PHOSPHIDO-BRIDGED IRIDIUM CLUSTERS - CRYSTAL AND MOLECULAR-STRUCTURE OF H(2))(3)(MU-BIS(DIPHENYLPHOSPHINO)METHANE)(CO)(4)] AND CORRELATION OF IR-IR DISTANCES WITH VALENCE ELECTRON COUNT

The 46 electron cluster, [Ir-3(mu-PPh(2))(3)(mu-dppm)(CO)(3)] (1), dpp m=bis(diphenylphosphino)methane, undergoes (reversible) addition of ca rbon monoxide to the 48 electron derivative, [Ir-3(mu-PPh(2))(3)(mu-dp pm)(CO)(4)] (2). This product crystallizes in the P $($) over bar$$ 1s pace group (Z=2) with two molecules in the asymmetric unit and with un it cell parameters: a=21.559(4), b=22.337(4), c=13.860(3) Angstrom, al pha=89.05(2), beta=95.46(2), gamma=108.66(1)degrees. The molecular str ucture consists of an iridium triangle with two edges bridged by rough ly coplanar phosphorus atoms. The third edge is also bridged by phosph orus but approximately perpendicular to the Ir-3 plane. The unique Ir- Ir distance is 2.707(3) Angstrom and the other distances average 2.989 (3) Angstrom. Comparison of the latter distance with corresponding dis tances in 1 and [Ir-3(mu-PPh(2))(3)(CO)(5)(t-BuNC)(2)] (3), shows a re gular increase in Ir-Ir length, 2.805(2), 2.989(3), and 3.188 Angstrom in 1-3, respectively, as the valence electron count changes from 46 t o 48 to 50 electrons. P-31 NMR data for the three molecules are also c onsistent with the changes in Ir-Ir distances.