1
Perhaps the most striking aspect of many coordination compounds of transition metals is that they have vivid colors. The UV-vis spectra of coordination compounds of transition metals involve transitions between the d orbitals of the metals (LF). 2
For many coordination compounds, the electronic absorption spectrum provides a convenient method for determining the magnitude of the effect of ligands on the d orbitals of the metal. Although in principle we can study this effect for any coordination geometry, we will focus on the most common geometry, octahedral, & will examine how the absorption spectrum can be used to determine the magnitude of the octahedral ligand field parameter, Δ o, for a variety of complexes. 3
11-1 Absorption of light (quantized & bands) S 1 ISC = intersystem crossing T 1 hν Absorption (excitation) Fluorescence hν'/k F Phosphorescence hν''/k P S 0 Non-radiative decay Radiative decay 4
λ absorbed versus color observed 400nm Violet absorbed, Green-yellow observed (λ = 560nm) 450nm Blue absorbed, Yellow observed (λ = 600nm) 490nm Blue-green absorbed, Red observed (λ = 620nm) 570nm Yellow-green absorbed, Violet observed (λ = 410nm) 580nm Yellow absorbed, Dark blue observed (λ = 430nm) 600nm Orange absorbed, Blue observed (λ = 450nm) 650nm Red absorbed, Green observed (λ = 520nm) 5
Because many coordination compounds contain two or more absorption bands of different energies & intensities, & thus the net color observed is the color predominating after the various absorptions are removed from white light. 6
It absorbs the light at 600-1000 nm (800 nm), & the color observed, blue, is the average complementary color of the light absorbed. 7
From Figure 11-1, this spectrum is a consequence of transitions between states of different energies & can provide valuable information about those states &, in turn, about the structures & bonding of the molecule or ion. 8
Cu 2+ 9
11-1-1 Beer-Lambert absorption law 10
The molar absorptivity (ε) is a characteristic of the species that is absorbing the light & is highly dependent on wavelength. 11
12
11-2 Quantum numbers of multielectron atoms Quantized : a transition from lower to higher energy states (absorption / excitation). Bands : observed absorption in Figure 11-1, with the energy of each band corresponding to the difference in energy between the initial & final states To gain insight into these states & the energy transitions between them (absorption / excitation), we first need to consider how electrons in atoms can interact with each other to gain different energy states. 13
Π c : a result of repulsion between electrons Π e : electrons tend to occupy separate orbitals & parallel spins C : 1s 2 2s 2 2p 2 (independently??) n = 2, l = 1 (quantum numbers defining 2p orbitals) m l = +1, 0, or -1 (three possible values) m s = +1/2 or -1/2 (two possible values) m l : orbital angular momentum m s : spin angular momentum 15 possible arrangements 14
Different arrangements have different energies due to the interactions of electrons. 15 possible arrangements 15
The 2p electrons are not independent of each other, however; the orbital angular momenta (m l ) & spin angular momenta (m s ) of the 2p electrons can interact in a manner called Russell-Saunders or LS coupling. Russell-Saunders or LS coupling : for lighter atoms Spin-orbit coupling : for heavier atoms 16
The interactions produce atomic states called microstates that can be described by new quantum numbers : M L = Σm l M S = Σm s (Total orbital angular momentum) (Total spin angular momentum) Russell-Saunders (LS) coupling 17
One possible set of values for the two electrons in the p 2 configuration would be First electron : m l = +1 & m s = +1/2 (+) Second electron : m l = 0 & m s = -1/2 (-) 1 + 0 - (m m l s) (each set of possible quantum numbers is called a microstate) 18
To tabulate the possible microstates: (1) To be sure that no two electrons in the same microstate have identical quantum numbers (the Pauli exclusion principle applies). (2) To count only the unique microstates. p 2 : 1 + 0 - & 0-1 + ; 0 + 0 - & 0-0 + are duplicates. sp : 0 + 0 - & 0-0 + (not duplicates). Electrons are indistinguishable. 19
p _ How to tabulate the possible microstates? ( m s ) +1 0-1 +1/2 (+), - 1/2 (-) ( m l ) C 6 5 2 6 2 = = 15 No. of microstates 20
