ADDITION OF SOME GOLD ELECTROPHILES TO AN OCTA-OSMIUM CARBONYL CLUSTER - THERMODYNAMIC VS KINETIC CONTROL

The reaction of the dianionic cluster [PPN]2[Os8(CO)22] {[PPN]+=[N(PPh 3)2]+} with Au2Cl2L {L=chelating phosphine, bis-(diphenylphosphino)met hane (dppm), 1,2-bis-(diphenylphosphino)ethane (dppe) or 1,4-bis-(diph enylphosphino)butane (dppb)} in the presence of an excess of the halid e acceptor TlPF6, affords the neutral digold-substituted clusters [Os8 (CO)22(Au2L)] {(1) L = dppm; (2) L = dppe; (3) L = dppb) in almost qua ntitative yield. The P-31{H-1} NMR spectra of these compounds indicate that the gold atoms are in non-equivalent environments and therefore cannot have a metal geometry similar to that previously reported for t he related cluster [Os8(CO)22(AuPPh3)2] (4). A reinvestigation of the reaction between [PPN]2 [Os8(CO)22] and monogold fragment 'AuPPh3+' ha s shown that, in addition to (4), a second isomer of [Os8(CO)22(AuPPh3 )2] (5) is formed, which on the basis of the spectroscopic data is tho ught to have the same metal geometry as compounds (1-3). A single crys tal X-ray diffraction study of the compound [Os8(CO)22(Au2dppb)] (3) h as shown that the metal core consists of a bicapped octahedron of osmi um atoms, with one of the gold atoms capping the osmium octahedron, an d the other gold atom bridging an edge of the osmium core.