Reduction of the organoditantalum allene complex (η-C5Me4R)2Ta2(μ-X)X3(μ-η1,η3-C3H4) (R = Me (Cp*), Et; X = Cl, Br) with sodium amalgam leads to the propynylidene complex (η-C5Me4R)2Ta2(μ-H)2X2(μ-HCCCH) by a formal double 1,3-C-H activation of the allene ligand. The solid-state molecular structure contains a planar HCCCH ligand bridging, in parallel coordination mode, the two tantalum atoms, with the HCCCH and Ta atoms coplanar. Key structural features are a Ta-Ta distance of 2.8817(7) Å, propynylidene C-C-C angle of 153.7(13)°, C-C distance of 1.370(8) Å, Ta-C(central) distance of 2.194(9) Å, and Ta-C(terminal) distance of 1.970(9) Å. Molecular orbital calculations on the complex at the RHF/SBK(d) and B3LYP/LanL2dz levels of theory demonstrate that the propynylidene ligand is best viewed formally as an allenediylidene(4-) ligand bonded to two d0 tantalum atoms via two Ta=C(terminal) double bonds and an unusual three-center, two-electron bridge bond involving both tantalum atoms and a lone pair on the planar, tetracoordinate central carbon. There is no net Ta-Ta bonding based on the orbital analysis.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry