Advanced Inorganic Chemistry ADVANCED INORGANIC CHEMISTRY Diagrams Orgel a Splitting of the Weak Field Dn Ground State Terms in an Octahedral Ligand Field DVANCED
Advanced Inorganic Chemistry ADVANCED INORGANIC CHEMISTRY Diagrams Orgel A Splitting of the weak field dn ground state terms in an octahedral ligand field DVANCED
Correlation of spectroscopic terms for dn
configuration in O complexes I
h NORGANIC
om c um er Terms in O At i N b h Term of states Symmetry
S 1 A1g P 3 T1g + C D 5 T2g Eg HEMISTRY F 7 T1g + T2g + A2g
Ground state determined by inspection of degeneracy of terms for given dn ADVANCED INORGANIC CHEMISTRY A
Orgel Diagrams DVANCED
3 T1g(P) 4 T1g(P) I 3P 3A 4P NORGANIC 2 2g 5 Eg 4 T2g T1g 3 T2g 2 3 4 4 5 D F F T2g D C
2 3 5 HEMISTRY T2g T1g Eg
1 3 ∆o 4 ∆o 2 ∆ d 4 d ∆ d d o A2g o
2 2 T2g E 3 3 g T1g T2g
3 3 T1g A2g
3 3 T1g T1g(P)
Ti3+ V2+ Cr3+ Mn3+ A DVANCED
The d-d bands of the d2 ion [V(H O) ]3+ 2 6 I NORGANIC C HEMISTRY The Tanabe-Sugano diagram ADVANCED INORGANIC CHEMISTRY A DVANCED Correlation diagrams between energies of atomic and
molecular terms can drawn as so-called I NORGANIC Tanabe-Sugano diagrams for each electron
configuration of free ions. C HEMISTRY Y. Tanabe, s. Sugano; J. Phys. Soc, Jap., 9, 753 (1954)
Energy values in a Tanabe-Sugano diagram are only given relative to the ground state (x-axis). A DVANCED The simple correlation diagram had multiples of B (the Racah parameter) on the energy axis to denote the relative energies of the atomic terms. In the I Tanabe-Sugano diagrams, the energy axis has units of E/B. The x-axis has NORGANIC units of ”o/B. Each Tanabe-Sugano diagram is given for only one specific B/C ratio (the best value). For example, the Tanabe-Sugano diagram for d3 C complexes is given for C=4.5 B. HEMISTRY non-crossing rule: Terms of the same symmetry cannot cross and will ‘repel’ each other. A DVANCED Racah Inter-electronic Repulsion Parameters (B, C)
1 S I NORGANIC
1G
3P E(3P) = A+7B C HEMISTRY
1D E(1D) = A - 3B + 2C
3F E(3F) = A - 8B
d2
3 F 3P = 15B 3F 1D = 5B + 2C Evidence for covalent bonding in metal-ligand interactions A DVANCED The Nephelauxetic Effect (“cloud expansion”)
Reduction in electron-electron repulsion upon complex formation I NORGANIC
Racah Parameter, B: electron-elctronic repulsion parameter
B is the inter- electronic repulsion in the gaseous Mn+ ion.
o C
n+ HEMISTRY B is the inter- electronic repulsion in the complexed MLx ion.
The smaller values for B in the complex compared to free gaseous ion is taken as evidence of smaller inter-electronic repulsion in the complex due to a larger “molecular orbital” on account of overlap of ligand and metal orbital, i.e. evidence of covalency (cloud expansion”).