Tabulate the Microstates s p 0 +1 0-1 2 C 6 1 1 C 12 microstates How about a d 2 configuration? 45 microstates 21
We have now seen how electronic quantum numbers m l & m s may be combined into atomic quantum numbers M L (Σm l ) & M S (Σm s ), which describe atomic microstates. 22
M L & M S, in turn, give atomic quantum numbers, L, S, & J. These quantum numbers collectively describe the energy & symmetry of an atom or ion & determine the possible transitions between states of different energies. These transitions account for the colors observed for many coordination complexes (d-d transitions/lf). 23
L & S describe collections of microstates, where M L & M S describe the microstates themselves. L & S are the largest possible values of M L & M S. M L is related to L much as m l to l, & the values of M S & m s are similarly related. 24
Atomic states characterized by S & L are often called free-ion terms (sometimes called R-S terms) because they describe individual atoms or ions, free of ligands. Their labels are often called term symbols. 2S+1 L 1 2 3 4 5 25
Free-ion terms are very important in the interpretation of the spectra of coordination compounds. same for 0-0 + Electrons are indistinguishable. 26
M S = -S +S p _ +1 0-1 +1/2 (+), - 1/2 (-) M L = -L +L number of microstates = (2S+1)(2L+1) = 6 27
Hund s rules : 1. The ground state term (term of lowest energy) has the highest spin multiplicity. 2. If two or more terms share the maximum spin multiplicity, the ground term is the one having the highest value of L. The ground state term for p 2 is 3 P, since L = +1, S = +1. +1 0-1 28
p _ +1 0-1 +1/2 (+), - 1/2 (-) L = 2 S = 0 1 D L = 1 S = 1 3 P p _ +1 0-1 +1/2 (+), - 1/2 (-) L = 0 S = 0 1 S 29
Therefore, p 2 configuration has 3 P, 1 D & 1 S terms. L=0, S=0, J=0 L=2, S=0, J=2 L=1, S=1, J=2, 1, 0 Less than half-filled, so 3 P 0 is the ground state term. See next page for the 3rd Hund s rule. 30
Hund s third rule : 3. For subshell (such as p 2 ) that are less than halffilled, the state having the lowest J value has the lowest energy ( 3 P 0 above); for subshells that are more than half-filled, the state having the highest J values has the lowest energy. 31
0 +1 0-1 12 microstates 32
Terms of p 1 d 1 (or d 2 ): 3 F 3 D 3 P 1 F 1 D 1 P m l +1 0-1 +2 +1 0-1 -2 p d Construct a microstate table. Reduce the microstate table to terms. Number of microstates: C 6 C 10 = 1 1 60 33
11-2-1 Spin-orbit coupling In multielectron atoms, the S & L quantum numbers combine into the total angular momentum quantum number J. The quantum number J may have the following values : J = L+S, L+S-1, L+S-2,, L-S ΔJ = ±1 34
Spin-orbit coupling can have significant effects on the electronic spectra of coordination compounds, especially those involving fairly heavy metals (atomic number > 40 (Zr)) L=0, S=0, J=0 L=2, S=0, J=2 L=1, S=1, J=2, 1, 0 35
11-3 Electronic spectra of coordination compounds p 2 d 2 configuration : 3 F, 3 P, 1 G, 1 D, & 1 S. 36
Absorption spectra of coordination compounds in most cases involves the d orbitals of the metal, & it is consequently important to know the free-ion terms for the possible d configurations. However, determining the microstates & free-ion terms for configurations of three or more electrons can be a tedious process (Table 11.5). In the interpretation of spectra of coordination compounds, it is often important to identify the lowest-energy term (highest S & L) / ground state. 37
38
Getting the ground state term (e.g., d 3 configuration in octahedral symmetry) 1. Sketch the energy levels (showing d electrons). 2. Spin multiplicity of lowest-energy state = # of unpaired electrons + 1 (2S + 1). 39
3. Determine the maximum possible value of M L (sum of m l values) for the configuration. This determines the type of free-ion term (i.e., S, P, D). 4. Combine results of steps 2 & 3 to get the ground term. 4 F 40
d 3 : 4 F 41
42
11-3-1 Selection rules The relative intensities of absorption bands are governed by a series of selection rules, on the basis of the symmetry & spin multiplicity of ground & excited electronic states. 43
