11-1 Absorption of Light Quantum Numbers of Multielectron Atoms Electronic Spectra of Coordination Compounds

Transcription

1 Chapter 11 Coordination Chemistry III: Electronic Spectra 11-1 Absorption of Light 11-2 Quantum Numbers of Multielectron Atoms 11-3 Electronic Spectra of Coordination Compounds

2 Chapter 11 Coordination Chemistry III: Electronic Spectra Vivid colors of coordination compound. Dyes, gems (rubies, emeralds), blood etc. Transitions between d orbitals of metals. We will need to look closely at the energies of these orbitals. The electronic absorption spectrum provides a convenient method for determining the magnitude of the effect of ligands on the d orbitals of the metal.

3 Absorption of Light Complementary colors: if a compound absorbs light of one color, we see the complement of that color.

4 Absorption of Light Complementary colors: if a compound absorbs light of one color, we see the complement of that color. 600 ~ 1000 nm Blue color

5 Absorption of Light ;Beer-Lambert Absorption Law Beer-Lambert Law log(i o /I) = A = εlc Wavelength, wavenumber Energy E = hv = hc/λ= hcν

6 Quantum Numbers of Multielectron Atoms Absorption of light results in the excitation of electrons from lower to higher energy states. We observe absorption in band with the energy of each band corresponding to the difference in energy between the initial and final states. We first need to consider electrons in atoms can interact with each other. Electrons tend to occupy separate orbitals Π c Electrons in separate orbitals tend to have parallel spins Π e

7 Quantum Numbers of Multielectron Atoms Carbon atom Energy levels for the p 2 electrons Five energy levels Each energy levels can be described as a combination of the m l and m s values of the 2p electrons. 2p electrons n = 2, l = 1 m l = +1, 0, or -1 m s = +1/2 or -1/2 The orbital angular momenta and the spin angular momenta of the 2p electrons interact in a manner called Russell-Saunders coupling (LS coupling).

8 Quantum Numbers of Multielectron Atoms Russell-Saunders coupling (LS coupling) Orbit-orbit coupling M L = m l L: total orbital angular momentum quantum number Spin-spin coupling M s = m s S: total spin angular momentum quantum number microstates Spin-orbit coupling J = L + S : total angular momentum quantum number How many possible combinations of m l and m s values? One possible set of values for the two electrons in the p 2 configuration would be First electron: m l = +1 and m s = +1/2 Second electron: m l = 0 and m s = -1/2 Notation p electrons n = 2, l = 1 m l = +1, 0, or -1 m s = +1/2 or -1/2

9 Quantum Numbers of Multielectron Atoms Tabulate the possible microstates 1. No two electrons in the same microstate have identical quantum numbers (the Pauli exclusion principle) 2. Count only the unique microstates ( and ) Electronic quantum # (m l and m s ) to atomic quantum # (M L and M S ) microstates 2p electrons n = 2, l = 1 m l = +1, 0, or -1 m s = +1/2 or -1/2

10 Quantum Numbers of Multielectron Atoms Electronic quantum # (m l and m s ) to atomic quantum # (M L and M S ) describe states of multielectron atoms Russell-Saunders coupling (LS coupling) Orbit-orbit coupling M L = m l L: total orbital angular momentum quantum number Spin-spin coupling M s = m s S: total spin angular momentum quantum number Spin-orbit coupling J = L + S : total angular momentum quantum number

11 Quantum Numbers of Multielectron Atoms L = 0, 1, 2, 3 S, P, D, F, L and S describe collections of microstates. M L and M S describe the microstates themselves. Atomic States Individual Electrons M L = 0, ±1, ± 2, ± L m l =, ±1, ± 2, ± l M S = S, S-1,. S m s = +1/2, -1/2 Term symbol Term Symbol 2S+1 L J

12 Quantum Numbers of Multielectron Atoms Electronic quantum # (m l and m s ) to atomic quantum # (M L and M S ) describe states of multielectron atoms Free-ion terms are very important in the interpretation of the spectra of coordination compounds. 1 S (singlet S) : S L = 0 M L = 0, 2S+1 =1 S =0 M S = 0 M S The minimum configuration of two electrons M L

13 Quantum Numbers of Multielectron Atoms 2 P (doublet P) : P L = 1 M L = +1,0,-1 2S+1 =2 S =1/2 M S = +1/2, -1/2 The minimum configuration of one electron -1/2 M S +1/ M L

