Slide 1

The Electronic Spectra of Coordination Compounds Slide 2 The UV/Vis spectra of transition metal complexes shows the transitions of the electrons. Analysis of these spectra can be quite complex. Slide 3 Electron Spectra The UV/Vis spectra are used to determine the value of o for the complex. The spectra arise from electronic transitions between the t 2g and e g sets if molecular orbitals. Electron- electron interactions can greatly complicate the spectra. Only in the case of a single electron is interpretation of the spectrum straightforward. Slide 4 Obtaining o For a d 1 configuration, only a single peak is seen. It results from the electron promotion from the t 2g orbitals to the e g orbitals. The toothed appearance of the peak is due to a Jahn-Teller distortion of the excited state. The energy of the peak = o. Slide 5 General Observations d 1, d 4, d 6 and d 9 usually have 1 absorption, though a side hump results from Jahn-Teller distortions. Slide 6 General Observations d 2, d 3, d 7 and d 8 usually have 3 absorptions, one is often obscured by a charge transfer band. Slide 7 General Observations d 5 complexes consist of very weak, relatively sharp transitions which are spin-forbidden, and have a very low intensity. Slide 8 Qualitative Explanation Consider a Cr(III) complex such as [Cr(NH 3 ) 6 ] 3+. The ground state configuration is: ________A transition from the d z 2 d x 2 -y 2 d xy to the d x 2 -y 2, or the d yz or d xz to the d z 2 ____ ____ ____orbitals involve a relatively d xy d yz d xz minor change in environment. Slide 9 Qualitative Explanation The transition from the d xz orbitals to the d z 2 orbitals involves a relatively minor change in the electronic environment. Slide 10 Qualitative Explanation Consider a Cr(III) complex such as [Cr(NH 3 ) 6 ] 3+. The ground state configuration is: ________A transition from the d z 2 d x 2 -y 2 d xy to the d z 2, or the d yz or d xz to the d x 2 -y 2 ____ ____ ____orbitals involve a major d xy d yz d xz change in environment. Slide 11 Qualitative Explanation The transition from orbitals in the xy plane to the d z 2 orbitals involves a fairly major change in the electronic environment. Slide 12 Qualitative Explanation Since the promotion of an electron from the t 2g set of orbitals to the e g set can involve differing changes in environment, several peaks will be seen in the spectrum. Slide 13 3d Multi-electron Complexes For complexes with more than one electron in the 3d (and 4s) orbitals of the metal, electron interactions must be considered. The electrons are not independent of each other, and the orbital angular momenta (m l values) and the spin angular momenta (m s values) interact. Slide 14 4d and 5d Metal Complexes The lower transition metals undergo further coupling (called j-j coupling or spin-orbit coupling). Slide 15 3d Multi-electron Complexes The interaction is called Russel-Saunders or L-S coupling. The interactions produce atomic states called microstates that are described by a new set of quantum numbers. M L = total orbital angular momentum =m l M S = total spin angular momentum = m s Slide 16 Determining the Energy States of an Atom A microstate table that contains all possible combinations of m l and m s is constructed. Each microstate represents a possible electron configuration. Both ground state and excited states are considered. Slide 17 Energy States Microstates would have the same energy only if repulsion between electrons is negligible. In an octahedral or tetrahedral complex, microstates that correspond to different relative spatial distributions of the electrons will have different energies. As a