Niobium metallacycles

Reactions other than Lewis acid-base associations/dissociations are frequently observed wit donor molecules, leading notably to solvolysis, oxygen or sulfur abstraction, insertion reaction and carbon-carbon coupling reactions. The tendency to form metal-element multiple bonds i remarkable in this respect the activation of dinitrogen by tantalum or niobium is unique. Th formation and chemistry of constrained reactive metallacycles open another promisin fast-developing area, on the frontier with organometallic chemistry. 

Although there are few five-membered metallacycles reported for group 5, there are some examples of metallacyclopentane compounds for niobium and tantalum. The reaction of the labile vanadium naphthalene complex [CpV(Ci0H8)] with ethene provided the unusual binuclear complex [Cp2V2(p-C4H8)2] (34 [Eq. (13)], Fig. 9) in which the two vanadium centres are bridged by two butanediyl groups.38... 

Metallacyclic (see Metallacycle) complexes of niobium and tantalum play an important role in understanding several catalytic and stoichiometric transformations of organic compounds. Some group 5 metallacycles are formed from the inter- or intramolecular hydride abstraction reactions. Most of the Nb and Ta metallacycles are prepared, however, from reductive coupling (see Reductive Coupling) of unsaturated organic substrates. To be included in this section, the metallacyclic ligand must have at least one M-C bond. 

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

Decreased activity of alkyne complexes containing 4-6 d electrons in the metallacyclic reaction may be explained by the bonding character of the c orbital (Figure 6.21). Examples are provided by niobium and molybdenum complexes d -d electron... 

Carbonyl Olefination Using Zirconium, Tantalum, Niobium, Molybdenum 1191 Tab. 4.18. Methylidenation of aldehydes with molybdenum metallacycles 57. 

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