14 Quantum Numbers of Multielectron Atoms M S -1/2 +1/2 +1 1x x M L 0 0x x -1-1 x - -1 x + Six microstates The spin multiplicity is the same as the # of microstates

15 Quantum Numbers of Multielectron Atoms Reduce microstate table into its commponent free-ion terms. The spin multiplicity is the same as the # of microstates. Each terms has different energies; they represent three states with different degrees of electron-electron interactions. Which term has the lowest energy. This can be done by using two of Hund s rules. 1. The ground term (term of lowest energy) has the highest spin multiplicity. (Hund s rule of maximum multiplicity) 2. If two or more terms share the maximum spin multiplicity, the ground term is one having the highest value of L.

16 Quantum Numbers of Multielectron Atoms

17 Quantum Numbers of Multielectron Atoms d 2 l = 2 m l = +2, +1 0, -1, -2 m s = +1/2 or -1/2 1 S 1 D 1 G 3 P 3 F M L x x xx xx xx x x M S 0 x xxx xxx xxxx xxxxx xxxx xxx xxx x -1 x x xx xx xx x x

18 Quantum Numbers of Multielectron Atoms d 6 l = 2 m l = +2, +1 0, -1, -2 m s = +1/2 or -1/2 High spin Low spin 5 D 1 I

19 Spin-Orbit Coupling Spin-orbit coupling acts to split free-ion terms into states of different energies. The spin and orbital angular momenta couple each other spin-orbit coupling J = L + S : total angular moment quantum number J may have the following values J = L+S, L+S-1, L+S-2,. L-S Term Symbol 2S+1 L J Spin-orbit coupling can have significant effects on the electronic spectra of coordination compounds, especially involving heavy metals.

20 Spin-Orbit Coupling J may have the following values J = L+S, L+S-1, L+S-2,. L-S Term Symbol 2S+1 L J Spin-orbit coupling acts to split free-ion terms into states of different energies. 1 S, 1 D 1 S 1 D 1 S 0 1 D 2 p 2 3 P 2 3 P 3 P 3 P 1 3 P 0

21 Spin-Orbit Coupling 3. For subshells that are less than half-filled, the state having the lowest J value has the lowest energy. For subshells that are more than half-filled, the state having the highest J value has the lowest energy. Half-filled subshells have only one possible J value. 1 S 1 S 0 1 S, 1 D 1 D 1 D 2 p 2 3 P 2 3 P 3 P 3 P 1 3 P 0 Total energy level diagram for the carbon atom. (five energy states) The state of lowest energy can be predicted from Hund s third rule.

22 Electronic Spectra of Coordination Compounds Microstates and free-ion terms for electron configurations Identify the lowest-energy term

23 Electronic Spectra of Coordination Compounds Identify the lowest-energy term 1. Sketch the energy levels, showing the d electrons. 2. Spin multiplicity of lowest-energy state = number of unpaired electrons Determine the maximum possible value of M L for the configuration as shown. This determines the type of free-ion term. 4. Combine results of steps 2 and 3 to get ground term. Spin multiplicity = 3+1=4 Max. of M L : =3 4 F

24 Electronic Spectra of Coordination Compounds: Selection Rules On the basis of the symmetry and spin multiplicity of ground and excited electronic states. Transitions between states of the same parity are forbidden (symmerty with respect to a center of inversion.: Laporte selection rule Between d orbitals are forbidde g g transition Between d and p orbitals are allowed; g u transition 4 A 2 and 4 T 1 : spin-allowed. Transitions between states of different spin multiplicities are forbidden: spin selection rule 4 A 2 and 2 T 2 : spin-forbidden

25 Electronic Spectra of Coordination Compounds: Selection Rules Some rules for relaxation of selection rules 1. Vibrations may temporarily change the symmetry(the center of symmetry is temporarily lost: vibronic coupling relax the first selection rule:d-d transition 2. Tetrahedral complexes often absorb more strongly than O h complexes. Metal-ligand sigma bonds can be described as involving a combination of sp 3 and sd 3 hybridization of the metal orbitals: relax the first selection rule 3. spin-orbit coupling provides a mechanism of relaxing the second selection rule

26 Electronic Spectra of Coordination Compounds: correlation diagrams To relate the electronic spectra of transition metal complexes to the ligand field splitting: correlation diagrams and Tanabe- Sugano diagrams 1. Free ions (no ligand field): d 2 ; 3 F, 3 P, 1 G, 1 D, 1 S. 2. Strong ligand field. t 2 t e 2 2g 2g e g g

27 Electronic Spectra of Coordination Compounds: correlation diagrams

28 Electronic Spectra of Coordination Compounds: correlation diagrams The free-ion terms will be split into states corresponding to the irreducible representation.

