The unit cell of the binuclear Ru(III) 4d(5) (low spin, S = 1/2) compound [(C-5(CH3)(5)Ru(mu-Cl)Cl](2) contains two isomers a and b which differ distinctly in the Ru-Ru separation (2.93 Angstrom (a), 3.75 Angstrom (b)) and in the Ru-Cl-Ru bridge angle (76 degrees (a), 100 degrees (b)). Magnetic susceptibilities have been determined in the temperature range 3 to 295 K in order to assess the intramolecular spin couplings. Isomer a shows a comparatively strong intramolecular antiferromagnetism (singlet-triplet splitting L 760 cm(-1)), whereas in b a weak ferromagnetic 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 change in spin state. The change in bonding character of the occupied orbitals allows an understanding of the bond length differences seen in the two isomers.