The reaction of a mixture of the [(eta(5)-C5H5)Co(eta(5)-MeEt(4)C(3)B(2))](1-) and nido-6-Me-5,6,9-C3B7H91- anions with MX(2) (M = Fe (1), Co (2), Ni, (3)) yields the new hybrid diborolyl/ tricarbadecaboranyl triple-decker complexes (eta(5)-C5H5)Co(mu-eta(5)-MeEt(4)C(3)B(2))M(MeC(3)B(7)H(9)), containing 30 (M = Fe), 31 (M = Co), and 30 (M = Ni) valence electrons, respectively. An X-ray structural study of 1 confirmed that the FeC3B7 fragment has a close geometry, consistent both with its 24-skeletal-electron count and with the cage structure previously confirmed for closo-1-(eta(5)-C5H5)Fe-(5-Me-2,3,5-C3B7H9). Structural studies of 2 and 3 confirmed that their MC(3)B(7) fragments have open cage distortions consistent with higher skeletal-electron counts. These complexes are the first triple-decker complexes in which the terminal cage ligands have open cage structures. As predicted on the basis of its 25-skeletal-electron count, in 2 the observed CoC3B7 cage structure is intermediate between those expected for 11-vertex close (24-electron) and nido (26-electron) frameworks. This open cage structure is likewise consistent with that found for 1-(eta(5)-C5H5)Co-(2-Me-2,3,5-C3B7H9) (4). In complexes 1 and 2, the MeC(3)B(7)H(9) ligand is functioning as a cyclopentadienyl analog, but electrochemical studies show that the tricarbadecaboranyl ligand is much more electron-withdrawing than the cyclopentadienyl ligand and can thus stabilize the formation of anionic species. If, as in 1 and 2, the MeC(3)B(7)H(9) cage in 3 is functioning as a 5-electron ligand, then 3 should be a paramagnetic 32-valence-electron system. However, 3 is diamagnetic and the structural and spectroscopic data suggest 30 valence electrons, with the MeC(3)B(7)H(9) ligand functioning as only an eta(4), 3-electron ligand similar to an eta(3) pi-allyl. This conclusion is also supported by structural studies of 1-(eta(3)-C3H5)Ni-(eta(6)-2-Me-2,3,5-C3B7H9) (5a), 1-(eta(3)-2-MeC(3)H(4))Ni-(eta(6)-2-Me-2,3,5-C3B7H9) (5b), and 9-(eta(5)-C5H5)Ni-(eta(4)-8-Me-7,8,10-C3B7H9) (6), which show that, in 5a and 5b, the NiC3B7H9 cages have closo-type structures (i.e. 24-skeletal-electrons) consistent with 5-electron donation to the nickel atom, whereas in 3 and 6 the NiC3B7H9 cages have nido-type structures (i.e. 26-skeletal-electrons), suggesting only 3 electrons are donated to the nickel atom.
