Niobium complexes reactions

Niobium, tris(diethyldilhiocarbamato)oxy-stereochemistry, 1,82 structure, 1, 83 Niobium, tris(oxa ato)oxy-stereochcmistry, 1, 82 Niobium, tris(phcnylcncdirhio)-structure, 1, 63 Niobium alanate, 3, 685 Niobium complexes alkyl alkoxy reactions, 2, 358 amides, 2,164 properties, 2, 168 synthesis, 2, 165 applications, 6,1014 carbamicacid, 2, 450 clusters, 3, 672,673,675 hexamethylbenzene ligands, 3, 669 cyanides synthesis, 2, 9 p-dinitrogen, 3, 418 fluoro... 

The first complexes of a-keto ylides and group 5 early transition metals have only recently been obtained by reaction of Nb(III) derivatives [[NbCl3(dme) (R C=CR")] with 25 (R = thiazolyl) (Scheme 16). The chelation of the ylide occurs through an N,0-coordination to the metal center and in presence of MeLi a deprotonation of a phenyl ring takes place with the loss of alkyne, leading to the formation of a new orfho-metallated binuclear compound 32. The two ylides involved in the complexation behave as tridentate anionic ligands and are mutually in a trans disposition in order to minimize the steric hindrance [71,72]. Another binuclear niobium complex 33 has been obtained from 25 (R = Me, Ph) with this time an 0-coordinated a-keto ylide [68]. 

Oxidation-Reduction Titrations. The extent of reduction resulting from reaction of niobium (V) chloride and bromide with pyridine was determined by indirect titration of crude reaction mixtures with standard ammonium tetrasulfato-cerate(IV) solution. Samples were stirred overnight in a stoppered flask with an excess of iron (III) ammonium sulfate. Any iron (II) formed by reaction with the niobium complex mixture was then titrated with the standard tetrasulfato-cerate(IV) solution using ferroin as indicator. Results of these determinations are given in Table III. 

Acetyl ligands, in niobium complexes, C-H BDEs, 1, 298 Achiral phosphines, on polymer-supported peptides, 12, 698 Acid halides, indium compound reactions, 9, 683 Acidity, one-electron oxidized metal hydrides, 1, 294 Acid leaching, in organometallic stability studies, 12, 612 Acid-platinum rf-monoalkynes, interactions, 8, 641 Acrylate, polymerization with aluminum catalysts, 3, 280 Acrylic monomers, lanthanide-catalyzed polymerization,... 

Enediynes, silane-initiated cascade cyclization, 11, 400 Ene reactions, allylsilanes, 9, 313 Energetics, key elementary reactions, 1, 617 Enes, niobium complexes, 5, 87 Ene-yne metathesis... 

In a third complex reaction, diphenylacetylene and acetonitrile have been observed to condense into the ty6-HC(QHs) C(QH5))3C(NH)(CH3) ligand, bonded to niobium (Fig. 13)2255. Among a number of resonance hybrids can be included the simplified form XIX, which contains a pentadienyl-metal interaction. Of course, the ligand could also be considered to be isoelectronic with hexatriene or the hexatriene dianion, depending on the metal oxidation state (cf., Fig. 6). A great deal of buckling is present in the con-... 

A related reaction is shown for the molybdenum-niobium complex (equation 9). As described in a review by Horowitz and Shriver on bridging carbonyl ligands, this complex is a rare example of a structure containing terminal, bridging, and r-bridging CO ligands. All such complexes, of necessity, include metal-metal bonding. 

Although many transition-metal derivatives react with cyclopropenes by ring opening (See Section 2.C. and ref 107 for examples), 1,2-dimclhylcyclopropene was regenerated from the complex 8 by reaction with triphenylphosphane or carbon disulfide, and a number of related nickel ° and niobium complexes of cyclopropenes which retain the three-membered ring have been reported. ... 

Several other azo compounds have been employed as spectrophotometric reagents for niobium. The reactions are carried out in the presence of complexants (tartrate, oxalate, H2O2). The coloured species produced are generally ternary niobium complexes. PAR (e = 3.6-10 ) [45-53] and Sulphochlorophenol S (e = 3.3-10 ) [54-61] are often employed. 

Doye s group [81] showed that a dinuclear titanium-sulfonamidate complex (Scheme 21), with a tetrahedral sulfur in the ligand backbone, can be used for intermo-lecular hydroaminoalkylation as well. This system gives mixtures of branched and linear products, although to date there has been no mechanistic rationale provided for the reduced regioselectivity of group 4 metal complexes in this transformation. There has been one report by Zi s group [44] that describes axially chiral bis(sulfonamidate) tantalum and niobium complexes for application as precatalysts for hydroamination and hydroaminoalkylation. Unfortunately, these complexes did not show any reactivity for either of these reactions. 

The niobium complex 562 is formed by successive treatment of CpNbCU with magnesium metal and phenylacetylene. The complex yields a mixture of 1,2,4-and 1,3,5-triphenylbenzene on standing (equation 58). The catalytic system CpNbCl4/Mg(0) mediates the reaction of the diyne 563 with hex-3-yne to give the benzene derivative 564. ... 

---