Synthesis, mixed valence aspects and non-linear optical properties of the triruthenium complexes [{(bpy) 2 Ru II } 3 (L)] 3+ and [{(phen) 2 Ru II } 3 (L)] 3+ (bpy = 2,2-bipyridine, phen = 1,10-phenanthroline and L 3- = 1,3,5-triazine-2,4,6-trithiol)