Group 5 V, Nb. and Ta

The results of EHMO calculations on various diene derivatives of TaCp and TaCp suggest they are best regarded as bent metallacyclopent-3-enes.The bonding and behaviour of these complexes is the subject of review.  

Methyl lithium reacts with VClCp 2 to form VMeCp 2- This compound undergoes hydrogenolysis to the hydride VHCp 2 and inserts with CO (1 atm.) to form V(C0Me)(C0)Cp 2 via the two intermediates 

Hydride to al)cene migration of NbH(CH2=CHR)Cp 2 in the presence of L (L = CNMe, CO) leads to the alkyls Nb(CH2CH2R)LCp 2, the reaction kinetics are interpretable in terms of concerted four centre cyclic transition states.- Alkynes with electron withdrawing substituents insert into the M-H bond of MH(CO)Cp2 resulting in the formation of the alkynyls M(CR=CHR)(C0)Cp2 (M = 

The reaction of sodium sand, TaCl, and PMe3 gives 

Cp Ta(H) Ti2.CH2PMe2 (dmpe), while reaction with MeX gives Cp TaX2 tl -CHIMe2) (X = halide) jhe reaction between WH Ti2-CH2lMe2) (PMe3)4 and Te results in the complex 

Reaction of the diene complexes Cp2M(butadiene) (M = Zr, Hf) with CpV(CO)4 leads to the vanadium carbene complex 23. In the case of M = Hf, complex 23 is in equililnium widi the carbene 24. These complexes react with various ketones to form nine-membeied mctallaoxy vanadium carbenes.2W 

There is some material of relevance in a review on niobium and tantalum coordination chemistry. 1 The thermal reactions of the ions M (M = Ta, Nb, NbO, NbN) with arenes in the gas phase results in cleavage of the arenes to form metallacyclopentadiene ion complexes via retro-alkyne cyclotrimerization mechanisms.  

Addition of 2-methy 1-2-butene to tranj-TaH(CO)4 PEt2P(CH2)2PPh2 results in hydrotantalation to form the alkyl tranj-Ta(CHMeCHMe2)(CO)4 PEt2P(CH2)2PPh2 mainly as the anti-Markovnikov addition product. 9 

The reduction of TaCl3(Dipp)2(OEt2) in the presence of the alkyne Bu CsCH gives the metallacyclopentadiene (Dipp)2ClTa CBuM HCH=CBuM. This is the kinetic product and 

Cp2Ta OCH2CH2 suggest the anti product is mote stable but that the initial product might be syn because of steric hindrance involving the Cp rings. 

Reaction of one equivalent of Me3SiCl with Na[Ta(CO)2(dmpe)2] affords the siloxycarbyne Ta(CO)(dmpe)2(=COSiMe3). This converts in the presence of NaCl to the CO-bridged complex (dmpe)2(OC)TaCOTa(Me3SiOCCOSiMe3)(dmpe)2.  

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Nineteen X-ray analyses have been performed, in our group (Table 1), on mono-and di-bridged bimetallic systems with one bent metallocene unit and with one phosphido bridge. A rhodium-niobocene structure with two phosphido bridges has been reported by Nikonov et al. Their structural parameters are listed in Table 1. The molecules are listed in an order which reflects the number of bridges and the nature of the metallocene (group 5 metals Nb(lll), Ta(Ill), d Nb(V), Ta(V), d group 6 metals Mo(lV), W(1V), d ). 

Na[M(CO)5], where M = V, Nb, and Ta, may also be prepared by the reduction of corresponding chlorides of the group 5 metals with sodium simultaneously utilizing high-energy ultrasounds. 

In theory, the cations of V, Nb, and Ta should be more tolerant towards polar functionalities than their more oxophilic Group 4 counterparts. However, only a few reports of Group 5 MMA polymerization initiators have appeared. Mashima and coworkers have synthesized 1,4-diaza-1,3-diene tantalum complexes and found that in the presence of Mc3Al, MMA may be polymerized in a living manner at -20 °C to —30 (rr = 71-78%). " Chen has also shown that Cp2TaMe3 polymerizes MMA when activated with two equivalents of AlMes. However, initiator efficiencies are low and molecular weight distributions are broad. ... 

As mentioned above the best known examples of this mechanism is diffusion of O, N, C, and H atoms interstitially dissolved in metals. By way of example oxygen and nitrogen atoms in bcc metals, e.g. in the group 5 metals V, Nb, and Ta, occupy octahedral sites, and in this case each interstitial atom has 4 nearest neighbour octahedral sites to which they may jump, thus... 

The crystal structures of both (PPN)[M(CO)6] derivatives have been determined.717 The coordination polyhedron is octahedral (Nb—C 2.098(5) A CNbC 89.2(2)°). The PPN moiety is constrained to be centrosymmetric, and thus linear. These compounds correspond to the lowest oxidation state of niobium and tantalum for which structural data are available. A single v(CO) is found in the IR (1854 and 1852 cm-1 for Nb and Ta respectively). Comparable spectra are observed for Na[M(CO)6] in pyridine, but in solvents of lower dielectric constants such as tetrahydrofuran, additional bands attributed to distortion of the anion by the countercation are observed. [Nb(CO)6] appears to be the most labile carbonyl of the group VA analogs. 

Oxidative addition reactions for group VA compounds within the context of the discussion found in 5.8.2.9.1 are presented in 10.3.3.1 and 10.3.3.2, including some cases of reactions that may not be true oxidative additions. The discussion on vanadium is separated from its heavier cousins, niobium and tantalum, since the chemistry of V is distinct from that of Nb and Ta. Oxidative additions across metal-metal multiple bonds and involving clusters are not covered. 

The chemistry of Nb and Ta is predominantly that of the -1-5 oxidation state. The heavier group 5 metals differ from V (see Section 21.6) in the relative instability of their lower oxidation states, their failure to form simple ionic compounds, and the inertness of the M(V) oxides. In contrast to V, it is not meaningful to assign ionic radii to Nb and Ta in their lower oxidation states since they tend to form hexanuclear clusters with metal-metal bonding (see later). For M(V), radii of 64 pm are usually tabulated for Nb " and but these are unreal quantities since... 

Pershina, V. 1998a. Solution chemistry of element 105-PartI Hydrolysis of group 5 cations Nb, Ta, and Pa. Radiochim Acta 80, 65-73. 

The chemistry of Nb and Ta is predominantly that of the -b5 oxidation state. The heavier group 5 metals differ from V... 

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