Niobium complexes bonding

Insertions of isocyanide into niobium-carbon bonds follow a path similar to that with vanadium, resulting in the formation of the 7]2-iminoacyl complexes, which can then be involved in further chemistry.175 176 The reaction of acetone with cyclopentadienyl complex 110 under a carbon monoxide atmosphere gives the if -acetone compound 111. Complex 111 subsequently undergoes either stepwise insertion of two isocyanides via 112 or double insertion of the isocyanide to give complex 113 (Scheme 48).177... 

The corresponding chemistry of analogous niobium complexes was inhibited by the requirement of a more complicated synthetic approach for the isolation of the niobaziridine hydride. The use of the isopropyl substitued —N(Pr )Ar amido ligand proved unsuitable for the stabilization of [Nb(H) N(P )Ar 2(ri -Me2CNAr)] because of insertion into the Nb—H bond. °2 These difficulties were overcome with use of the N(CH2Bu )Ar substituent and a synthetic approach based upon [Nb(O) N(CH2Bu )Ar 3]3 which enabled the isolation of [Nb(H) N(CH2Bu )Ar 2( ri -CH(Bu )=NAr)] via reduction. The synthesis of this species has opened routes to some unusual chemistry as shown in Scheme 6.5. ... 

Hydrogenation of aromatic rings will not be described in detail in this review since it is mostly concerned with isolated double bonds. This is however an area of active research, as recently demonstrated by the use of catalytic niobium complexes for the selective hydrogenation of aryl phosphines (Scheme 3)56. 

Some complexes are known in which carbon dioxide is bonded to a transition metal. Aresia era/. [19) isolated the nickel complex (1)in which the CO ligand is coordinated througli the carbon atom and one of the oxygen atoms. Fachinetti et al. [20] found the polymeric cobalt complex (2), in which carbon dioxide is C bonded to cobalt and 0-bonded to two different potassium ions. Lappert isolated tlie remarkably stable niobium complex (3). which was the first authentic CO complex of an early d-block metal [21 j. 

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. 

Figure 12 Six-coordinate molybdenum and seven-coordinate niobium complexes which appeared to display the phenomenon of bond-stretch isomerism...

In contrast to the tantalum compounds considered above, the niobium complexes can exist as conformers of different stability at low temperatures. For example, in the H NMR spectrum of complex 4, the intensity ratio between the signals of the conformers is about 3 1 [ 32 ]. The crystal structures of the bis-amido compound 5 and the methylamido compound 6 all show that the lone pair of the amido ligand lies preferentially in the equatorial binding plane of the cyclopentadienyl and imido ligands, a conformation due to the ir bonding interaction of the lone pair with the niobium centre. If rotation about the Nb-Namido bond becomes slow on the NMR time scale then two conformers will be observed, one with the amido proton oriented toward the cyclopentadienyl ring and one with it oriented away. It is supposed that the major conformer is that with the amido proton oriented toward the cyclopentadienyl ring, which minimizes the steric interactions present [32]. 

Dinuclear niobium complexes of general formula [ NbCl2L 2( i-Cl)2( i-L)j (L = C HgS) have been prepared from the reaction of diniobium (V)decachloride [NfajClj ,] with magnesium in tetrahydroth-iophene [218]. This complex was successfully used in the catalytic cyclotrimerization of alkynes. In the solid-state structure, the Nb-Nb bond distance found to be 2.6956(9) A that coincides within the range of atomic distances of Nb -Nb double bonds, 2.60-2.86 A [218]. 

The most common oxidation state of niobium is +5, although many anhydrous compounds have been made with lower oxidation states, notably +4 and +3, and Nb can be reduced in aqueous solution to Nb by zinc. The aqueous chemistry primarily involves halo- and organic acid anionic complexes. Virtually no cationic chemistry exists because of the irreversible hydrolysis of the cation in dilute solutions. Metal—metal bonding is common. Extensive polymeric anions form. Niobium resembles tantalum and titanium in its chemistry, and separation from these elements is difficult. In the soHd state, niobium has the same atomic radius as tantalum and essentially the same ionic radius as well, ie, Nb Ta = 68 pm. This is the same size as Ti ... 

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