Niobium complexes carbonylation reactions

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. 

R. R.Schrock, The Reaction of Niobium and Tantalum Neopentylidene Complexes with the Carbonyl Function, J. Am. Chem. Soc. 98, 5399-5400 (1976). 

Carbonyl Complexes of the Transition Metals Mechanisms of Reaction of Organometallic Complexes Niobium Tantalum Organometallic Chemistry. 

The Wittig-type olefination of carbonyl compounds is one of the characteristic reactions of carbene complexes. High-valent carbene complexes of early transition metals show ylide-like reactivity to vards carbonyl compounds. In 1976, Schrock first demonstrated that niobium and tantalum neopentylidene complexes 1 and 2, the typical nucleophilic Schrock-type carbene complexes, olefinate various carbonyl compounds including carboxylic acid derivatives [4]. 

Vanadium, Niobium, and Tantalum.—High-pressure carbonylation of [CpNbCy in the presence of Na-Cu-Al affords high yields ca. 90%) of [CpNb(CO)4], and the preparation can be carried out on a large scale ca. 35 g) irradiation of this product in solution in THF affords [CpNb(CO)3(THF)], which is a useful intermediate for the preparation of complexes of the type [CpNb(CO)3L] e.g., reactions with CO and to afford a labelled tetracarbonyl. A study of... 

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