Question

A solution of [Co(H₂O)₆]²⁺ is pink (light red), and a solution of [Fe(H₂O)₆]³⁺ is yellow. [Co(H₂O)₆]²⁺ absorbs -- energy photons and has a -- crystal field splitting than [Fe(H₂O)₆]³⁺.

a) Higher, smaller
b) Lower, smaller
c) Higher, larger
d) Lower, larger

Answer 1

The [Co(H2O)6]2+ complex absorbs relatively higher energy photons and thus has a larger crystal field splitting than the [Fe(H2O)6]3+ complex, which means the correct answer is (b) Lower, smaller.

Step-by-step explanation:

The question concerns the energy of photons absorbed by coordination complexes and the crystal field splitting of the complexes [Co(H2O)6]2+ and [Fe(H2O)6]3+. The color of a complex is due to the light that is transmitted or reflected, which is complementary to the light absorbed. Photon energy is inversely proportional to wavelength, hence, a complex that absorbs lower energy photons will absorb light with a longer wavelength and vice versa.

Since [Co(H2O)6]2+ is pink, it absorbs photons of green light, which have relatively higher energy compared to the yellow light absorbed by the [Fe(H2O)6]3+ complex. Furthermore, the smaller crystal field splitting in [Fe(H2O)6]2+ as compared to [Co(H2O)6]2+ implies that less energy is required for d-electron transitions in the case of iron(II) complex. Hence, the correct answer is (b) Lower, smaller.