Gold-Niobium Compounds

While only a few compounds with Au-Cr bonds have been structurally characterized, for the heavier elements of Group 6 there are many more complexes for which the molecular structure have been established by X-ray diffraction and the diversity of the compounds is also greater. 

All characterized compounds contain either Cr(CO) or Cr(L)(CO) (L = cyclopenta-dienyl derivative, PPh3) fragments bound to gold. Unsupported Au-Cr bonds are found in the neutral (Ph3P)AuCr(CO)3(Ti5-C5H4CH2CH2NMe2) 129 [2.6291 (11) A] [113] or the bent anion of [NBu4][Au Cr(ri5-Cp)(CO)3 2] 130 [Au-Cr 2.641 

The structures of several compounds with Au-Mn and Au-Re bonds have been reported and all are neutral or ionic species without further association through metal-metal interaction resulting in supramolecular architectures. 

The structural chemistry of compounds containing Au-Group 8 metal bonds, and espedally that of dusters, is by far the best represented in the literature in comparison with those of the other transition metals [224]. There are many similarities between the heterometallic duster cores in compounds with Au-Fe, Au-Ru and Au-Os bonds, but also some differences which will be emphasized in the subsequent discussion. 

Metal frameworks similar to that observed in cluster 235 were found in several other Au2 Ru4 [346, 356-358] or Au2Os4 [327] derivatives. 

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Hafnium had lain hidden for untold centuries, not because of its rarity but because of its dose similarity to zirconium (16), and when Professor von Hevesy examined some historic museum specimens of zirconium compounds which had been prepared by Julius Thomsen, C. F. Rammelsberg, A. E. Nordenskjold, J.-C. G. de Marignac, and other experts on the chemistry of zirconium, he found that they contained from 1 to 5 per cent of the new element (26, 27). The latter is far more abundant than silver or gold. Since the earlier chemists were unable to prepare zirconium compounds free from hafnium, the discovery of the new element necessitated a revision of the atomic weight of zirconium (24, 28). Some of the minerals were of nepheline syenitic and some of granitic origin (20). Hafnium and zirconium are so closely related chemically and so closely associated in the mineral realm that their separation is even more difficult than that of niobium (columbium) and tantalum (29). The ratio of hafnium to zirconium is not the same in all minerals. 

Few gold compounds contain vanadium-carbonyl fragments or dicyclopentadienyl-niobium fragments, respectively, and no additional interactions between discrete heterometalic cores are present in the crystal of any of them. 

In contrast with the difluorides, the distribution of trifluorides extends to the third series of the transition metals, where iridium and gold trifluorides are fully characterized. In the second series, trifluorides are known for the elements from niobium to rhodium, with the exception of technetium, and in the first series, from titanium to cobalt. All the trifluorides have been characterized structurally, with earlier reports based on X-ray powder-diffraction data, since the compounds were not prepared in single-crystal form until more recently, when high-temperature, crystal-growth techniques became available. 

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