result, distinguishable energy levels, called terms are seen. Slide 18 Energy States To obtain all of the terms for a given electron configuration, a microstate table is constructed. The table is a grid of all possible electronic arrangements. It lists all of the possible values of spin and orbital orientation. It includes both ground and excited states, and must obey the Pauli Exclusion Principle. Slide 19 Constructing a Microstate Table Consider an atom of carbon. Its highest occupied orbital has a p 2 electron configuration. Microstates correspond to the various possible occupation of the p x, p y and p z orbitals. Slide 20 Constructing a Microstate Table m l =+1 0-1 microstate: Configurations:___ ___ ___ (1 +,0 + ) ___ ___ ___ (0 +,-1 + ) ___ ___ ___ (1 +,-1 + ) These are examples of some of the ground state microstates. Others would have the electrons (arrows) pointing down. Slide 21 Constructing a Microstate Table m l =+1 0-1 microstate: Configurations:___ ___ ___ (1 +,1 - ) ___ ___ ___ (0 +,0 - ) ___ ___ ___ (-1 +,-1 - ) These are examples of some of the excited state microstates. Slide 22 Microstate Table for p 2 For the carbon atom, M L will range from +2 down to -2, and M S can have values of +1 (both electrons pointing up), 0 (one electron up, one electron down), or -1 (both electrons pointing down). Slide 23 Microstate Notation For the carbon atom, M L will range from +2 down to -2, and M S can have values of +1 (both electrons pointing up), 0 (one electron up, one electron down), or -1 (both electrons pointing down). Slide 24 Microstate Table for p 2 M S MLML +10 +2:1+1-1+1- +1:1+0+1+0+ 1+0-1-0+1+0-1-0+ 1-0-1-0- 0:-1 + 1 + -1 + 1 - 0 + 0 - -1 - 1 + -1 - 1 - -1:-1 + 0 + -1 + 0 - -1 - 0 + -1 - 0 - -2:-1 + -1 - The table includes all possible microstates. Slide 25 Constructing a Microstate Table Once the microstate table is complete, the microstates are collected or grouped into atomic (coupled) energy states. Slide 26 Constructing a Microstate Table For two electrons, L = l 1 + l 2, l 1 + l 2 -1, l 1 + l 2 -2,l 1 - l 2 For a p 2 configuration, L = 1+1, 1+1-1, 1-1. The values of L are: 2, 1 and 0. L is always positive, and ranges from the maximum value of l. Slide 27 Constructing a Microstate Table For two electrons, S = s 1 + s 2, s 1 + s 2 -1, s 1 + s 2 -2,s 1 - s 2 For a p 2 configuration, S = + , + -1. The values of S are: 1 and 0. Slide 28 Atomic Quantum Numbers Quantum numbers L and S describe collections of microstates, whereas M L and M S describe the individual microstates themselves. Slide 29 Constructing a Microstate Table The microstate table is a grid that includes all possible combinations of L, the total angular momentum quantum number, and S, the total spin angular momentum quantum number. For two electrons, L = l 1 + l 2, l 1 + l 2 -1, l 1 + l 2 -2,l 1 - l 2 S = s 1 + s 2, s 1 + s 2 -1, s 1 + s 2 -2,s 1 - s 2 Slide 30 Constructing a Microstate Table Once the microstate table is complete, all microstates associated with an energy state with specific value of L and S are grouped. It doesnt matter which specific microstates are placed in the group. Microstates are grouped and eliminated until all microstates are associated with a specific energy state or term. Slide 31 Term Symbols Each energy state or term is represented by a term symbol. The term symbol is a capitol letter that is related to the value of L. L = 01234 Term Symbol SPDFG Slide 32 Term Symbols The upper left corner of the term symbol contains a number called the multiplicity. The multiplicity is the number of unpaired electrons +1, or 2S+1. Slide 33 Microstate Table for p 2 M S MLML +10 +2:1+1-1+1- +1:1+0+1+0+ 