The 2-D INEPT-INADEQUATE spectrum for 7 (Figure 2) is also irrefutable evidence for structural identification.7 Note that 8 (Figure 3b) would show a connected to b and b connected to c. The spectrum clearly shows a directly bonded to both b and c, with two-bond coupling between b and c and three-bond coupling between c and d.10•11

Reduction of (q-C5Me4R)TaX4 (R = Me, Et; X = CI, Br) with two equivalents of sodium amalgam in toluene affords the reactive tantalum-tantalum doubly-bonded species (V-C5Me R)2Ta2(p-X)4 in 70% yield; the solid-state structure of ( C5Me5)2Ta2(p-Br)4 consists of a Ta=Ta double bond, 2.748(2) 1 in length, bridged symmetrically by four bromide ligands. $ The C5Me5 lH n.m.r. chemical shifts in C6D6 for the TaV precursors (C5Me5)TaBr4 and (CSMeS)TaC14 are 6 2.26 and 2.15, respectively.

Reductive oligomerization of (C5Me4R)VX3 or addition of (C5Me4R)SnBu3 to VX3L3 (L = thf, tht) yields the (peralkylcyclopentadienyl)vanadium(III) halides (η5-C5Me4R)2V2(μ-Br)4 and (η5-C5Me4R)3V3(μ-Cl)6 (R = Me, Et), which can be halogenated to afford (C5Me4R)VX3. Paramagnetic (C5Me4Et)2V2(μ-Br)4 is a four-legged piano-stool dimer in the solid state with a nominal V−V bond (2.565(1) Å), while the spin-frustrated, antiferromagnetic (C5Me4Et)3V3(μ-Cl)6 is a piano-stool cyclotrimer with two μ-chlorines per nonbonded V···V edge (3.3732[63] Å).

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