The model for absorption of electromagnetic radiation which is most often applicable in spectroscopy is the electric dipole model (the intensity of an electronic transition is proportional to the dipole strength, D) Operator : dipole moment vector ( ) dτ : volume element 44
1. Laporte s Rule states that in centrosymmetric environments, transitions can occur only between states of opposite parity (g u) [the dipole moment transforms as x, y, & z, which in any centrosymmetric point group must be odd (u)]. symmetry forbidden : d(g) d(g), p(u) p(u) symmetry allowed : d(g) p(u) - + - + 奇函數 45
2. Spin selection rule (spin conservation) : transitions may occur only between energy states with the same spin multiplicity. spin forbidden : singlet triplet spin allowed : singlet (triplet) singlet (triplet) 46
Some of the most important mechanisms are as follows against the Laporte & spin selection rule: 1. Vibronic coupling : the bonds in transition metal complexes are not rigid but undergo vibrations that may temporarily change the symmetry. Vibronic coupling provides a way to relax the first selection rule. 47
In fact, d-d transitions can still exist, but low with absorptivities (ε), 10-50 Lmol -1 cm -1. Allowed transition : ~ 10 4 Forbidden transition : 1-10 2 48
2. Tetrahedral complexes often absorb more strongly than octahedral complexes of the same metal in the same oxidation state (sp 3 /sd 3 ). The mixing of p-orbital character (of u symmetry) with d-orbital character provides a second way of relaxing the first selection rule. And T d is not centrosymmetric. 49
3. Spin-orbit coupling in some cases provides a mechanism of relaxing the spin selection rule, with the results that transitions may be observed from a ground state of one spin multiplicity to an excited state of different spin multiplicity. Such absorption bands for first-row transition metal complexes are usually very weak, with typical molar absorptivities (ε) less than 1 Lmol -1 cm -1. For complexes of second- & third-row transition metals, spin-orbit coupling can be more important. 50
11-3-2 Correlation diagrams Correlation diagrams make use of two extremes, 1. Free ions (no ligand field) : d 2 3 F, 3 P, 1 G, 1 D, and 1 S, with the 3 F term having the lowest energy. These terms describe the energy levels of a free d 2 ion in the absence of any interactions with ligands. In correlation diagrams, these free-ion terms are shown on the far left (weak field limit). 51
2. Strong ligand field : there are three possible configurations for two electrons in an octahedral field (d 2 ) : t 2g2 e g0 (g.s.), t 2g1 e g1 (e.s. 1 ), & t 2g0 e g 2 (e.s. 2 ). In correlation diagrams, these states are shown on the far right, as the strong field limit. Here, the effect of the ligands is so strong that it completely overrides the effect of the LS coupling. Ground state Excited state 1 Excited state 2 52
Heavy lines : same spin multiplicity Dashed lines : different spin multiplicity LS coupling due to d n configurations Look up Table 11-6 LF 53
Degeneracy 1 3 5 7 9 11 13 54
(1) Method - weak interaction (left)
(2) Direct product : strong interaction (right) 57
t 2g2 (t 2g x t 2g ) : 1 A 1g, 1 E g, 1 T 2g, 3 T 1g t 2g e g (t 2g x e g ) : 1 T 1g, 1 T 2g, 3 T 1g, 3 T 2g e g2 (e g x e g ) : 1 A 1g, 1 E g, 3 A 2g The states derived from these strong-field configurations are the direct products of the orbitals occupied. Assigning the spin multiplicity : J. Chem. Educ. 1972, 49, 336; 1977, 54, 147. 58
59
Notes : 1. The free-ion states (LS coupling) are shown on the far left. 2. The extremely strong-field states are shown on the far right. 3. Both the free-ion & strong-field states can be reduced to irreducible representations. Each freeion i.r. is matched with (correlates with) a strongfield i.r. having the same symmetry (same label). 60
11-3-3 Tanabe-Sugano diagrams T-S diagrams are special correlation diagrams that are particularly useful in the interpretation of electronic spectra of coordination compounds. In T-S diagrams, the lowest-energy state (G.S.) is plotted along the horizontal axis; consequently, the vertical distance above this axis is a measure of the energy of the excited state above the ground state. 61
Interpretation of Tanabe-Sugano diagrams d 2-3 F ( 3 F, 3 P, 1 G, 1 D, & 1 S; weak field limit) 3 T 1g (F) (strong field limit) arises from 3 F. In the T-S diagram, this line is made horizontal; it is labeled 3 T 1g (F) & is shown to arise from the 3 F in the free ion limit (left side of diagram). 62
