF were prepared from Pd(OAc)₂ and H₂S, and followed by addition of polymers (142).

Thin films of PdS and PtS have been deposited on GaAs substrate from [M{S₂CNMe(c-Hex)}₂] (M = Pd or Pt) by low pressure MOCVD (143). The complexes also serve as precursors for the growth of nanocrystals of PdS and PtS which are formed by thermolysis of the complex in trioctylphosphine oxide (143).

PdS thin films have been deposited onto Si and quartz substrates at 10⁻² torr from a single source precursor [Pd(S₂COPrⁱ)₂]. Two different vapour deposition processes: photochemical (308 nm laser irradiation) and thermal (350°C) were employed (144). The PdS films are in the polycrystalline tetragonal phase. Palladium sulfide in a polymer matrix has been used for the manufacture of semiconductors and solar cells (145). Semiconducting and photoelectrochemical properties of PdS have also been investigated (146).

PtS₂ nanoclusters synthesised from PtCl₄ and (NH₄)₂S in inverse micelles show an indirect band gap of 1.58 eV as compared to 0.87 eV for bulk PtS₂. Nanoclusters with double the mass show a band gap of 1.27 eV (147).

Aqueous dispersions of PdS particles have been prepared from PdCl₂ or Na₂PdCl₄ with Na₂S solutions. Uniform spherical particles of diameter 20–30 nm were obtained in acidic medium in the presence or absence of surfactants. Surfactants of the AVANEL S series enhanced deposition of PdS particles on an epoxy circuit board (148).

Thin films (0.05–1 μm thick) of pyrite-type RuS₂ (polycrystalline or epitaxial) have been grown on various substrates such as silica, sapphire and GaAs, by MOCVD using ruthenocene and H₂S as precursors (149). The films have been characterised by X-ray diffraction, microprobe and SIMS analysis, and electrical and optical measurements.

The BARC group has recently designed several molecular precursors for the synthesis of palladium chalcogenides (150–154). The compound [Pd(Spy){S₂P(OPrⁱ)₂}(PPh₃)] containing a chelating dithiophosphate group (Figure 6) undergoes a three-stage decomposition leading to the formation of PdS₂ at 357°C (150). The dimeric methyl- allyl palladium complexes, [Pd₂(μ-ER)₂(η³-C₄H₇)₂] afford polycrystalline Pd₄E (E = S or Se), see Scheme, and amorphous Pd₃Te₂, the former at moderately low temperatures (refluxing xylene) and the latter at room temperature (151). Thermo- gravimetric analysis of [PdCl(SeCH₂CH₂NMe₂)]₃ and [PdCl(SeCH₂CH₂NMe₂)(PR₃)] (PR₃ = PPh₃ or Ptol₃) reveals that these compounds undergo a two-step decomposition leading to polycrystalline Pd₁₇Se₁₅ (153). The thermogravimetric analysis of [PdCl{Te(3-MeC₅H₃N)}(PPh₃)], see Figure 7 shows that the compound decomposes in a single step at 290°C to give PdTe (by XRD, Figure 8) as aggregates of microcrystals (by SEM, Figure 9) (154).

Fig. 6

Molecular structure with atomic numbering scheme for (Pd(Spy)[S2P(OPri)2)(PPh3)] (150) View this figure

Fig. 7

Structure of [PdCl{Te(3-MeC5H3N)}(PPh3)] (154) View this figure

Fig. 8

XRD pattern of PdTe obtained from [PdCl{Te(3-MeC5H3N)}(PPh3)] (154) View this figure

Fig. 9

SEM picture of PdTe obtained from [PdCl{Te(3-MeC5H3N)}(PPh3)] (154) View this figure

Scheme

View this figure

Conclusions

The great structural diversity and catalytic applications (HDS, HDN, hydrogenation, etc.) of platinum group metal chalcogenides, seem to offer new avenues for further research. Clear trends seem to be emerging in molecular precursor chemistry for the preparation of metal chalcogenides. It is believed that such trends would provide opportunities to isolate not only known stable or metastable phases at low temperatures (for instance palladium allyl complexes (151)) but also as yet unknown stoichiometries. It is hoped that this brief review will serve as an interface between scientists working in areas such as mineralogy, catalysis, materials science and solid state structural chemistry.