1+0-1-0+1+0-1-0+ 1-0-1-0- 0:-1 + 1 + -1 + 1 - 0 + 0 - -1 - 1 + -1 - 1 - -1:-1 + 0 + -1 + 0 - -1 - 0 + -1 - 0 - -2:-1 + -1 - Eliminate microstates with M L =+2 -2, with M s =0. Slide 34 Microstate Table for p 2 M S MLML +10 +2:1+1-1+1- +1:1+0+1+0+ 1+0-1-0+1+0-1-0+ 1-0-1-0- 0:-1 + 1 + -1 + 1 - 0 + 0 - -1 - 1 + -1 - 1 - -1:-1 + 0 + -1 + 0 - -1 - 0 + -1 - 0 - -2:-1 + -1 - Eliminate microstates with M L =+2 -2, with M s =0. Slide 35 Microstate Table for p 2 M S MLML +10 +2:1+1-1+1- +1:1+0+1+0+ 1+0-1-0+1+0-1-0+ 1-0-1-0- 0:-1 + 1 + -1 + 1 - 0 + 0 - -1 - 1 + -1 - 1 - -1:-1 + 0 + -1 + 0 - -1 - 0 + -1 - 0 - -2:-1 + -1 - These microstates are associated with the term 1 D. Slide 36 Microstate Table for p 2 M S MLML +10 +2:1+1-1+1- +1:1+0+1+0+ 1+0-1-0+1+0-1-0+ 1-0-1-0- 0:-1 + 1 + -1 + 1 - 0 + 0 - -1 - 1 + -1 - 1 - -1:-1 + 0 + -1 + 0 - -1 - 0 + -1 - 0 - -2:-1 + -1 - Eliminate microstates with M L =+1 -1, with M s =+1 -1 Slide 37 Microstate Table for p 2 M S MLML +10 +2:1+1-1+1- +1:1+0+1+0+ 1+0-1-0+1+0-1-0+ 1-0-1-0- 0:-1 + 1 + -1 + 1 - 0 + 0 - -1 - 1 + -1 - 1 - -1:-1 + 0 + -1 + 0 - -1 - 0 + -1 - 0 - -2:-1 + -1 - Eliminate microstates with M L =+1 -1, with M s =+1 -1 Slide 38 Microstate Table for p 2 M S MLML +10 +2:1+1-1+1- +1:1+0+1+0+ 1+0-1-0+1+0-1-0+ 1-0-1-0- 0:-1 + 1 + -1 + 1 - 0 + 0 - -1 - 1 + -1 - 1 - -1:-1 + 0 + -1 + 0 - -1 - 0 + -1 - 0 - -2:-1 + -1 - These microstates are associated with the term 3 P. Slide 39 Microstate Table for p 2 M S MLML +10 +2:1+1-1+1- +1:1+0+1+0+ 1+0-1-0+1+0-1-0+ 1-0-1-0- 0:-1 + 1 + -1 + 1 - 0 + 0 - -1 - 1 + -1 - 1 - -1:-1 + 0 + -1 + 0 - -1 - 0 + -1 - 0 - -2:-1 + -1 - One microstate remains. It is associated with the term 1 S. Slide 40 Term States for p 2 The term states for a p 2 electron configuration are 1 S, 3 P, and 1 D. The term symbol with the greatest multiplicity and highest value of M L will be the ground state. 3 P is the ground state term for carbon. Slide 41 Determining the Relative Energy of Term States 1. For a given electron configuration, the term with the greatest multiplicity lies lowest in energy. (This is consistent with Hunds rule.) 2. For a term of a given multiplicity, the greater the value of L, the lower the energy. Slide 42 Determining the Relative Energy of Term States For a p 2 configuration, the term states are 3 P, 1 D and 1 S. The terms for the free atom should have the following relative energies: 3 P< 1 D < 1 S Slide 43 Determining the Relative Energy of Term States The rules for predicting the ground state always work, but they may fail in predicting the order of energies for excited states. Slide 44 Energy States for a d 2 Configuration A microstate table for a d 2 electron configuration will contain 45 microstates (M L = 4 -4, and M S =1, 0 or -1) associated with the following terms: 1 S, 1 D, 1 G, 3 P, and 3 F Slide 45 Energy States for a d 2 Configuration Problem: Determine the ground state of a free atom with a d 2 electron configuration, and place the terms in order of increasing energy. Problem: Determine the ground state of a free atom with a d 2 electron configuration, and place the terms in order of increasing energy. 