Nephelauxetic Ratio, β = B
Bo NAEPHELAUXETICDVANCED INORGANICEFFECT CHEMISTRY Ionic Large β + Large β 2 Ionic Small overlap ~ Mn~ + 2 O < F 2 < Pt < H + 4 3 < V + 2 < Ni < en < NH< < en + - 3 2 4 O 2 < Cr + 3 < Fe + Series 3 < NCS < C ~Ir Cl + 3 MetalSeries Ligand < Rh + 3
< Co I < Br < CN < Br< CN < I
Small β Small Covalent +
4 Small β Small
Covalent
Pt Large overlap
Nephelauxetic Nephelauxetic A DVANCED Empirical Racah parameters, h, k β = 1– [h(ligand) x k(metal)] I NORGANIC 3+ Cr(NH3)6 β = 1 –hk β = 1 –(1.4)(0.21)
= 0.706 C HEMISTRY
3- Cr(CN)6 β = 1 –hk β = 1 –(2.0)(0.21) = 0.580
Bo - B = hligands x kmetal ion
Bo A DVANCED I NORGANIC
Typical ” o and »max values for octahedral (ML6) d-block metal complexes ______-1 -1 Complex ” o cm ~ »max (nm) Complex ” o cm »max (nm) ______[Ti(H O) ]3+ 20,300 493 [Fe(H O) ]2+ 9,400 1064 2 6 2 6 C 3+ 3+ [V(H2O)6] 20,300 493 [Fe(H2O)6] 13,700 730 HEMISTRY 2+ 3- [V(H2O)6] 12,400 806 [Fe(CN)6] 35,000 286 3- 4- [CrF6] 15,000 667 [Fe(CN)6] 33,800 296 3+ 3- [Co(H2O)6] , l.s. 20,700 483 [Fe(C2O4)3] 14,100 709 2+ 3- [Cr(H2O)6] 14,100 709 [Co(CN)6] l.s. 34,800 287 3+ 3+ [Cr(H2O)6] 17,400 575 [Co(NH3)6] l.s. 22,900 437 3+ 2+ [Cr(NH3)6] 21,600 463 [Ni(H2O)6] 8,500 1176 3+ 2+ [Cr(en)3] 21,900 457 [Ni(NH3)6] 10,800 926 3- 2+ [Cr(CN)6] 26,600 376 [Ni(en)3] 11,500 870 ______Example of the use of Tanabe-Sugano Diagrams
For the use of Tanabe-Sugano diagrams we will be using Tables 17.1 and 17.2 (see the resources for Test 3). 10 Dqo = f x g. Let us consider the complex 2+ Co(NH3)6 . The oxidation state of the cobalt is +2, so the the metal isconsidered 7 a d . To figure out 10 Dqo (also known as delta octahedral), from Table 17.1 we multiply f from the ligand column by g from the metal ion column. This gives -1 1.25 x 9000 = 11,250 cm which is the size of 10 Dqo. The next step is to determine the reduced Racah parameter for the complex. The reduced Racah parameter is called beta.
. beta=(Bcomplex)/(B free ion) = 1 - h k
The quantities h and k can also be found in Table 17.1 for many ligands and metal centers. For the current example
. beta=(Bcomplex)/(B free ion) = 1 - h k = 1 - (1.4)(0.09) = 0.874 From this it easy to rearrange things to get Bcomplex and use the value of Bfree ion for Co2+ from Table 17.2 -1 -1 (beta)(Bfree ion) = Bcomplex = (0.874)(971 cm ) = 849 cm
To use a Tanabe-Sugano diagram, you mustdivide the value of 10 Dqo by Bcomplex. -1 -1> (10 Dqo)/Bcomplex = 11,250 cm /849 cm = 13.25 This is the value that will be read on the x-axis of the Tanabe-Sugano diagram. Using the correct Tanabe-Sugano diagram (d7 in this case) is critical. Looking at the Tanabe- Sugano diagram quickly reveals that the term symbol for a free Co2+ ion is 4F. Also looking at the Tanabe Sugano diagram, we notice that the value of 13.25 is to the left of the point of inflection. This means that the 2+ complex Co(NH3)6 is a high spin complex (if the value was to the left of the inflection point, it would be a low spin complex). Spin allowed ttransitions from the ground state will therefore all be from quadruplet to quadruplet. The allowed transitions are:
4 4 4 4 4 4 T1g -----> T2g T1g -----> T1g T1g -----> A2g Reading straight up from 13.25 on the x-axis until it crosses the line corresponding to the other quadruplet states will give us E/Bcomplex on the y- axis.
