A solution absorbs light of wavelength 620 nm…

Spin-only magnetic moment: $\mu = \sqrt{n(n+2)}\ \text{BM}$, with $n$ unpaired electrons. An ion absorbs the light whose photon energy matches its d-d gap: $E = 119\,627 / \lambda$ in kJ/mol, with $\lambda$ in nm. An ion disproportionates if $E^\circ_{cell} = E^\circ(\text{reduced}) - E^\circ(\text{oxidised})$ is positive, and $\Delta G^\circ = -nFE^\circ$ with $F = 96\,500\ \text{C/mol}$.