Tantalum derivatives

A more reactive cationic dimethyl tantalum derivative is produced from Ta(OEP)Me3 using one equivalent of the weak acid HNMe2Ph BPh4]. I Ta(OEP)Me2] reacts cleanly with CO to produce a cationic enediolate compound, containing the MeC(0 )==C(0 )Me ligand which results from both insertion and... 

The first tantalum nitrene was obtained in 1959 by thermolysis of [Ta(NEt2)]5-288 This class of compounds is presently accessible by several routes, including hydrogen abstraction from the mono- or di-alkylamides, reaction of metallacarbenes with organic imines, oxidation of low valent species by organic azides, or reductive coupling of nitriles (Table 13). The tantalum derivatives are usually stabler than those of niobium. 

Both neutral and ionic niobium and tantalum chloride azides are known. The neutral azides [MN3C14]2 were obtained from MC1S and C1N3.312 They both explode spontaneously, although the tantalum derivative is somewhat stabler. The more stable ionic [MN3X5]- (X = C1, Br) species were synthesized from the hexahalometallates(V) and iodoazide or from the pen-tahalides and phosphonium or arsonium azides.73,258... 

The monomeric carbonyl complexes of d M1 isolated so far are mainly the seven-coordinated [MX(CO)2(dmpe)2] compounds (M = Nb, X = Cl, Br, N3 697 M = Ta, X = H, Cl, Br, CN, Me, Et, Pr ) (53) has been obtained by reduction of [TaCl2(dmpe)2] with one equivalent of sodium naphthalenide under CO the other tantalum derivatives were generally prepared by oxidative additions to [Ta(CO)2(dmpe)2] formed in situ by reduction of (53 Scheme 10). The niobium analogs were formed by reductive carbonylation of [NbCU(dmpe)2].698... 

Bis(arene) [M(jj6-C6H3R3)2] niobium (R = H3, H2Me or 1,3,5-Me3), and less stable tantalum derivatives (R = H3) were prepared in a similar way.714 They are extremely sensitive to oxygen, but relatively stable to water. The photoelectron spectra of the volatile bis(i76-arene) niobium display low values for the first ionization potentials (5.18-5.57 eV), indicating that the compounds are electron rich. The IR spectra of [Nb(C6H3R3)2] (R = H, Me) in an argon matrix at 80 K are in agreement with a symmetrical sandwich structure. 

Recently, the anodic oxidation of metals (method 2) was also applied for the preparation of the niobium and tantalum derivatives of M(OR)5 series (R= Me, Et, Pr, Bu ) [1478, 1616, 1639]. It should be mentioned that a crystalline oxoisopropoxideTa20(OPri)g,iPrOH (Fig. 4.1 c), was isolated from the PrOH-based electrolyte. It is destroyed on heating in vacuo, yielding Ta(OPri)J. 

Reactions between MClj and hydrogen phosphonates (HL = (RO)2P(0)H R = Et, Pr", Bu, Ph) afforded [MCls L ] (jc = 1-3), wWch were reported to be polymeric through —O— P—O bridges. NbXs when allowed to react with Na[S2P(OR)2] (R = Me, Et, cyclohexyl) gave [Nb(OMe)2Cl S2P(OR)2 2] recrystallization from the appropropriate alcohol afforded complexes having other alkoxo groups. The reaction of dithiophosphates with TaXs did not afford the analogous tantalum derivatives. 

Information on the carbonyl chemistry of niobium and tantalum is, to date, very meager. The main difficulty appears to be the reduction of the usual pentavalent derivatives of these metals to the very low formal oxidation states of metal carbonyl derivatives. Nevertheless, the yellow anions [M(C0)6] (M = Nb, Ta) have been obtained by a method analogous to, but more difficult than, one of the preparations of the [VCCO) ]" anion. The method involves reduction of the pentachlorides with sodium metal in diglyme in the presence of high pressures of carbon monoxide (63). The niobium and tantalum derivatives are much more air-sensitive than the analogous vanadium derivative. The niobium derivative has not yet been obtained analytically pure (63). No chemistry of the [Nb(CO)J and the [Ta(CO)6] ions has been reported, even conversion to the neutral carbonyl derivatives [M(CO) (M = Nb or Ta = 1 or 2) or to the carbonyl hydride derivatives HM(CO)6 (M = Nb, Ta) still presenting unsolved problems. 

High-Valence Metal Clusters. Structural properties of selected hexanuclear high-valence cluster complexes are described in Table 2.5. A series of molybdenum and tantalum derivatives of type [(M6Y8)X6] and [(M6Ys)L8] (M = Mo or W X = halide or alkoxide Y = halide or other monovalent anion and L = neutral Lewis-base) are known. The structures of these species are like that of the anion [(Mo6Cl8)Cl6] illustrated in Fig. 2.19 in which the molybdenum atoms are in the vertex of an octahedron. Metal-metal distances of about... 

Fricke, B., Johnson, E., Rivera, G.M. Ionization potentials and radii of atoms and ions of element 105 (unnilpentium) and ions of tantalum derived firom multiconfiguration Dirac-Fock calculations. Radiochim. Acta 62, 17-25 (1993)... 

---