Advanced Inorganic Chemistry

Advanced Inorganic Chemistry

Orgel Diagrams

Correlation of spectroscopic terms for d n configuration in O h complexes Atomic Term Splitting of the weak field d n ground state terms in an octahedral ligand field Number of states Terms in O h Symmetry S 1 A 1g P 3 T 1g D 5 T 2g + E g F 7 T 1g + T 2g + A 2g Ground state determined by inspection of degeneracy of terms for given d n

Orgel Diagrams 2 E g 2 D 2 T 2g d 1 o 3 P 3 F 3 T 1g (P) 4 T 1g (P) 3 A 4 2g P 4 T 5 T 2g 1g 3 T 2g 4 F 4 T 5 2g D 3 T 5 1g E g o 4 o A 2g d 2 d3 o d 4 2 T 2g 2 E g 3 T 1g 3 T 2g 3 T 1g 3 A 2g 3 T 1g 3 T 1g (P) Ti 3+ V 2+ Cr 3+ Mn 3+

The d-d bands of the d 2 ion [V(H 2 O) 6 ] 3+

The Tanabe-Sugano diagram

Correlation diagrams between energies of atomic and molecular terms can drawn as so-called Tanabe-Sugano diagrams for each electron configuration of free ions. 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).

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 Tanabe-Sugano diagrams, the energy axis has units of E/B. The x-axis has 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 complexes is given for C=4.5 B. non-crossing rule: Terms of the same symmetry cannot cross and will repel each other.

Racah Inter-electronic Repulsion Parameters (B, C) 1 S 1 G 3 P 1 D 3 F E( 3 P) = A+7B E( 1 D) = A - 3B + 2C E( 3 F) = A - 8B d 2 3 F 3 P 3 F 1 D = 15B = 5B + 2C

Evidence for covalent bonding in metal-ligand interactions The Nephelauxetic Effect ( cloud expansion ) Reduction in electron-electron repulsion upon complex formation Racah Parameter, B: electron-elctronic repulsion parameter B o is the inter- electronic repulsion in the gaseous M n+ ion. B is the inter- electronic repulsion in the complexed ML x n+ 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 B o

Nephelauxetic Ligand Series I < Br < CN < Cl < NCS < C 2 O 2-4 < en < NH 3 < H 2 O < F Small β Covalent Nephelauxetic Metal Series Large β Ionic NEPHELAUXETIC ADVANCED INORGANIC EFFECT CHEMISTRY Pt 4+ < Co 3+ < Rh 3+ ~Ir 3+ < Fe 3+ < Cr 3+ < Ni 2+ < V 4+ < Pt 2+ ~ Mn 2+ Small β Large overlap Covalent Large β Small overlap Ionic

Empirical Racah parameters, h, k β = 1 [h(ligand) x k(metal)] Cr(NH 3 ) 6 3+ Cr(CN) 6 3- β = 1 hk β = 1 hk β = 1 (1.4)(0.21) = 0.706 β = 1 (2.0)(0.21) = 0.580 B o - B = h ligands x k metal ion B o

Typical o and» max values for octahedral (ML 6 ) d-block metal complexes Complex o cm -1 ~» max (nm) Complex o cm -1» max (nm) [Ti(H 2 O) 6 ] 3+ 20,300 493 [Fe(H 2 O) 6 ] 2+ 9,400 1064 [V(H 2 O) 6 ] 3+ 20,300 493 [Fe(H 2 O) 6 ] 3+ 13,700 730 [V(H 2 O) 6 ] 2+ 12,400 806 [Fe(CN) 6 ] 3-35,000 286 [CrF 6 ] 3-15,000 667 [Fe(CN) 6 ] 4-33,800 296 [Co(H 2 O) 6 ] 3+, l.s. 20,700 483 [Fe(C 2 O 4 ) 3 ] 3-14,100 709 [Cr(H 2 O) 6 ] 2+ 14,100 709 [Co(CN) 6 ] 3- l.s. 34,800 287 [Cr(H 2 O) 6 ] 3+ 17,400 575 [Co(NH 3 ) 6 ] 3+ l.s. 22,900 437 [Cr(NH 3 ) 6 ] 3+ 21,600 463 [Ni(H 2 O) 6 ] 2+ 8,500 1176 [Cr(en) 3 ] 3+ 21,900 457 [Ni(NH 3 ) 6 ] 2+ 10,800 926 [Cr(CN) 6 ] 3-26,600 376 [Ni(en) 3 ] 2+ 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 Dq o = f x g. Let us consider the complex Co(NH 3 ) 6 2+. The oxidation state of the cobalt is +2, so the the metal isconsidered a d 7. To figure out 10 Dq o (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 1.25 x 9000 = 11,250 cm -1 which is the size of 10 Dq o. The next step is to determine the reduced Racah parameter for the complex. The reduced Racah parameter is called beta. beta=(b complex )/(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=(b complex )/(B free ion ) = 1 - h. k = 1 - (1.4)(0.09) = 0.874

From this it easy to rearrange things to get B complex and use the value of B free ion for Co 2+ from Table 17.2 (beta)(b free ion ) = B complex = (0.874)(971 cm -1 ) = 849 cm -1 To use a Tanabe-Sugano diagram, you mustdivide the value of 10 Dq o B complex. (10 Dq o )/B complex = 11,250 cm -1 /849 cm -1> = 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 (d 7 in this case) is critical. Looking at the Tanabe- Sugano diagram quickly reveals that the term symbol for a free Co 2+ ion is 4 F. 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 complex Co(NH 3 ) 6 2+ 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: by 4 T 1g -----> 4 T 2g 4 T 1g -----> 4 T 1g 4 T 1g -----> 4 A 2g

