MAGNETISM AND ELECTRONIC-STRUCTURE OF THE (PENTAMETHYLCYCLOPENTADIENYL)DICHLORORUTHENIUM DIMERS

The unit cell of the binuclear Ru(III) 4d(5) (low spin, S = 1/2) compo und [(C-5(CH3)(5)}Ru(mu-Cl)Cl](2) contains two isomers a and b which d iffer distinctly in the Ru-Ru separation (2.93 Angstrom (a), 3.75 Angs trom (b)) and in the Ru-Cl-Ru bridge angle (76 degrees (a), 100 degree s (b)). Magnetic susceptibilities have been determined in the temperat ure range 3 to 295 K in order to assess the intramolecular spin coupli ngs. Isomer a shows a comparatively strong intramolecular antiferromag netism (singlet-triplet splitting L 760 cm(-1)), whereas in b a weak f erromagnetic coupling (triplet-singlet splitting approximate to 24 cm( -1)) via the chlorine bridge is deduced. Extended Huckel calculations provide a qualitative explanation for the observed geometries and spin states of the two isomers. The electronic picture in these t(2g)(5)t( 2g)(5) compounds is very similar to that in the e(g)(3)e(g)(3) copper dimers. Orbital crossing as a function of bridging angle leads to a ch ange in spin state. The change in bonding character of the occupied or bitals allows an understanding of the bond length differences seen in the two isomers.