Crystal Structure and Energy Transfer in Double-Complex Salts Composed of Tris(2,2‘-bipyridine)ruthenium(II) or Tris(2,2‘-bipyridine)osmium(II) and Hexacyanochromate(III)

In crystals of double-complex salts [M(bpy)3]2[Cr(CN)6]Cl·8H2O (M2+ = Ru2+, Os2+; bpy = 2,2‘-bipyridine),
luminescence from 3CT state of [M(bpy)3]2+ is partially quenched by [Cr(CN)6]3- at 77 K and room temperature
(RT). This quenching is attributed to intermolecular excitation energy transfer from the 3CT state of [M(bpy)3]2+
to the 2Eg state of [Cr(CN)6]3-. Crystal structure and crystal parameters of [Os(bpy)3]2[Cr(CN)6]Cl·8H2O: 
monoclinic, C2, a = 22.384(4) Å, b = 13.827(4) Å, c = 22.186(3) Å, β = 90.70(2)°, V = 6866(2) Å3, Z = 4,
R = 0.0789, Rw = 0.1932:  are almost the same as those of [Ru(bpy)3]2[Cr(CN)6]Cl·8H2O:  monoclinic, C2, a =
22.414(2) Å, b = 13.7686(15) Å, c = 22.207(2) Å, β = 90.713(8)°, V = 6852.9(12) Å3, Z = 4, R = 0.0554, Rw
= 0.1679. Moreover, these double complex salts have the same distance and relative orientation between donor
and acceptor. The rate of intermolecular energy transfer from [M(bpy)3]2+ to [Cr(CN)6]3- was evaluated by the
decay time of luminescence from 3CT state of [M(bpy)3]2+ in single- and double-complex salts. The rate of
energy transfer in [Os(bpy)3]2[Cr(CN)6]Cl·8H2O (4.9 × 107 s-1) is about eight times larger than that in [Ru(bpy)3]2[Cr(CN)6]Cl·8H2O (6.0 × 106 s-1) at 77 K. The difference of energy transfer rate is brought about by
only the spectral overlap between the normalized luminescence spectrum from the 3CT state of donor ([M(bpy)3]2+)
and the normalized excitation spectrum of the 2Eg state of acceptor ([Cr(CN)6]3-) in the salts. Decay rates of the
3CT state in [M(bpy)3]2[Cr(CN)6]Cl·8H2O were measured as a function of temperature. A large enhancement of
a decay rate from the 3CT state was obtained for [Ru(bpy)3]2[Cr(CN)6]Cl·8H2O as the temperature was increased.
This result implies that an additional path from the 3CT state of [Ru(bpy)3]2+ to the 2T2g state of [Cr(CN)6]3-
would be opened for energy transfer with a rise in temperature in [Ru(bpy)3]2[Cr(CN)6]Cl·8H2O.