4 4 T1g -----> T2g E/Bcomplex =12.4
4 4 T1g -----> T1g E/Bcomplex = 25.6
4 4 T1g -----> A2g E/Bcomplex = 25.6
To get the energy of the transitions in cm-1, each of these must be multiplied by Bcomplex
4 4 -1 -1 T1g -----> T2g E/Bcomplex =12.4 ; 12.4 x 849 cm = 10,528 cm
4 4 -1 -1 T1g -----> T1g E/Bcomplex = 25.6; 25.6 x 849 cm = 21,734 cm
4 4 -1 -1 T1g -----> A2g E/Bcomplex = 25.6, 25.6 x 849 cm = 21,734 cm The last step is to convert the wave number (reciprocal centimeters, cm-1) to namometers
4 4 -1 -1 -5 -5 T1g -----> T2g 10,528 cm ; 1/(10,528 cm ) = 9.50 x 10 cm; (9.50 x 10 cm)(107 nm/cm) = 950 nm
4 4 -1 -1 -5 -5 T1g -----> T1g 21,734 cm ; 1/(21,734 cm ) = 4.60 x 10 cm; (4.60 x 10 cm)(107 nm/cm) = 460 nm
4 4 -1 -1 -5 -5 T1g -----> A2g 21,734 cm ; 1/(21,734 cm ) = 4.60 x 10 cm; (4.60 x 10 cm)(107 nm/cm) = 460 nm
All of these transitions are d-d transitions. The first transition at 950 nm is in the near IR just above the red portion of the visible spectrum. The two transitions at 460 nm correspond to an absorbance of blue (very slightly shaded to green) light in the visible spectrum. A
Use of Tanabe-Sugano Diagrams for Interpretation DVANCED of UV/Visible Absorption Spectra of Complexes I NORGANIC C d3 HEMISTRY ADVANCED INORGANIC CHEMISTRY ADVANCED INORGANIC CHEMISTRY ADVANCED INORGANIC CHEMISTRY ADVANCED INORGANIC CHEMISTRY A DVANCED I NORGANIC C HEMISTRY
26.5 A DVANCED I NORGANIC C HEMISTRY
26.5
A DVANCED I NORGANIC C HEMISTRY Charge-Transfer Spectra Charge Transfer Transitions A DVANCED
• As well as ‘d-d’ transitions, the electronic spectra of transition metal I complexes may 3 others types of electronic transition: NORGANIC
Ligand to metal charge transfer (LMCT) Metal to ligand charge transfer (MLCT) C
Intervalence transitions (IVT) HEMISTRY
• All complexes show LMCT transitions, some show MLCT, a few show IVT Ligand to Metal Charge Transfer A DVANCED
• These involve excitation of an electron from a ligand-based orbital into a d- orbital I NORGANIC O O visible light
M O M O C O O HEMISTRY O O
• This is always possible but LMCT transitions are usually in the ultraviolet • They occur in the visible or near-ultraviolet if
metal is easily reduced (for example metal in high oxidation state) ligand is easily oxidized
If they occur in the visible or near-ultraviolet, they are much more intense than ‘d-d’ bands and the latter will not be seen Ligand to Metal Charge Transfer A DVANCED
•They occur in the visible or near-ultraviolet if I NORGANIC metal is easily reduced (for example metal in high oxidation state) C
3- 2- - HEMISTRY TiO2 VO4 CrO4 MnO4 Ti4+ V5+ Cr6+ Mn7+
d0 in far UV ~39500 cm-1 ~22200 cm-1 ~19000 cm-1 white white yellow purple
more easily reduced Metal to Ligand Charge Transfer A DVANCED
• They occur in the visible or near-ultraviolet if I NORGANIC metal is easily oxidized and ligand has low lying empty orbitals C
N HEMISTRY
N N N M N N N N N N
M = Fe2+, Ru2+, Os2+
2+ 6 • Sunlight excites electron from M (t2g) into empty ligand π* orbital
method of capturing and storing solar energy Intervalence Transitions A DVANCED
• Complexes containing metals in two oxidation states can be coloured due to
excitation of an electron from one metal to another I NORGANIC C HEMISTRY
“Prussian blue” contains Fe2+ and Fe3+
• Colour arises from excitation of an electron from Fe2+ to Fe3+ 3) Charge transfer bands A DVANCED • Similar to d-d transitions, charge-transfer (CT) transitions also involve the metal d-orbitals. CT bands are observed if the energies of empty and filled ligand- and metal-centered orbitals are similar.
