NEW DEVELOPMENTS IN THE COORDINATION CHEMISTRY OF INORGANIC SELENIDE AND TELLURIDE LIGANDS

Even a cursory examination of the main body of the article gives a sense of the state of the metal selenide and telluride field. A large percentage of the references in this article have been published in the last 5 years, verifying that the field is expanding at an enormous rate. Most of the progress has been largely descriptive, whereby researchers combine various chalcogenide sources with simple metal complexes and analyze the products, usually by crystallography. The development of several new classes of reagents, notably soluble Zintl polynuclear anions, trimethylsilyl compounds, organophosphine chalcogenides, and molten alkali metal polychalcogenides, has allowed synthetic exploration to proceed at an ever expanding rate. Despite the lack of maturity of the field several trends are beginning to suggest themselves. There appears to be nothing inherently unstable about metal chalcogenide bonds in any sector of the periodic table. Most of the elements investigated have yielded stable and interesting selenide and telluride complexes of various types. The most obvious gaps that do exist seem only to be a result of a lack of effort directed toward those areas. Also there seems to be no inherent instability of metal complexes containing polyselenide or -telluride ligands. Extremely stable compounds ligating di-, tri-, and tetrachalcogenides are being reported at an accelerating pace. In fact mild pyrolytic conditions are often employed to enhance reactivity and crystallization. The complexes seem to show no tendency to extrude elemental selenium or tellurium and form polymeric metal monochalcogenides. This is in marked contrast to organopolyselenides and -tellurides. The metal complexes generally do not seem to show much photochemical sensitivity either, again in contrast to organic compounds. There are several obvious differences between the complexes of selenium and tellurium and their closest relatives, and metal sulfides. The coordinated chains of the heavier elements are somewhat shorter than those of polysulfides. Only a few coordinated pentaselenides are known, and tetratellurides are the longest coordinated tellurium chains known, unlike the polysulfides where coordinated chains of nine sulfur atoms have been reported. There are some metal selenide and telluride complexes which are direct analogs to known sulfur complexes. However, the heavier chalcogenides more often form complexes which have considerably different structural and electronic properties. This is especially so for complexes of tellurium, where its large size and diffuse orbitals often leads to complexes which are often extremely unusual. Indeed these attributes make it unlikely that sulfur analogs to compounds such as NbTe103− will ever be isolated. What are the future directions of this field? It appears that the synthetic explorations will continue unabated for some time. In fact the richness of this work has made it difficult to successfully realize a termination point for this article. Many of the new synthetic techniques have simply not been applied to all the metal systems. Interestingly, there do not appear to be many “sinks” in these systems. Therefore reaction of soluble polyanions with a particular metal salt often does not lead to the same products as does reaction of trimethylsilyl reagents or organophosphine reagents with the same metal starting material. The systematic variation of counterions and solvents will also continue to lead to new products, but Goddess Fortune can sometimes cast a baleful eye in this direction. The approach can be frustrated by the fact that simple salts of the starting materials such as Se52− and Te42− often crystallize far better than the desired metal complexes. A much higher success rate will be obtained as more new reagents and conditions are developed. For example, mixed polychalcogenides are only now beginning to emerge. The difference in size and electronegativity from sulfur to tellurium will lead to distinctive coordination chemistry of mixed ligands. Use of mixed 15/16 clusters as starting material further accentuates this difference and leads to even more novel coordination chemistry. The reactivity of the new metal complexes is just beginning to be probed. The use of neutral metal carbonyl complexes in cluster building reactions has been quite rewarding. Also conversion of metal complexes to dense binary solids has begun to bear fruit. However, use of the new complexes in organic transformations and catalytic processes has been essentially ignored. An obvious use for these new complexes is their application in new materials with novel electronic, magnetic and optical properties. However, an unfortunate situation emerges here. Most of the compounds recently characterized are molecular species which can be readily crystallized by using large organic counterions, organophosphines as co-ligands, and other related techniques. However, to obtain bulk properties like those described above, these large carbon rich counterions must be eliminated. This does indeed lead to extended solids, but they are often difficult to characterize and control chemically. Therefore new synthetic strategies must be developed to lead to pure, characterizable (i.e. crystalline) bulk solids which are subject to chemical control. This desire has spawned the use of several nontraditional techniques aimed at dissolving the real and conceptual boundaries between molecules and bulk solids. These include synthesis of very large clusters (using trimethylsilyl reagents, for example), use of molten polychalcogenides as solvents and reagents, and synthesis in superheated solvents. It is techniques like these which will be responsible for the continued rapid expansion of this exciting area of coordination chemistry. © 1993, American Chemical Society. All rights reserved.