1 S, 1 D, 1 G, 3 P, and 3 F Slide 46 Determining the Ground State Term We only need to know the ground state term to interpret the spectra of transition metal complexes. This can be obtained without constructing a microstate table. The ground state will a) have the maximum multiplicity b) have the maximum value of M L for the configuration obtained in part (a). Slide 47 The Splitting of Terms In an octahedral field, the free ion terms will split due to their differing spatial orientations. Term # of StatesTerms in O h Field S1A 1g S1A 1g P3T 1g P3T 1g D5T 2g + E g D5T 2g + E g F7T 1g + T 2g + A 2g F7T 1g + T 2g + A 2g G9A 1g + E g +T 1g +T 2g G9A 1g + E g +T 1g +T 2g Slide 48 Correlation Diagrams The diagrams show the free ion terms on the left, and the effect of a strong octahedral field on the right. This diagram is for a d 2 ion. Slide 49 Correlation Diagrams The terms converge on the right side of the diagram in three clusters. Each of these represents the possible electron configurations for a d 2 ion in a strong octahedral field. Slide 50 Correlation Diagrams At the left, the free- ion terms (due to L-S coupling) predominate. At the right, the electron configurations predominate. The diagram shows the intermediate cases in which both factors need to be considered. Slide 51 Correlation Diagrams This correlation diagram shows the ground state and spin- allowed transitions in bold lines. Slide 52 Selection Rules There are several selection rules that govern the intensities of the absorption bands seen in transition metal complexes. 1. Transitions between states of the same parity (g g or u u) are forbidden. This is the Laporte Rule. This rule would forbid electronic transitions between d orbitals, since all d orbitals are gerade. Slide 53 Selection Rules 1. Transitions between states of the same parity (g g or u u) are forbidden. This is the Laporte Rule. This rule is relaxed due to vibrations of the complex that cause a loss of the center of symmetry. As a result, molar absorbitivities of 10-50 L/mol-cm are observed. This rule is relaxed due to vibrations of the complex that cause a loss of the center of symmetry. As a result, molar absorbitivities of 10-50 L/mol-cm are observed. Slide 54 Selection Rules 2. Transitions between states of different multiplicities are forbidden. This is called the spin selection rule. This rule can be relaxed very slightly for the first row transition metals by spin-orbit coupling. Typical molar absorbitivities are less than 1 L/mol-cm, with very pale color observed. This rule can be relaxed very slightly for the first row transition metals by spin-orbit coupling. Typical molar absorbitivities are less than 1 L/mol-cm, with very pale color observed. Slide 55 Spin-Forbidden Transitions Mn 2+ (and Fe +3 ) usually have a high spin d 5 configuration. As a result, all electronic transitions are spin-forbidden. Mn(II) compounds are sometimes a very pale pink, and Fe(III) compounds a very pale green due to relaxing of the selection rule. Slide 56 Correlation Diagrams This diagram shows the possible transitions that do not violate the spin selection rule. A d 2 complex should have 3 possible transitions. Slide 57 Correlation Diagrams A non-crossing rule is observed in correlation diagrams. A non-crossing rule is observed in correlation diagrams. Terms or energy states of the same symmetry interact so that their energies never cross. Slide 58 Tanabe-Sugano Diagrams In order to accurately interpret the electronic spectra of transition metal complexes, a series of diagrams have been created. These diagrams are used to assign transitions (initial energy state and final energy state) to peaks observed in the spectra, and to calculate the value of o. Slide 59 Tanabe-Sugano Diagrams Tanabe-Sugano diagrams have the lowest energy state (the ground state) plotted along the horizontal axis. The energy of excited states can then be readily compared to the ground state. Slide 60 Tanabe-Sugano Diagrams Slide 61 Many tables eliminate, or use dotted lines for excited states that are spin- forbidden. Slide 62 Tanabe-Sugano Diagrams Also, since the d orbitals are all gerade, the g subscript is usually