• The direction of the electron transfer is determined by the relative energy levels of these orbitals: I NORGANIC - 2- i) ligand-to-metal charge transfer (LMCT) like in MnO4 , CrO4 etc. or ii) metal-to ligand charge 2+ transfer (MLCT) like in [Fe(bpy)3] . The simplified diagrams below are the modified versions of what we had in Lecture 26. Bold arrows show possible CT transitions. VII 2-
Mn 4 O C
II HEMISTRY 0 3t π d 2 Fe 3 bpy -GO's d6 bpy = 2a1 N N (p) t2 3t2g t2g(π*) (s) a1 2t2 29500 eg 44400 e (d) e e g t2 t1 t1(n) ∆ 17700 ο t 30300 t 2g(π) 2(σ) 2t a1 2g t 1t2 2g
1t2g 1a1 A
Charge transfer spectra DVANCED
Metal character I NORGANIC
LMCT
Ligand character C HEMISTRY
Ligand character
MLCT Metal character
Much more intense bands Charge-Transfer Spectra A DVANCED 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 I NORGANIC These absorptions may be much more intense than d-d transitions (which for octahedral complexes usually have µ values of 20 L mol-1 cm-1 or less): molar absorptivities of 50,000 L mole-1 cm-1 or greater are not C uncommon for these bands HEMISTRY
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) Charge-Transfer Spectra A DVANCED For example, consider an octahedral d6 complex with Ã-donor ligands I NORGANIC C HEMISTRY
The possibility exists that electrons can be excited, not only from the t2g level to the eg but also from the Ãorbitals originating from the ligands to the eg
The latter excitation results in a charge-transfer transition; it may be designated as charge transfer to metal (CTTM) or ligand to metal charge transfer (LMCT)
This type of transition results in formal reduction of the metal. A CTTM excitation involving a cobalt (III) complex, for example, would exhibit an excited state having cobalt (II) Charge-Transfer Spectra A DVANCED 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 À-acceptor ligands I NORGANIC In these cases, empty À* orbitals on the ligands become the acceptor orbitals on absorption of light C HEMISTRY
CTTL results in oxidation of the metal; a CTTL excitation of an iron(III) complex would give an iron(IV) excited state. CTTL most commonly occurs with ligands having empty À* orbitals, such as CO, CN-, SCN-, - bipyridine, and dithiocarbamate (S2CNR2 ) Charge-Transfer Spectra A DVANCED
In complexes such as Cr(CO)6 which have both Ã-donor and À-acceptor orbitals, both types of charge transfer are possible I It is not always easy to determine the type of charge transfer in a given NORGANIC coordination compound
Many ligands give highly colored complexes that have a series of C overlapping absorption bands in the ultraviolet part of the spectrum as HEMISTRY well as the visible
In such cases, the d-d transitions may be completely overwhelmed and essentially impossible to observe Charge-Transfer Spectra A DVANCED Finally, the ligand itself may have a chromophore and still another type of absorption band an intraligand band, may be observed I These bands may sometimes be identified by comparing the spectra of NORGANIC complexes with the spectra of free ligands
However, coordination of a ligand to a metal may significantly alter the C energies of the ligand orbitals, and such comparisons may be difficult, HEMISTRY especially if charge-transfer bands overlap the intraligand bands
Also, it should be noted that not all ligands exist in the free state: some ligands owe their existence to the ability of metal atoms to stabilize molecules that are otherwise highly unstable 3)CN- Example: A a) HOMO = σ-bonding electron DVANCED pair donor to metal ion I b) LUMO = π-bonding electron NORGANIC pair acceptor from metal ion
c) The π* orbitals are higher in C HEMISTRY energy than the metal t2g orbitals having the correct symmetry to overlap with
d) The energy match is good enough for overlap to occur
e) π-bonding results i. 3 new bonding t2g MO’s receive A the d-electrons DVANCED
ii. 3 new antibonding t2g* MO’s formed I NORGANIC iii. The eg* MO’s from the σ-bond MO treatment are nonbonding C iv.Ligands like this increase ∆o by lowering HEMISTRY the energy of t2g MO’s favoring low spin complexes v. CN- is a strong field ligand vi. Metal to Ligand (M L) or π- back bonding to π-acceptor ligand vii. Transfer of electron density away from M+ stabilizes the complex over σ-bonding only 4)F- example
a) Filled p-orbitals are the only orbitals A capable of π-interactions DVANCED i) 1 lone pair used in σ-bonding ii) Other lone pairs π-bond I NORGANIC b) The filled p-orbitals are lower in energy
than the metal t2g set C
c) Bonding Interaction HEMISTRY i. 3 new bonding MO’s filled by Fluorine electrons
ii. 3 new antibonding MO’s form t2g* set contain d-electrons
iii. ∆o is decreased (weak field)
d) Ligand to metal (L M) π-bonding
i. Weak field, π-donors: F, Cl, H2O ii. Favors high spin complexes ADVANCED INORGANIC CHEMISTRY A DVANCED I NORGANIC C HEMISTRY END ADVANCED INORGANIC CHEMISTRY ADVANCED INORGANIC CHEMISTRY ADVANCED INORGANIC CHEMISTRY ADVANCED INORGANIC CHEMISTRY ADVANCED INORGANIC CHEMISTRY ADVANCED INORGANIC CHEMISTRY ADVANCED INORGANIC CHEMISTRY ADVANCED INORGANIC CHEMISTRY ADVANCED INORGANIC CHEMISTRY