PHOSPHIDO-BRIDGED IRIDIUM CLUSTERS - CRYSTAL AND MOLECULAR-STRUCTURE OF [IR-3(MU-PPH(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), dppm=bis(diphenylphosphino)methane, undergoes (reversible) addition of carbon monoxide to the 48 electron derivative, [Ir-3(mu-PPh(2))(3)(mu-dppm)(CO)(4)] (2). This product crystallizes in the P $($) over bar$$ 1space group (Z=2) with two molecules in the asymmetric unit and with unit cell parameters: a=21.559(4), b=22.337(4), c=13.860(3) Angstrom, alpha=89.05(2), beta=95.46(2), gamma=108.66(1)degrees. The molecular structure consists of an iridium triangle with two edges bridged by roughly coplanar phosphorus atoms. The third edge is also bridged by phosphorus 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 distances in 1 and [Ir-3(mu-PPh(2))(3)(CO)(5)(t-BuNC)(2)] (3), shows a regular 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 to 48 to 50 electrons. P-31 NMR data for the three molecules are also consistent with the changes in Ir-Ir distances.