left off. Slide 63 Tanabe-Sugano Diagrams The vertical axis is E/B, where B is a Racah parameter. B is a measure of repulsion between terms of the same multiplicity. Slide 64 Tanabe-Sugano Diagrams The horizontal axis is o /B. In order to determine o, we need to determine the value of B, or mathematically eliminate it. Slide 65 Tanabe-Sugano Diagrams The diagrams for configurations d 4 -d 6 have a vertical break in the middle of the diagram. This is due to the shift from a high spin (weak field) complex to a low spin (high field) complex. Slide 66 Symmetry Labels and Electron Configurations At the far right side of the diagrams, at an infinitely strong octahedral field, the symmetry labels correspond to the electron configuration of the complex. T designates a triply degenerate asym- metrically occupied state. or Slide 67 Symmetry Labels and Electron Configurations An E label designates a doubly degenerate asymmetrically occupied state. An A or B label designates a non-degenerate state. or Slide 68 Interpretation of Spectra d 1 & d 9 There is only 1 spin-allowed transition, with the energy absorbed equal to the value of o. Slide 69 Interpretation of Spectra d 1 & d 9 The d 1 excited state exhibits a strong Jahn-Teller distortion, as seen in the UV/Vis spectrum. Slide 70 Interpretation of Spectra d 1 & d 9 The d 9 ground state exhibits a strong Jahn-Teller distortion. The result is a side peak in the UV/Vis spectrum. Slide 71 Interpretation of Spectra d 4 & d 6 (high spin) The Tanabe-Sugano diagrams show only one spin-allowed transition for either complex. The frequency of the absorption equals o. Slide 72 Interpretation of Spectra d 4 & d 6 (high spin) The single peak shows distortion from octahedral geometry due to the Jahn-Teller effect. Slide 73 Interpretation of Spectra d 4 & d 6 (high spin) The ground state of Cr 2+ (d 4 ) and the excited state of Fe 2+ (d 6 ) should exhibit strong Jahn- Teller distortions. Slide 74 Interpretation of Spectra d 3 & d 8 For a d 3 ground state, the first transition, from 4 A 2g (F) to 4 T 2g (F) corresponds to o. o LFSE =.6 o -.8 o = -.2 o LFSE = -1.2 o Slide 75 Interpretation of Spectra d 3 & d 8 The frequency of the lowest energy transition provides the value of o. The third peak is obscured by a very intense charge transfer band. o Slide 76 Interpretation of Spectra d 3 & d 8 The curvature of the 4 T 1 states is a result of the non-crossing rule. Since the terms wont cross, they mix, and curve away from each other. Slide 77 Interpretation of Spectra d 3 & d 8 The Tanabe-Sugano diagram for d 8 is the same as that for d 3, except the multiplicity is different. Three peaks are expected, with the lowest energy absorption equal to o. Slide 78 Interpretation of Spectra d 3 & d 8 The peaks are jagged due to distortions from octahedral geometry. oooo Slide 79 Interpretation of Spectra d 3 & d 8 The Tanabe-Sugano diagram can be used to assign transitions to each absorption. Slide 80 Interpretation of Spectra d 3 & d 8 The first peak is due to the 4 A 2g (F) 4T 2g (F) transition. 11 11 Slide 81 Interpretation of Spectra d 3 & d 8 The second peak is due to the 4 A 2g (F) 4 T 1g (F) transition. 11 22 22 Slide 82 Interpretation of Spectra d 3 & d 8 The third peak is due to the 4 A 2g (F) 4 T 1g (P) transition. 