Lecture 29 Electronic Spectra of Coordination Compounds ML x (x = 4,6) 1) Electron repulsion B’ and   parameters for d 3 & d 8 O h species The d-electron-d-electron repulsions present in d X metal complexes with x > 1 make the relationship between the d-orbital splitting parameter  and absorption frequencies complex. The following equations relate , the electron repulsion parameter B’ and transition frequencies 1, 2, 3 ( 1 < 2 < 3 ) for d 3 and d 8 octahedral complexes: 1 =  2 = 7.5B’  [225 B’ 2 +  2 -18B’  ] 1/2  3 = 7.5B’  [225 B’ 2 +  2 -18B’  ] 1/2 For example, for d 3 chromium(III) complex CrF 6 3- the absorptions are at 14900, and cm -1. From the formulas above  = cm -1, = 15B’ + 3  = Therefore, 15B’ = and B’ ≈ 827 cm -1.

2) Tanabe-Sugano diagrams. d 2 octahedral complexes Tanabe-Sugano diagrams have been calculated for the cases of all ligand field strengths for all octahedral d n -metal complex configurations. The diagrams allow for estimation of a complete set of parameters included in the calculations: i) the (apparent) electron repulsion parameter B’, ii) , iii) transition frequencies. To calculate these parameters we need to have at least two experimentally found transition frequencies. Consider an example of a d 2 metal complex and the appropriate Tanabe-Sugano diagram. From 3 bands expected for V(H 2 O) 6 3+ (d 2 ) in its UV spectrum we have one at 1 = ( 3 T 1g (F)  3 T 2g ) and another at 2 = ( 3 T 1g (F)  3 T 1g (P)) cm -1. The third expected transition 3 ( 3 T 1g (F)  3 A 2g ) is far in the UV region and is masked by other absorptions. We can calculate the 3. For 2 / 1 = 1.49 on diagram we find 1 /B ≈ 25; 2 /B= 37; 3 / B = 52;  /(10B) = 2.7. Then B ≈ 691cm -1 ;  = cm -1 ; 3 ≈ cm -1.

3) Terms of d 5 metal complexes Terms of free d 5 metal ions are 6 S, 4 G, 4 F, 4 D, 4 P, 2 I, 2 H, 2 G, 2 G, 2 F, 2 F, 2 D, 2 D, 2 D, 2 P, 2 S (16 terms, 252 microstates). The lowest energy term is 6 S. In the octahedral ligand field the 6 S term will NOT be split. It gives rise to a single 6 A 1g term. The 6 A 1g term is the ground state term at weak ligand fields. NO terms of the same multiplicity exists and thus NO spin-allowed e-e transition is possible. At strong ligand fields spin pairing occurs (t 2 3 e 2  t 2 5 ). As a result, the ground state term and the multiplicity change from 6 A 1g to 2 T 2g (I).

4) Weak ligand fields, all d n configurations: the number and intensity of bands The number of the spin-allowed electronic transitions for all d n metal configurations for the case of weak ligand fields is given in table below. The intensity of absorption bands can be evaluated on the basis of the selection rules. The symmetry forbidden d-d transitions for d 1 -d 4, d 6 -d 9 complexes below have  in the range of 1 – 10 3 L/(mol cm). Both symmetry and spin forbidden transitions for d 5 complexes have  of – 1 L/(mol cm). Configuration (example)Ground stateExcited states w/same S# Abs.bands d 1 oct (Ti(H 2 O) 6 3+ ), d 9 tetr. 2T22T2 2E22E2 1 d 2 oct (V(H 2 O) 6 3+ ), d 8 tetr. 3 T 1 (F) 3 T 2, 3 T 1 (P), 3 A 2 3 d 3 oct (Cr(H 2 O) 6 3+ ), d 7 tetr. 4A24A2 4 T 2, 4 T 1 (F), 4 T 1 (P)3 d 4 oct (Cr(H 2 O) 6 2+ ), d 6 tetr. 5E25E2 5T25T2 1 d 5 oct (Mn(H 2 O) 6 2+ ) or tetr. 6A16A1 none0 d 6 oct (Fe(H 2 O) 6 2+ ), d 4 tetr. 5T25T2 5E25E2 1 d 7 oct (Co(H 2 O) 6 2+ ), d 3 tetr. 4 T 1 (F) 4 T 2, 4 T 1 (P), 4 A 2 3 d 8 oct (Ni(H 2 O) 6 2+ ), d 2 tetr. 3A23A2 3 T 2, 3 T 1 (F), 3 T 1 (P)3 d 9 oct (Cu(NH 3 ) 6 2+ ), d 1 tetr. 2E22E2 2T22T2 1

5) Line width in electronic spectra A regular absorption band in electronic spectra is of several thousands cm -1 wide. The width is associated with: a) combination of vibrational and electronic (vibronic) transitions; b) distortion of the complex geometry (ligands shift) with or without change of initial symmetry. The related change of  and/or term splitting pattern affect the position of the “immediate” absorption maximum. In rare cases bands are as narrow as of few hundreds of cm -1. This is the case when the energy gap between the ground and excited states is not a function of .

6) Line width in electronic spectra Terms which are not affected by metal complex vibrations appear as horizontal lines on Tanabe- Sugano diagrams. For high-spin d 5 complexes these are in particular the terms 4 A 1g, 4 E g (G). The electronic transitions to these levels produce narrow absorption lines. Narrow lines corresponding to these transitions can be seen in a spectrum of d 5 high spin MnF 2 (Mn II ion has an octahedral environment in the crystal lattice): 4 T 1g (G) 4 T 2g (G) 4 A 1g (G) 4 E g (G)