NEW NIOBIUM COMPLEXES WITH ALKYNES .3. AN EXTRAORDINARY HEXANIOBIUM COMPOUND CONTAINING A CENTRAL NB3 CLUSTER AND 3 OUTER NB ATOMS BOUND TO ETA-4-TETRAPHENYLCYCLOBUTADIENE LIGANDS

By reduction of NbCl5 with (n-Bu)3SnH in toluene, followed by addition of PhCCPh and then a suitable workup, a crystalline compound of composition (Bu3Sn)[Nb6Cl19C122(C6H5)12]·2C7H8 was obtained in 35% yield. The structure of this material has been determined crystallographically. It forms monoclinic crystals in space group C2/c with unit cell parameters of a = 35.250 (7) Å, b= 24.048 (2) Å, c = 27.956 (6) Å, β = 102.74 (2)°, V= 23115 (13) Å3, and Z = 8. With the use of 9017 independent data collected at -80 °C, 1225 parameters were refined to R = 0.062, Rw = 0.085. The Bu3Sn unit has an effectively trigonal-planar structure with Cl atoms occupying axial positions (Sn-Cl, 2.74 Å) to complete a trigonal bipyramid. The hexanuclear entity consists of a central triangular Nb3(μ3-Cl)(μ-Cl)3 cluster with a Cl3NbCl2(c-C4Ph4) unit of roughly octahedral shape (if c-C4Ph4 is treated as one ligand) attached to each central Nb atom by three mutually cis Cl atoms. The Nb-Nb distances in the central cluster have an average value of 2.87 [1] Å, indicative of Nb-Nb single bonds, while each Nb(inner) to Nb(outer) distance is about 3.50 Å, indicative of no bonding. The c-C4Ph4 moieties have square rings (C-C, 1.45 [2] Å) symmetrically bonded to the outer Nb atoms. © 1990, American Chemical Society. All rights reserved.