Tantalum dimer, formation

Inner-Transition Metal to Transition and Inner-Transition Metal Bond 67 9.2.8. Formation of Heavy Transition Metal Group V Metal-Metal Bonds 9.2.8.3. In Discrete Niobium and Tantalum Dimeric and Trimeric Complexes... 

Niobium and tantalum compounds form adducts with virtually all types of neutral ant anionic donors. The coordination chemistry of the higher halides is widely developed, and thei activity as Friedel-Crafts catalysts is another manifestation of their Lewis acidity. The stron acceptor capacity of the high valent metal compounds tends to favor the formation of dimers and sometimes of higher condensation products, which competes with coordination with othe donor molecules. Numerous simple anionic or heteropolyanionic species, but little cationi chemistry, and no simple metal salts, are known. 

Scheme 17. Formation of supracyclopentadienyl derivatives from the alkylidyne moiety in the dimeric tantalum complex and alkynes (13). Drawings of molecules are schematic.

Monomeric species M OR-tert)x have been characterized for titanium, vanadium, chromium, zirconium, and hafnium (x = 4) and for niobium and tantalum (x == 5). With chromium it was found that limiting Cr(III) to coordination number 4 in the dimeric Cr2(OBu )e caused instability and a remarkable facility toward valency disproportionation or oxidation to the stable quadricovalent Cr(OBu )4 (8, 9). In contrast, molybdenum formed a stable dimeric tri-tert-butoxide (Bu O)3Mo=Mo-(OBu )3 which is diamagnetic and presumably bound by a metal-metal triple bond (10, II). Yet another interesting feature of chromium is the synthesis of a stable diamagnetic nitrosyl Cr(NO) (OBu )3 in which the nitric oxide is believed to act as a three-electron donor with formation of a four-coordinated low spin chromium (II) compound (12). The insta-bihty of Cr2(OBu )e and the stability of both Cr(NO) (OBu )3 and Cr(OBu )4 must result from the steric effects of the tertiary butoxo groups since the less bulky normal alkoxo groups form very stable polymeric [Cr(OR)3]a. compounds in which the Cr(III) has its usual coordination number of 6 (octahedral). 

The cycloolefin-tantalum complex (r] -C5Me5)Ta(cyclooctene)Cl 48 is capable of dimerization of various olefins into a mixture of the head-to-tail and head-to-head dimers 49 and 50 (Scheme 21) [27]. Some representative results are summarized in Table 10. It was shown that the dimerization process proceeds through series of reversible metallacycle formations such as tantalacy-clopentanes and tantalacyclobutanes, and jS-hydrogen eliminations. The dimerization ends in reductive elimination of the organyl moiety from the intermediate alkylhydridotantalum species. 

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