Dinuclear coordination of a novel four-electron reduced μ-η1, η3-allene ligand with β-agostic C-H•••Ta interaction:: Direct addition of allene to (C5Me5)2Ta2( [μ-X)4 (X = Cl, Br), and molecular structure of (C5Me5)2Ta2(μ-η1, η3-C3H4)(μ-Cl)Cl3

The organoditantalum complexes (eta-C5Me5)(2)Ta-2(mu-X)(4) (X = Cl, Br) react with 1-5 equiv. of allene under mild conditions to afford the fluxional allene adducts (C5Me5)(2)Ta-2(mu-eta(1),eta(3)-C3H4)(mu-X)X-3 as shown by spectroscopic and spectrometric data. Low-temperature-limit proton NMR data at -95 degrees C for (C5Me5)(2)Ta-2(mu-eta(1),eta(3)-C3H4)(mu-Cl)Cl-3 show a large chemical shift range for the inequivalent allene hydrogens, with a high-field value for one hydrogen at delta-0.36, and inequivalent Cp* groups. The low-temperature-limit C-13 NMR resonances for the allene ligand are found at delta 204.4 (central C), delta 70.3 (CH2, (1)J(CH) = 154 Hz), and the substantially upfield value of delta 42.9. The latter resonance has remarkably different coupling constants ((1)J(CH) = 173, 154 Hz). The allene coordinates in a novel alkylidene-diyl fashion in the solid-state, as shown by X-ray diffractometry on (C5Me5)(2)Ta-2-(mu-eta(1),eta(3)-C3H4)(mu-Cl)Cl-3 at 200 K. One tantalum is doubly-bonded to the central allenic carbon (Tal=C22 distance, 2.011(7) angstrom) and interacts with an allene hydrogen in a probable beta-agostic manner (Ta1-H21A distance of 2.25(10) angstrom). The second tantalum is coordinated to the allene in a hybrid of eta(3)-allylic and sigma bonding, with Ta2-C distances to the allene methylenes of 2.206(8) and 2.276(9) angstrom and a Ta2-C22 bond distance to the central allenic carbon of 2.253(7) angstrom. The allene hydrogens are oriented above and below the C-C-C plane, consistent with appreciable sigma-bonding between the Ta(2) and the allene methylenes. The bent allene ligand has a C-C-C angle of 106.0(6)degrees and can be viewed as reduced by four electrons, with concomitant oxidation of the two d 2 tantalum centers of the organoditantalum(111) reactant. The long (TaTa)-Ta-... nonbonded distance of 3.3052(8) angstrom is consistent with two do tantalum centers in an organoditantalum(V) product. (c) 2005 Elsevier Ltd. All rights reserved.