Unit 3 Summary

Crystal Field Theory x z y M n Which d-orbitals are effected the most?

z M x d z 2 along z-axis z x x d x 2 - y 2 along x-y axis M These two d-orbitals constitute the higher energy e g set

d zx orbital has lobes between z-x axis z x M Likewise interactions for d xy and d yz Less repulsion These three d-orbitals constitute the lower energy t 2g set. ----

Octahedral Tetrahedral t 2g egeg e t2t2 ΔoΔo ΔtΔt Use diagram/spectrochemical series to explain: Absorption wavelengths due to electronic excitation Paramagnetic properties (unpaired electrons)

λ AbsorbedColor observed 400nm Violet absorbedGreen-yellow observed (λ 560nm) 450nm Blue absorbedYellow observed (λ 600nm) 490nm Blue-green absorbedRed observed (λ 620nm) 570nm Yellow-green absorbedViolet observed (λ 410nm) 580nm Yellow absorbedDark blue observed (λ 430nm) 600nm Orange absorbedBlue observed (λ 450nm) 650nm Red absorbedGreen observed (λ 520nm) The colors are determined by Δ. Different ligands generate crystal fields of different strength. When the molecules absorb visible light, excited electrons jump from lower energy t 2g to the higher energy e g orbital. The Δ (difference between energies of the two orbitals) is equal to the energy of the absorbed photon, and related inversely to the wavelength of the light. Weaker field ligands with smaller Δ emit light of longer λ and thus lower v. Similarly, stronger field ligands with larger Δ emit light of shorter λ and thus higher v. Color in coordination complexes

Problems 1. (a) When water ligands in [Ti(H 2 O) 6 ] 3+ are replaced by CN - ligands to give [Ti(CN) 6 ] 3-, the maximum absorption shifts from 500 nm to 450 nm. Is this shift in the expected direction? Explain. What color do you expect to observe for this ion? CN - is a stronger field ligand than H 2 O. Therefore the energy separation between the t 2g and e g levels is greater. Ti 3+ is a d 1 metal ion.  o = hc/. If  is larger, is smaller. A( max ) = 500 nm Solution appears red A( max ) = 450 nm Solution appears yellow oo [Ti(CN) 6 ] 3- egeg t 2g oo [Ti(H 2 O) 6 ] 3+ egeg t 2g

(b) The [Fe(H 2 O) 6 ] 3+ ion has a pale purple color, and the [Fe(CN) 6 ] 3- ion has a ruby red color. What are the approximate wavelengths of the maximum absorption for each ion? Is the shift of wavelength in the expected direction? Explain. CN - is a stronger field ligand than H 2 O. Therefore the energy separation between the t 2g and e g levels is greater. Fe 3+ is a d 5 metal ion.  o = hc/. If  is larger, is smaller.