The T-S diagram also shows excited states. In the d 2 diagram, the excited states of the same spin multiplicity (in blue) as the ground states are the 3 T 2g, 3 T 1g (P), and 3 A 2g. 3 T 1g (F) is the ground state. 63
B : Racah parameter to express interelectronic repulsion 15 B 64
The quantities plotted in a T-S diagram : Horizontal axis : Δ o /B, where Δ o is the octahedral ligand field splitting. B : Racah parameter, a measure of the repulsion between terms of the same multiplicity (e.g. for d 2, the energy difference between 3 F & 3 P is 15B). 65
E/B : where E is the energy (of excited states) above the ground state. As mentioned, one of the most useful characteristics of T-S diagrams is that the ground electronic state is always plotted along the horizontal axis; this makes it easy to determine the values of E/B above the ground state. 66
Example : [V(H 2 O) 6 ] 3+ (d 2 ) Three bands are expected in UV-visible spectrum. 67
ν 1 17,800 cm -1 (562 nm) ν 2 25,700 cm -1 (389 nm) ν 3 ~ 38,000 cm -1 (263 nm) 68
Summary : 1. In [V(H 2 O) 6 ] 3+ (d 2 ), ground state is 3 T 1g (F); under ordinary conditions this is the only electronic state that is appreciably occupied. 2. There are three transitions from 3 T 1g (F) to 3 T 2g, 3 T 1g (P), & 3 A 2g (spin conservation). 69
3. Three allowed transitions are expected. However, two bands are readily observed at 17,800 (562 nm) &25,700 cm -1 (389 nm). 4. A third band, at approximately 38,000 cm -1 (263 nm), is apparently obscured in aqueous solution by charge transfer bands (in the solid state, however, a band attributed to the 3 T 1g 3 A 2g transition is observed at 38,000 cm -1.) 70
Other electron configurations d 4 HS LS 71
At the dividing line, the ground state changes from 5 E g to 3 T 1g. The spin multiplicity changes from 5 to 3 to reflect the change in the number of unpaired electrons (spin crossover). d 4 High spin Low spin Too complicated!! (low-spin d 4 )? 72
[M(H 2 O) 6 ] n+ Why is absorption by [Mn(H 2 O) 6 ] 2+ so weak? First-row metals Figure 11-8 73
To answer this question, it is useful to examine the T-S diagram, in this case for a d 5 configuration. We expect [Mn(H 2 O) 6 ] 2+ to be a high-spin complex, because H 2 O is a rather weak-field ligand. The G.S. for weak-field d 5 is the 6 A 1g. There are no excited states of the same spin multiplicity, & consequently there can be no spin-allowed absorptions. 74
Too complicated!! (low-spin d 5 ) 75
11-3-4 Jahn-Teller distortions and spectra Why Jahn-Teller effect exists?? larger +1/2 δ -1/2 δ smaller +2/3 δ -1/3 δ 76
By virtue of the simple d- electron configuration for d 1 & d 9, to expect each to exhibit one absorption band corresponding to excitation of an electron from t 2g to the e g levels seems too oversimiplied. In fact, the spectra of [Ti(H 2 O) 6 ] 3+ (d 1 ) & [Cu(H 2 O) 6 ] 2+ (d 9 ) show two closely overlapping absorption bands rather than a single band (Jahn-Teller effect ). Figure 11-8 77
In 1937, Jahn & Teller showed that nonlinear molecules having a degenerate electronic state should distort to lower the symmetry of the molecules & to reduce the degeneracy. High-spin d 4, low-spin d 7, & d 9 ions have significant Jahn-Teller effects. When degenerate orbitals are unevenly occupied, Jahn-Teller distortions are likely. 78
Symmetry is lowered to reduce degeneracy. 79
Symmetry labels for configurations 80
E A T 81
2 E g (g.s.) 2 T 2g (e.s.) How about the ground state term of d 1? 82
[Cu(H 2 O) 6 ] 2+ (d 9 ) : two separate bands If the distortion is strong enough, therefore, two separate absorption bands may be observed in the visible region, B 1g B 2g or E g (too low in energy to be observed in the visible spectrum for B 1g A 1g ). See Figure 11-9 83
In summary The 2 D free-ion term is split into 2 E g & 2 T 2g by a field of O h symmetry, & further split on distortion to D 4h symmetry. The labels of the states resulting from the free-ion term (Figure 11-10) are in reverse order to the labels on the orbitals (Figure 11-9). d 1 & d 9 are reverse to each other. d 9 d 1 Figure 11-10 84
d 1 [Ti(H 2 O) 6 ] 3+ two overlapping bands Excited state distortion should be more important, because it is along Ti-O bonds. (e g : dx 2 -y 2, dz 2 ) 85
