Tantalum complexes oxidation states

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 ... 

The heavier metal tantalum is distinctly less inclined than niobium to form oxides in lower oxidation states. The rutile phase TaOz is known but has not been studied, and a cubic rock-salt-type phase TaO with a narrow homogeneity range has also been reported but not yet fully characterized. TazOs has two well-established polymorphs which have a reversible transition temperature at 1355°C but the detailed structure of these phases is too complex to be discussed here. 

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

Even the addition of 4 equiv. of dimethylzinc to a bis(dicarbollide)tantalum dichloride, Scheme 23, produced only the monomethyl complex 26 in an isolated yield of 75%.63 In these reactions, the oxidation state of the transition metal and the steric bulk of its ligands obviously play a role in the degree of alkylation. 

Aspects of borole complex reactivity have been studied in detail, including the behavior of tantalum sandwiches bearing alkyl ligands on the metal.15-17 Complexes such as 6 are best regarded as resonance hybrids where strong B-N 7t-overlap lowers the formal oxidation state of the metal 15 16... 

The benzyl derivative Cp Ta( j -02)Bn was structurally characterized by X-ray diffraction the 02-ligand is side-on coordinated and lies in the equatorial plane of the bent metallocene fragment. The 0-0 distance of 1.477(8) A and the 0-0 stretching frequency (vo-o = 863cm ) are consistent with a peroxo hgand (02 ) coordinated to tantalum in its highest possible formal oxidation state (+V). Notably, base appears to stabihze these complexes i.e., in the presence of triethylamine Cp Ta( 7 -02)Me did not decompose even when heated to 80 °C. 

Compounds (17) and (19) are formally 14-electron species. By contrast, the /z-dinitrogen complexes [ M (N2)(Cp )2 2(t/-N2)] (M = Ti, Zr) are respectively 16-electron and 18-electron species, and the low oxidation state of the metal is preserved. The spectroscopic data also illustrate the differences between the tantalum and these Tin or Zr11 /4-dinitrogen compounds. The l5N spectrum of (16) shows a singlet at 414p.p.m.,306 a chemical shift comparable to that... 

A study has been made of the reactivity of high-oxidation state tantalum-alkylidene complexes stabilized by the aryldiamine ligand [C6H3(CH2NMe2)2-2, 6] , and an unusual rearrangement involving the unique activation of an aryl C—H bond by the alkylidenetantalum moiety has been observed.531... 

We were quite optimistic in the beginning as the second reduction process corresponds to the formation of a black deposit which was potentially the first electrochemical route to make thick tantalum layers. After having washed off all ionic liquid from the sample we were already a bit sceptical as the deposit was quite brittle and did not look metallic. The SEM pictures and the EDX analysis supported our scepticism and the elemental analysis showed an elemental Ta/Cl ratio of about 1/2. Thus, overall we have deposited a low oxidation state tantalum choride. Despite the initial disappointment we were still eager to obtain the metal and found some old literature from Cotton [122], in which he described subvalent clusters of molybdenum, tungsten and tantalum halides. In the case of tantalum the well-defined Ta6Cli22+ complex was described with an average oxidation number of 2.33 and thus with a Ta/Cl molar ratio very close to 1/2. Such clusters are depicted in Figure 4.15. 

Alkylidene complexes are of two types. The ones in which the metal is in a low oxidation state, like the chromium complex shown in Fig. 2.4, are often referred to as Fischer carbenes. The other type of alkylidene complexes has the metal ion in a high oxidation state. The tantalum complex is one such example. For both the types of alkylidene complexes direct experimental evidence of the presence of double bonds between the metal and the carbon atom comes from X-ray measurements. Alkylidene complexes are also formed by a-hydride elimination. An interaction between the metal and the a-hydrogen atom of the alkyl group that only weakens the C-H bond but does not break it completely is called an agostic interaction (see Fig. 2.5). An important reaction of alkylidene complexes with alkenes is the formation of a metallocycle. 

In general, Group 5 fluorides tend to adopt a preference for higher oxidation states, and as a consequence, over the past few years, there have been virtually no reported examples of low valent fluoro complexes of vanadium, niobium or tantalum. 

In contrast, Schrock carbenes are electron deficient [10 to 16 valence electrons (VE)] early transition metal complexes with the metal atom in a high oxidation state and carbene substituents that are limited to alkyl groups and hydrogen [131]. Their bonding situation can be described in terms of the interaction of a triplet carbene with a triplet metal fragment resnlting in a covalent double bond [132], Tantalum complexes like [(np)3Ta=CHBu ] and [Cp2(Me)Ta=CH2] are representative of Schrock carbenes. 

No derivative in which niobium or tantalum displays an oxidation state of —II has been reported so far, while carbonyl complexes derived from [M(CO)s] represent the only coordination compounds for these metals in oxidation state —III. 

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