STERIC EFFECTS AND EXCITED-STATE GEOMETRY - THE RESONANCE RAMAN-SPECTROSCOPY OF [RU2X3(TACN)(2)](2+) (X = CL, BR AND I) AND [RU2X3(ME(3)TACN)(2)](2+) (X = CL, BR OR I)

The mixed-valence complexes [L3RUII1/(X3RuL3)-X-2-L-II1/2](2+), where X = halide and L = amine, commonly referred to as the 'ruthenium blues ,' all exhibit a strong absorption band which dominates the visible re gion of their electronic spectra. The resonance Raman (RR) spectra of [Ru2X3(tacn)(2)](2+) (tacn = 1,4,7-triazacyclononane, X = Cl, Br or I) and [Ru2X3(Me(3)tacn)(2))](2+) (Me(3)tacn = 1,4,7-trimethyl-1,4,7-tri azacyclononane, X = Cl or Br) were recorded at room and liquid helium temperatures using excitation wavelengths which fall within the contou r of the main visible absorption bands. Unlike their ammine analogues, [Ru2X3(NH3)(6)](2+)(X = Cl or Br), whose RR spectra contain only a fe w bands which are relatively straightforward to assign, the RR spectra of some of the tacn and Me,tacn complexes are spectacular, exhibiting long progressions involving a number of different combination bands. This paper presents the detailed assignments of RR spectra of the abov e complexes. Analysis of the low-temperature RR spectra yielded qualit ative information about the geometry changes of the complexes upon ele ctronic excitation. (C) 1998 John Wiley & Sons, Ltd.