d 1 [Ti(H 2 O) 6 ] 3+ two overlapping bands 2 E g A 1g 2 D B 1g The order is inverse if the M-OH 2 bond is elongated along z-axis. 2 T 2g E g B 2g 86
Previously, the T-S diagrams, as shown in Figure 11-7, assume O h symmetry, in excited states as well as ground states. The consequence is that the diagrams are useful in predicting the general properties of spectra. In fact, many complexes do have sharply defined bands that fit the T-S diagrams well (e.g., d 2, d 3, & d 4 ). [M(H 2 O) 6 ] n+ J-T distortion not significant. 87
However, distortions from pure octahedral symmetry are rather common, & the consequence can be the splitting of bands or, in some cases of several distortion, situations in which the bands are difficult to interpret. 88
11-3-5 Examples of applications of T-S diagrams : determining Δ o from spectra. Absorption spectra of coordination compounds can be used to determine the magnitude of ligand field splitting (Δ o ). The problem is that absorption spectra often have overlapping bands; to determine the positions of the bands accurately requires an appropriate mathematical technique for reducing overlapping bands into their individual components. d 3 89 89
The ease with which Δ o can be determined depends on the d-electron (d n ) configuration of the metal; in some cases, Δ o can be read easily from a spectrum, but in other cases a more complicated analysis is necessary. 90
d 1, d 4 (high spin), d 6 (high spin), d 9 Each of these cases corresponds to a simple excitation of an electron from a t 2g to an e g orbital, with the spin multiplicity unchanged (initial & final configuration). In each case, there is a single excited state of the same spin multiplicity as the ground state. 91 91
There is a single excited state of the same spin multiplicity as the ground state. HS HS 92 92
Consequently, there is a single spin-allowed absorption, with the energy of the absorbed light equal to Δ o, i.e. [Ti(H 2 O) 6 ] 3+ (d 1 ), [Cr(H 2 O) 6 ] 2+ (high-spin d 4 ), [Fe(H 2 O) 6 ] 2+ (high-spin d 6 ), & [Cu(H 2 O) 6 ] 2+ (d 9 ). Each of these complexes exhibits essentially a single absorption band (Figure 11-8). In some cases, splitting of bands due to Jahn-Teller distortion is observed. 93
e g d 1, d 4 (high-spin), d 6 (high-spin), d 9 Δ o t 2g Ε= Δ o = absorption energy = T 2g E g 94
d 3 & d 8 configurations (see Fig. 11-7) The electron configurations have a ground state (4&3) F term (L = 3). In an octahedral ligand field, an F term splits into three terms, an A 2g, a T 1g, & a T 2g (Table 11-6, A 2g is of lowest energy for d 3 or d 8 ). For these configurations, the difference in energy between the two lowest-energy terms, the A 2g & T 2g, is equal to Δ o. Two excitation may be observed (2Δ o ). 95
d 3, d 8 (lowest-energy transition) [Cr(H 2 O) 6 ] 3+ & [Ni(H 2 O) 6 ] 2+ Δ o ~17,500 cm -1 Δ o ~8,500 cm -1 (571 nm) (1,176 nm) t 2g1 e g 2 t 2g2 e g 1 2Δ o Δ o = 4 A 2g 4 T 2g lowest absorption energy t 2g 3 45 o 96
Shoulders are due to Jahn-Teller distortion of excited states. 97
In the d 3 case, the ground state is a 4 A 2g state. There are three excited quartet states : 4 T 2g, 4 T 1g (from 4 F term), & 4 T 1g (from 4 P term). Note the two states of the same symmetry may mix. The consequence of such mixing is that, as the ligand field is increased, the states appear to repel each other; the lines in the T-S diagram curve away from each other (orbital mixing/ configuration interaction). 98
orbital mixing 99
100
However, this causes no difficulty in obtaining Δ o for a d 3 complex, because the lowest-energy transition ( 4 A 2g 4 T 2g ) is not affected by such curvature (The T-S diagram shows that the energy of the 4 T 2g state varies linearly with the strength of the ligand field). 101
For d 2, the free-ion 3 F term is also split into 3 T 1g + 3 T 2g + 3 A 2g ( 3 T 1g is the ground state). It seems to simply determine the energy of the 3 T 1g (F) 3 T 2g & assign this as the value of Δ o (Figure 11-3). 3 T 1g (F) t 2g2 ; 3 T 2g t 2g1 e g1 ; 3 A 2g e g2. Δ o 2Δ o In principle, the difference between these 3 T 1g (F) & 3 T 2g states should give Δ o. 102
