Synthesis, Solid‐State Structure and Multinuclear NMR Studies of the New Polyhydrido Iridium Carbonyl Cluster Ir4H4(μ‐H)4(CO)4(PPh3)4

The carbonyl hydride cluster Ir4H8(CO)4(PPh3)4 (1) has been formed in high yield and purity by heating a mixture of Ir4(CO)12 and PPh3 in toluene at 90 °C under hydrogen at atmospheric pressure. The product has been characterised by microanalysis, spectroscopy (IR, 1H and 31P NMR) and X‐ray diffraction. In the solid state one hydrogen, one CO and one triphenylphosphane ligand are terminally bonded to each iridium atom, whereas four hydride ligands are bridging. All the hydrogen atoms were directly located by X‐ray analysis at an average H−Ir distance of 1.55 Å (for the terminal H) and 1.75 Å (for the μ‐H). As shown by 1H and 31P NMR spectra, two isomeric forms (1A and 1B) of the cluster are present in CD2Cl2 solution at room temperature, and they do not interconvert in the explored temperature range. However, if a solid sample of the cluster is dissolved at 188 K, only 1A is detected by NMR spectroscopy. The structure of 1B could be proposed on the basis of the T1 relaxation times of the various hydrides: it results by localised exchange of H and CO around two vertices of the tetrahedral frame. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)