29 Electronic Spectra of Coordination Compounds: correlation diagrams

30 Electronic Spectra of Coordination Compounds: correlation diagrams Irreducible representations may be obtained for the strong-field limit configurations. Each free-ion irreducible representation is matched with a strong-field irreducible representation. The spin multiplicity of the ground state.

31 Electronic Spectra of Coordination Compounds: correlation diagrams

32 Electronic Spectra of Coordination Compounds: Tanabe-Sugano diagrams = Racah parameter, a measure of he repulsion between terms of the ame multiplicity; the energy ifference between 3 F and 3 P is 15B. is the energy above the ground tate.

33 Electronic Spectra of Coordination Compounds: Tanabe-Sugano diagrams

34 Electronic Spectra of Coordination Compounds: Tanabe-Sugano diagrams High spin vs low spin High spin Low spin Ground state and spin multiplicity changed

35 Electronic Spectra of Coordination Compounds: Tanabe-Sugano diagrams

36 Jahn-Teller Distortions and Spectra d 1 d 9 complexes: might expect each to exhibit one absorption band: excitation from the t 2g to the e g levels. e g e g t 2g t 2g Two closely overlapping absorption bands.

37 Jahn-Teller Distortions and Spectra To lower the symmetry of the molecule and to reduce the degeneracy. Distortion from O h to D 4h : results in stabilization of the molecule. The most common distortion observed is elongation along z axis.

38 Jahn-Teller Distortions and Spectra : Symmetry labels for configurations Electron configurations have symmetry labels that match their degeneracies. T E Triply degenerate asymmetrically occupied state Doubly degenerate asymmetrically occupied state A or B Nondegenerate state

39 Jahn-Teller Distortions and Spectra : Symmetry labels for configurations 2 D term for d 9 ymmetry label Lower energy the opposite of the order of energies of the orbitals Higher energy 2 E g 2 T 2g Distortions can be splitting of bands. Too weak

40 Tanabe-Sugano Diagrams: Determining o from Spectra;d 1, d 4 (high spin), d 6 (high spin), d 9

41 Tanabe-Sugano Diagrams: Determining o from Spectra

42 Tanabe-Sugano Diagrams: Determining o from Spectra;d 3, d 8 Ground state F term To find o, we simply find the energy of the lowestenergy transition

43 Tanabe-Sugano Diagrams: Determining o from Spectra;d 2, d 7 (high spin Ground state F term e g 2 t 2g e g t 2g 2 3 T 1g state arising from the 3 P free-ion terms, causing a slight curvature of the both in the Tanabe-Sugan diagram. An Alternative way.

44 Tanabe-Sugano Diagrams: Determining o from Spectra;d 2, d 7 (high spin t 2 t e 2 2g 2g e g g

45 Tanabe-Sugano Diagrams: Determining o ;d 5 (high spin), d 4 to d 7 (low spin d 5 Low spin from d 4 to d 7

46 Electronic Spectra of Coordination Compounds: Tanabe-Sugano diagrams

47 Tetrahedral Complexes The lack of a center of symmetry: makes transitions between d orbitals more allowed; much more intense absorption bands. Hole formalism: d 1 O h configuration is analogous to the d 9 T d configuration: the hole in d 9 results in the same symmetry as the single electron in d 1. We can use the correlation diagram for d 10-n configuration in O h geometry e g t 2 hole t 2g e

48 Charge-Transfer Spectra Charge-transfer absorptions is much more intense than d-d transitions. Involve the transfer of electrons from molecular orbitals that are primarily ligand in character to orbitals that are primarily metal in character (or vice versa) Formal reduction of the metal: Co(III) to Co(II) LMCT

49 Charge-Transfer Spectra IrBr 6 2- (d 5 ): two band IrBr 6 3- (d 6 ): one band Why? Formal reduction of the metal: Co(III) to Co(II) LMCT

50 Charge-Transfer Spectra MLCT π-acceptor ligand (π* orbitals): CO, CN -, SCN -, bipyridine.. Oxidation of the metal d-d transitions may be completely overwhelmed and essentially impossible to observe. Formal oxidation of the metal: Fe(III) to Fe(IV) MLCT