However, the 3 T 1g (F) state can mix with the 3 T 1g (P) state arising from the 3 P free-ion term, causing a slight curvature of both in the T-S diagram. This curvature can lead to some error in using the ground state to obtain values of Δ o. (P) (F) 103
d 2, d 7 (high-spin) Δ o = lowest absorption energy? (Δ o ) 3 T 1g (F) / 3 T 1g (P) mixing 104
3 T 1g (F) t 2g2 ; 3 T 2g t 2g1 e g1 ; 3 A 2g e g2. This means that we can use the difference between 3 T 2g (t 2g1 e g1 ) & 3 A 2g (e g2 ) to calculate Δ o. 105
106
The difficulty with this approach is that two lines cross in the T-S diagram. ν 1 is easily assigned, but ν 2 and ν 3 may be in question. 107
In addition, the second & third absorption bands may overlap, making it difficult to determine the exact positions of the bands (the apparent positions of absorption maxima may be shifted if the bands overlap). In such cases a more complicated analysis, involving a calculation of the Racah parameter, B, may be necessary. 108
Example : 109 109
ν 3 /ν 1 2 110
ν 2 /ν 1 111
ν 2 : 42 = E/B (Δ 0 /B=31, check from Fig 11-13 ) ν 1 : 29 = E/B Or average!! 112
d 5 (high spin) : spin-forbidden transition d 4 -d 7 (low spin) : too difficult (too many excited states of the same spin multiplicity as the ground state, Figure 11-7). 113
11-3-6 6 Tetrahedral complexes (1) Octahedral complexes (even lower absorptivity) (2) Tetrahedral complexes (lower absorptivity) 114
A useful comparison can be drawn between these by using what is called the hole formalism. It can be shown that, in terms of symmetry, the 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. 115
In practical terms, this means that, for tetrahedral geometry, we can use the correlation diagram for the d 10-n configuration in octahedral geometry to describe the d n configuration in tetrahedral geometry. 116
11-3-7 Charge-transfer spectra Example: [Solvent][X 2 ] Charge-transfer absorption, a strong interaction between a donor solvent & a halogen molecule, X 2, leads to the formation of a complex in which an excited state (primarily of X 2 character, LUMO) can accept electrons from a HOMO (primarily of solvent character) on absorption of light of suitable energy : 117
X 2. donor [donor + ][X 2- ] The absorption band, known as a charge-transfer band, can be very intense; it is responsible for the vivid colors of some of the halogens in donor solvents. 118
It is extremely common for coordination compounds also to exhibit strong charge-transfer absorptions, typically in the ultraviolet and/or visible portions of the spectrum. These absorptions may be much more intense than d-d transitions (ε ~ 20 Lmol -1 cm -1 or less); ε ~ 50,000 Lmol -1 cm -1 or greater are not uncommon for CT bands. 119
Such absorption bands 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). 120
The possibility exists that electrons can be excited, not only from the t 2g to e g level (d-d transition) but also from the σ orbitals originating from the ligands to the e g. The latter excitation results in a chargetransfer transition; it may be designed as chargetransfer to metal (CTTM) or ligand to metal charge transfer (LMCT). This type of transition results in formal reduction of the metal. 121
Similarly, it is possible for there to be charge-transfer to ligand (CTTL), also known as metal to ligand charge transfer (MLCT), transitions in coordination compounds having π-acceptor ligands. In these cases, empty π* orbitals on the ligands become the acceptor orbitals on absorption of light. 122
In IrBr 6 2- (d 5 ), two bands appear at ca. 600 & 270 nm. In IrBr 6 3- (d 6 ), only one band appears at ca. 250 nm. LMCT results in reduction on the metal. 123
A common example of tetrahedral geometry is the permanganate ion, MnO 4-, which is intensely purple because of a strong absorption involving charge transfer from orbitals derived primarily from the filled oxygen p orbitals to empty orbitals derived primarily from the manganese(vii). 124
CTTL (MLCT) results in oxidation on the metal. 125
CTTL most commonly occurs with ligands having empty π* orbitals, such as CO, CN -, SCN -, bipyridine, and dithiocarbamate (S 2 CNR 2- ). In Cr(CO) 6, which has both σ-donor and π-acceptor orbitals, both types of charge transfer are possible. d-d, MLCT & LMCT (charge-transfer complexes) & IL (intraligand transition) These make our world colorful & wonderful. 126