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UV Photoelectron Spectra of Some Bent Bis(r;5-cyclopentadienyl)Niobium and Tantalum Complexes Henk van Dam*, Andries Terpstra, and Ad Oskam** Anorganisch Chemisch Laboratorium, University of Amsterdam, J. H. van't Hoff Instituut, Nieuwe Achtergracht 166, 1018 W V Amsterdam, The Netherlands Jan H. Teuben Laboratorium voor Anorganische Chemie, Rijksuniversiteit, Nijenborgh 16, 9747 AG Groningen, The Netherlands Z. Naturforsch. 36b, 420-425 (1981); received December 29, 1980 UV PE Spectra, Niobium and Tantalum Complexes He(I) and He(II) photoelectron

, project No. 2/3100/23, and the Slovak–French Science and Technology Co-operation PROGRAMME STEFANIK are acknowledged for financial support. References 1. A. V. Vik, Structure and Solubility of Niobium and Tantalum Complexes in Molten Alkali Fluorides; Ph.D. Thesis, Norwegian University of Science and Technology, Trondheim (2000). 2. V. I. Konstantinov, Elektrovydelenie tantala, niobia a ich splavov (Electrodeposition of tantalun, niobium and their alloys). Metalurgia, Moskva (1977), p. 41. 3. V. Van, A. Silný, J. Hı́veš, and V. Daněk, Electrochem. Commun. 7 (1999

, zirconium, hafni- um, niobium, tantalum (d°) 20-33, molybdenum, tung- sten (d1) 8-11 and iron (d5) 37, 38 complexes. Indeed, strongly splitted multiplets in the ranges 8 . 8 4 1 - 1 0 . 4 2 0 MHz and 6 . 4 3 8 - 1 0 . 3 3 8 MHz were found in the spectra of niobium and tantalum complexes, respectively; the high-lying frequencies should be assigned to the chlo- rine atoms involved in secondary bonding. Unfortunate- ly, this part of the spectrum could not be recorded for all complexes due to the low frequency intensity. Type III. Dimeric Complexes with a Chlorine

complexes is considerably more activated than in end-on mononuclear ones. For some of the complexes the structure M=N-N=M seems to be more appropriate than the structure with NN triple bond M-NIN-M. They illustrate very well four-electron mechanism of dinitrogen reduction discussed earlier. The results with protonation of bridging dinitrogen confirm the conclusion that dinitrogen ligand is strongly activated. Niobium and tantalum complexes form hydrazine quantitatively when protonated by HC1. Vanadium, molybdenum and tungsten complexes produce mainly ammonia, the

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Niobium and Tantalum Complexes UV Photoelectron Spectra of Some Bent Bis ( - cy clopentadieny 1) Niobium and Tanta- lum Complexes 420 Nitrophenols Electrochemical Behavior of the Nitrophenols and their Reduction Products at Graphite Electrodes 840 Nitrosyl- and Hydroxylamido(O.N) Complexes of Vanadium On the Mechanism of the Formation of Nitrosyl vanadium Complexes from Hydroxyl- amine : Reversible, Intramolecular Conversion of Coordinated Nitric Oxide to a Hydroxyl- amido(l—) Ligand 683 Nitrosylation Transition Metal Sulfur Ylide Complexes, XI . Structure

(Dihapto) Ligands 351 Vanadium, Niobium, Tantalum Irradiation of (r/5-C5H5)V(CO)4 with disubstituted acetylenes yields the complex (f/5-C5H5)V(CO)2(RC^CR). Since a single acetylene replaces two carbon monoxide ligands, both π-systems of the acetylene must participate in bonding, and the ligand is thus a four-electron donor. In the complex (f75-C5H5)2V(CF3C=CCF3) the acetylene ligand acts as a two-electron donor. Niobium and tantalum complexes of diphenylacetylene are implicated as inter- mediates in the formation of cyclo butadiene complexes (Chapter 13