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/B complex on the y- axis. 4 T 1g -----> 4 T 2g E/B complex =12.4 4 T 1g -----> 4 T 1g E/B complex = 25.6 4 T 1g -----> 4 A 2g E/B complex = 25.6 To get the energy of the transitions in cm -1, each of these must be multiplied by B complex 4 T 1g -----> 4 T 2g E/B complex =12.4 ; 12.4 x 849 cm -1 = 10,528 cm -1 4 T 1g -----> 4 T 1g E/B complex = 25.6; 25.6 x 849 cm -1 = 21,734 cm -1 4 T 1g -----> 4 A 2g E/B complex = 25.6, 25.6 x 849 cm -1 = 21,734 cm -1

The last step is to convert the wave number (reciprocal centimeters, cm -1 ) to namometers 4 T 1g -----> 4 T 2g 10,528 cm -1 ; 1/(10,528 cm -1 ) = 9.50 x 10-5 cm; (9.50 x 10-5 cm)(10 7 nm/cm) = 950 nm 4 T 1g -----> 4 T 1g 21,734 cm -1 ; 1/(21,734 cm -1 ) = 4.60 x 10-5 cm; (4.60 x 10-5 cm)(10 7 nm/cm) = 460 nm 4 T 1g -----> 4 A 2g 21,734 cm -1 ; 1/(21,734 cm -1 ) = 4.60 x 10-5 cm; (4.60 x 10-5 cm)(10 7 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.

Use of Tanabe-Sugano Diagrams for Interpretation of UV/Visible Absorption Spectra of Complexes d 3

26.5

26.5

Charge-Transfer Spectra

Charge Transfer Transitions As well as d-d transitions, the electronic spectra of transition metal complexes may 3 others types of electronic transition: Ligand to metal charge transfer (LMCT) Metal to ligand charge transfer (MLCT) Intervalence transitions (IVT) All complexes show LMCT transitions, some show MLCT, a few show IVT

Ligand to Metal Charge Transfer These involve excitation of an electron from a ligand-based orbital into a d- orbital O O O M O visible light 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 O O O M O 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 They occur in the visible or near-ultraviolet if metal is easily reduced (for example metal in high oxidation state) d 0 TiO 2 Ti 4+ VO 4 3- V 5+ Cr 6+ Mn 7+ CrO 4 MnO 4 in far UV ~39500 cm -1 ~22200 cm -1 ~19000 cm -1 white white yellow purple 2- - more easily reduced

Metal to Ligand Charge Transfer They occur in the visible or near-ultraviolet if metal is easily oxidized and ligand has low lying empty orbitals N N N N M N N N N N N M = Fe 2+, Ru 2+, Os 2+ Sunlight excites electron from M 2+ (t 2g ) 6 into empty ligand π* orbital method of capturing and storing solar energy

Intervalence Transitions Complexes containing metals in two oxidation states can be coloured due to excitation of an electron from one metal to another Prussian blue contains Fe 2+ and Fe 3+ Colour arises from excitation of an electron from Fe 2+ to Fe 3+

(p) (s) (d) 3) Charge transfer bands 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) ligand-to-metal charge transfer (LMCT) like in MnO 4-, CrO 4 2- etc. or ii) metal-to ligand charge transfer (MLCT) like in [Fe(bpy) 3 ] 2+. The simplified diagrams below are the modified versions of what we had in Lecture 26. Bold arrows show possible CT transitions. t 2 a 1 e t 2 Mn VII 4 O 2- d 0 3t 2 2a 1 2t 2 29500 44400 e t 1 17700 30300 1t 2 t 1 (n) t 2 a 1 (σ) ο Fe II d 6 e g t 2g 3 bpy π-go's 3t 2g N N e g 2t 2g bpy = t 2g (π*) t 2g (π) 1a 1 1t 2g

Metal character Ligand character Charge transfer spectra Ligand character LMCT Metal character MLCT Much more intense bands

Charge-Transfer Spectra 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 (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 uncommon for these bands 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 For example, consider an octahedral d 6 complex with Ã-donor ligands The possibility exists that electrons can be excited, not only from the t 2g level to the e g but also from the Ãorbitals originating from the ligands to the e g 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 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 In these cases, empty À* orbitals on the ligands become the acceptor orbitals on absorption of light 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 (S 2 CNR 2- )

Charge-Transfer Spectra In complexes such as Cr(CO) 6 which have both Ã-donor and À-acceptor orbitals, both types of charge transfer are possible It is not always easy to determine the type of charge transfer in a given coordination compound Many ligands give highly colored complexes that have a series of overlapping absorption bands in the ultraviolet part of the spectrum as well as the visible In such cases, the d-d transitions may be completely overwhelmed and essentially impossible to observe

Charge-Transfer Spectra Finally, the ligand itself may have a chromophore and still another type of absorption band an intraligand band, may be observed These bands may sometimes be identified by comparing the spectra of complexes with the spectra of free ligands However, coordination of a ligand to a metal may significantly alter the energies of the ligand orbitals, and such comparisons may be difficult, 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) HOMO = σ-bonding electron pair donor to metal ion b) LUMO = π-bonding electron pair acceptor from metal ion c) The π* orbitals are higher in energy than the metal t 2g 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 t 2g MO s receive the d-electrons ii. 3 new antibonding t 2g * MO s formed iii. The e g * MO s from the σ-bond MO treatment are nonbonding iv.ligands like this increase o by lowering the energy of t 2g 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 capable of π-interactions i) 1 lone pair used in σ-bonding ii) Other lone pairs π-bond b) The filled p-orbitals are lower in energy than the metal t 2g set c) Bonding Interaction i. 3 new bonding MO s filled by Fluorine electrons ii. 3 new antibonding MO s form t 2g * 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, H 2 O ii. Favors high spin complexes

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