Nature of Bis(2,2′-bipyridine)rhodium(I) in Aqueous Solutions

Spectrophotometric studies of aqueous solutions of bis(2,2′-bipyridine)rhodium(I) (Rh(bpy)₂⁺) as a function of pH and rhodium(I) concentration provide evidence for four species as shown in eq 1–3. At room temperature log K₁ = 4.0 ± [formula omitted] 0.2, log K₂ = 7.3 ± 0.1, and log K₃ = 9.3 ± 0.3. Both monomer and dimer in eq 1 are purple; λ_max(monomer) = 505 nm (ϵ ≈ 1.0 × 10⁴ M⁻¹ cm⁻¹) and λ_max(dimer) ≈ 530 nm (ϵ/rhodium ≈ 1.0 × 10⁴ M⁻¹ cm⁻¹). The rhodium(III) hydride (eq 2) does not absorb in the visible region but the “protonated dimer” (eq 3) is brown: λ_max (ϵ/rhodium) = 960 nm (2.3 × 10³ M⁻¹ cm⁻¹), ∼500 nm (4.2 × 10³ M⁻¹ cm⁻¹, sh), 420 nm (4.8 × 10³ M⁻¹ cm⁻¹). The properties elucidated for these species are compared with those of other d⁸ systems.