MIXED TRANSITION METAL-MAIN GROUP ATOM CLUSTERS AS POTENTIAL MODELS FOR M2X (010) AND M4 (100) SURFACES - SYNTHESIS AND SPECTROSCOPIC AND STRUCTURAL CHARACTERIZATION OF M4(CO)13(MU-3-PPH) (M = RU, OS) - PYRAMIDAL CLUSTERS WITH M3P SQUARE FACES

The clusters HM3(CO)10(mu-PPh2) (M = Ru (1a),Os (1b)) are shown to be convenient precursors of the phosphinidene-stabilized clusters nido-M4(CO)13(mu3-PPh) (M = Ru (2a), Os (2b)) via P-C(Ph) activation, reductive elimination of benzene, and condensation. Heating a toluene solution of launder a purge of carbon monoxide forms Ru4(CO)13(mu3-PPh) (40%) as the major product. Formation of Os4(CO)13(mu3-PPh) requires more forcing conditions. Heating a solid sample of lb at 215-degrees-C for 8 min affords 2b in 20-25% yield. Both 2a and 2b have been characterized by spectroscopy and by accurate single-crystal X-ray structure analyses. Crystals of 2a and 2b are isomorphous, crystallizing in the orthorhombic space group P2(1)2(1)2, with the following unit cell dimensions: 2a, a = 11.031(2), b = 12.366(2), and c = 18.094 angstrom, with Z = 4; 2b, a = 11.055(3), b = 12.344(3), and c = 18.016(5) angstrom, with Z = 4. The molecular structures of 2a and 2b possess M4P cluster frameworks containing a butterfly arrangement of metal atoms stabilized by a mu3-phosphinidene fragment capping an open triangular face. Both clusters adhere to the effective atomic number rule (62 electrons) and are associated with nido octahedral M4P core geometries when the skeletal bonding electrons are considered (seven skeletal electron pairs, rive vertices). In the latter instance the ''PPh'' fragment is located in the square basal plane of the pyramidal framework. An alternative, less time-consuming procedure for the preparation of RU4(CO)13(mu3-PPh) (2a) is also described. Dropwise addition of dichlorophenylphosphine to a concentrated THF solution of the dianion K2[RU4(CO)13] affords 2a in reasonable yields. Variable-temperature C-13 NMR studies of 2a revealed four independent dynamic processes involving localized CO exchange, and a direct comparison is drawn between 2a and (mu-H)2Ru4(CO)12(mu3-PPh). At temperatures in excess of 296 K, we have observed the onset of total intermetallic CO scrambling. The potential of clusters 2a and 2b to serve as molecular models for catalytically active transition metal and transition metal-main group surfaces as well as potential precursors to square planar metal clusters stabilized by mu4-phosphinidene ligands are discussed.