HYBRID DIBOROLYL TRICARBADECABORANYL TRIPLE-DECKERS - (ETA(5)-CYCLOPENTADIENYL)COBALT (MU-ETA(5)-1,3-DIBOROLYL)METAL (ETA(6)-TRICARBADECABORANYL AND ETA(4)-TRICARBADECABORANYL) COMPLEXES (M=FE, CO, NI) - TRIPLE-DECKER COMPLEXES WITH OPEN CAGE DISTORTIONS/

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), C o (2), Ni, (3)) yields the new hybrid diborolyl/ tricarbadecaboranyl t riple-decker complexes H5)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 elect rons, respectively. An X-ray structural study of 1 confirmed that the FeC3B7 fragment has a close geometry, consistent both with its 24-skel etal-electron count and with the cage structure previously confirmed f or 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 distor tions consistent with higher skeletal-electron counts. These complexes are the first triple-decker complexes in which the terminal cage liga nds have open cage structures. As predicted on the basis of its 25-ske letal-electron count, in 2 the observed CoC3B7 cage structure is inter mediate between those expected for 11-vertex close (24-electron) and n ido (26-electron) frameworks. This open cage structure is likewise con sistent 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 c yclopentadienyl analog, but electrochemical studies show that the tric arbadecaboranyl ligand is much more electron-withdrawing than the cycl opentadienyl ligand and can thus stabilize the formation of anionic sp ecies. 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-ele ctron system. However, 3 is diamagnetic and the structural and spectro scopic data suggest 30 valence electrons, with the MeC(3)B(7)H(9) liga nd functioning as only an eta(4), 3-electron ligand similar to an eta( 3) pi-allyl. This conclusion is also supported by structural studies o f 1-(eta(3)-C3H5)Ni-(eta(6)-2-Me-2,3,5-C3B7H9) (5a), (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 c ages have closo-type structures (i.e. 24-skeletal-electrons) consisten t 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.