CHEMISTRY OF IRIDIUM CARBONYL CLUSTER COMPLEXES - SYNTHESIS AND X-RAYCRYSTAL-STRUCTURE OF [PPH4]2[IR12(MU-CO)5(CO)19], CONTAINING AN UNPRECEDENTED METAL SKELETON

The new dodecanuclear carbonyl cluster [Ir12(CO)24]2- can be prepared in good yields by thermal treatment of [Ir6(CO)15{Cu(NCMe)}]- in reflu xing tetrahydrofuran: the anion has been characterized as the tetraphe nylphosphonium salt by a crystallographic study. [PPh4]2[Ir12(CO)24] c rystallizes in the triclinic space group P1BAR with cell constants a = 14.614(5) angstrom, b = 25.435(6) angstrom, c = 11.977(5) angstrom, a lpha = 90.30(3)-degrees, beta = 112.94(3)-degrees, gamma = 104.33(2)-d egrees, V = 3946(5) angstrom3, and Z = 2. Data were collected at room temperature to a maximum 2theta = 46-degrees, giving 10 311 unique ref lections. The structure was solved by direct methods. The final discre pancy indices were R = 0.050 and R(w) = 0.057 for 5562 independent ref lections with I > 3sigma(I). The metal frame work of the anion is a nu 2 (14-atom) trigonal bipyramid, lacking one apical and one equatorial vertex. The Ir-Ir distances are scattered in the range 2.654-(1)-2.939 (1) angstrom. Average bond distances for Ir-C(terminal), Ir-C(bridging ), C-O(terminal), and C-O(bridging) are 1.82, 2.05, 1.19, and 1.19 ang strom, respectively. Electrochemistry shows that the dianion undergoes , in acetonitrile solution, a series of single-stepped two-electron tr ansfers encompassing the sequence 2+/0/2-/4-/6-, mostly leading to tra nsient congeners. Only the dianion/tetraanion redox change appears to be chemically reversible, but its electrochemical features suggest tha t [Ir12(CO)24]2- must experience severe stereochemical reorganization upon the addition of two electrons.