11 33 22 33 Slide 83 Interpretation of Spectra d 3 & d 8 The first peak [ 4 A 2g (F) 4T 2g (F)] has an energy equal to o. 11 11 Slide 84 Interpretation of Spectra d 2 & d 7 The interpretation of spectra from d 2 or d 7 (high spin) complexes is the most complicated due to curvature in the ground state of the Tanabe-Sugano diagrams. Since the ground state and an excited state have the same symmetry ( 4 T 1g ), they mix and curve away from each other. Slide 85 Interpretation of Spectra d 2 & d 7 Slide 86 The repulsion of like terms means that the energy of the ground state fluctuates with field strength. oo oo Slide 87 Interpretation of Spectra d 2 & d 7 If 1 and 3 are both seen in the spectrum, the difference between the two absorptions = o. oo oo 33 11 Slide 88 Interpretation of Spectra d 2 & d 7 The transition corresponding to 3 is often quite weak, as it involves the simultaneous excitation of two electrons, and is therefore less probable. oo oo 33 11 Slide 89 Interpretation of Spectra d 2 & d 7 It is not easy to assign the absorptions due to several complications: 1. Lines cross in the Tanabe-Sugano diagram, making assignment difficult 2. The second and third absorptions may overlap, making it difficult to determine the actual position of the peaks Slide 90 Interpretation of Spectra d 2 & d 7 An additional problem arises from the crossing of lines. Assignment of the absorptions is not obvious. Slide 91 Interpretation of Spectra d 5 (high spin) There are no spin allowed transitions for d 5 high spin configurations. Extinction coefficients are very low, though the selection rule is relaxed by spin-orbit coupling. Slide 92 Interpretation of Spectra d 5 (high spin) Mn 2+ compounds are white to pale pink in color. Slide 93 Charge Transfer Spectra Many transition metal complexes exhibit strong charge-transfer absorptions in the UV or visible range. These are much more intense than d d transitions, with extinction coefficients 50,000 L/mol-cm (as compared to 20 L/mol-cm for d d transitions). Slide 94 Charge Transfer Spectra Examples of these intense absorptions can be seen in the permanganate ion, MnO 4 -. They result from electron transfer between the metal and the ligands. Slide 95 In charge transfer absorptions, electrons from molecular orbitals that reside primarily on the ligands are promoted to molecular orbitals that lie primarily on the metal. This is known as a charge transfer to metal (CTTM) or ligand to metal charge transfer (LMCT). The metal is reduced as a result of the transfer. Charge Transfer Spectra Slide 96 _ _ e g d _ _ _ _ _ _ _ _ t 2g _ _ _ _ _ _ _ _ _ _ _ _ free metal octahedral complex ligand orbitals Ligand to metal charge transfer Slide 97 Ligand to Metal Charge Transfer LMCT occurs in the permangate ion, MnO 4 1-. Electrons from the filled p orbitals on the oxygens are promoted to empty orbitals on the manganese. The result is the intense purple color of the complex. Slide 98 Ligand to Metal Charge Transfer LMCT typically occurs in complexes with the metal in a fairly high oxidation state. It is the cause of the intense color of complexes in which the metal, at least formally, has no d electrons (CrO 4 2-, MnO 4 1- ). Slide 99 Metal to Ligand Charge Transfer MLCT typically occurs in complexes with acceptor ligands. The empty * orbitals on the ligands accept electrons from the metal upon absorption of light. The result is oxidation of the metal. Slide 100 Charge Transfer Spectra _ _ _ _ _ _ _ _ _ _ _ _ * _ _ e g d _ _ _ _ _ _ _ _ t 2g free metal octahedral complex ligand * orbitals Metal to ligand charge transfer Slide 101 Metal to Ligand Charge Transfer Examples of LMCT include iron(III) with acceptor ligands such as CN - or SCN 1-. The complex absorbs light and oxidizes the iron to a +4 oxidation state. Slide 102 Metal to Ligand Charge Transfer The metal may be in a low oxidation state (0) with carbon monoxide as the ligand. Many of these complexes are brightly colored, and some appear to exhibit both types of electron transfer.