NEW NICKEL-ANTIMONY CARBONYL CLUSTERS - STEREOCHEMICAL ANALYSES OF THE [NI-10(SBR)(2)(CO)(18)](2-) DIANIONS (R=ME, ET, PR-I, BU-T, P-FC6H4)CONTAINING EMPTY 1,12-NI10SB2 ICOSAHEDRAL CAGES AND OF THE UNPRECEDENTED STIBINIDO-BRIDGED 34-ELECTRON NI-2(CO)(4)(MU(2)-SBBUT(2))(2) DIMER

Reactions of [Ni-6(CO)(12)](2-) (1) with R2SbBr (R = Me, Et, Pr-i) and R2SbCl (R = Bu-t, p-FC6H4) have given rise to the new [Ni-10(SbR)(2)( CO)(18)](2-) complexes (R = Me (3), Et (4), Pr-i (5), Bu-t (6), p-FC6H 4 (7)) as the only major Ni-Sb product in each reaction (30-55% yields isolated as [NMe4](+) salts). These dianions, each of which has a cry stallographically imposed inversion center, possess a common noncenter ed 1,12-Ni10Sb2 icosahedral cage that is surrounded by a 20-vertex lig and polyhedron composed of 10 terminal, 4 doubly bridging, and 4 tripl y bridging COs and the two Sb-attached R substituents. The formulation of each dianion was unambiguously established from X-ray crystallogra phic and infrared studies. An elemental analysis of [NMe4](2)[3] subst antiated its stoichiometry; magnetic susceptibility measurements concl usively showed this salt to be diamagnetic at room temperature. Cyclic voltammetric measurements of 3 and 5 in acetonitrile solutions simila rly displayed only irreversible oxidation waves at ca. +0.60 V. A nega tive-ion electrospray ionization (ESI) mass spectrum of [NMe4](2)[5] r evealed the doubly charged ion-parent (M2-) signal for the dianion (5) as well as 11 other assigned doubly charged high-range m/z signals. I n light of the previously known phenylstibinidene [Ni-10(SbPh)(2)(CO)( 18)](2-) (2) prepared from reactions of 1 with either PhSbCl2 or Ph2Sb Cl, reactions of 1 with PhMeSbX (X = Br, I) were carried out; only 3 w as isolated. Limiting slow-exchange C-13{H-1} spectra were obtained at -60 degrees C for both 3 and 5 in acetone-d(6); a variable-temperatur e line-shape analysis of the terminal-bridge carbonyl exchange process in 5 gave an estimated activation energy of 52.5 kJ/mol. One achieved objective was to assess the resulting steric effects due to a systema tic change of the Sb-attached R substituents upon both the geometries of the common empty 1,12-Ni10Sb2 cage and the encapsulating 20-vertex ligand polyhedron. Another goal was to determine the possible existenc e for E = Sb of the other two hypothetical [Ni-9(ER)(3)(CO)(15)](2-) a nd [Ni-8(ER)(4)(CO)(12)](2-) dianions containing empty 1,2,12-Ni9E3 an d 1,2,9,12-Ni8E4 icosahedral cages, respectively; previous work had sh own the existence of all three electronically equivalent (isolobal) di anions for E = P and As but only 2 for E = Sb with R = Ph. Despite ext ensive efforts to detect such species, no evidence for their existence was uncovered. However, a neutral 34-electron Ni-2(CO)(4)(mu(2)-(SbBu 2)-Bu-t)(2) dimer (8) containing a planar Ni2Sb2 core was isolated as a side product and structurally characterized; this dimer is the first example of a stibinido-bridged analogue of the known 34-electron Ni-2 (CO)(4)(mu(2)-PR2)(2)-type dimer. A structural/bonding analysis with o ther 34-, 32-, and 30-electron Ni-2(mu(2)-ER2)(2) dimers (E = P, As) c ontaining analogous planar Ni2E2 rings provides a self-consistent basi s that a strong Ni-Ni' bonding interaction between two Ni(I) plays a d ominant role in dictating the observed geometry of each of these ligan d-bridged dimers. This comparison also indicates that an essential ing redient for the existence of 8 as a stable entity is the bulky Sb-atta ched Bu-t substituents which apparently protect the dimer from air oxi dation.