Transition Metals, Compounds and Complexes or Electronic Spectroscopy of Transition Metal Complexes Dr. E.R. Schofield Lecture 6: Tanabe-Sugano diagrams and Charge Transfer Transitions Tanabe-Sugano diagrams Calculating D, E and B d0 and d10 ions Charge Transfer transitions

Tanabe-Sugano diagram for d3 ions n1 = 17 400 cm-1 visible n2 = 24 500 cm-1 visible n3 = obscured by CT transition [Cr(H2O)6]3+: Three spin allowed transitions E/B 24 500 = 1.41 17 400 = 24 D/B = 24 n3 = 2.1n1 = 2.1 x 17 400  n3 = 36 500 cm-1 D/B

Calculating n3 n1 = 17 400 cm-1 n2 = 24 500 cm-1 E/B When n1 = E =17 400 cm-1 E/B = 24 so B = 725 cm-1 When n2 = E =24 500 cm-1 E/B = 34 so B = 725 cm-1 E/B = 34 cm-1 E/B = 24 cm-1 If D/B = 24 D = 24 x 725 = 17 400 cm-1 D/B = 24

n1 = 17 400 cm-1 visible n2 = 24 500 cm-1 visible n3 = obscured by CT transition Energy diagram for octahedral d3 complex 4T1g 10 Dq 2 Dq 6 Dq x 15 B' For Oh d3, Do = n1 = 17 400 cm-1 Do / B = 24 B = 725 cm-1 4T1g 4T2g 4A2g

Charge Transfer Transitions d0 and d10 ions d0 and d10 ion have no d-d transitions Zn2+ d10 ion white TiF4 d0 ion TiCl4 d0 ion TiBr4 d0 ion TiI4 d0 ion white white orange dark brown [MnO4]- Mn(VII) d0 ion [Cr2O7]- Cr(VI) d0 ion extremely purple bright orange [Cu(MeCN)4]+ Cu(I) d10 ion [Cu(phen)2]+ Cu(I) d10 ion colourless dark orange Charge Transfer Transitions

Metal-to-ligand charge transfer MLCT transitions Ligand-to-metal charge transfer LMCT transitions Charge Transfer Transitions d-d transitions Lp* eg* t2g* Md Lp Ls

Selection rules CT transitions are spin allowed and Laporte allowed Transitions occur between metal based orbitals with d-character and ligand based orbitals with p-character Dl = ± 1 Transitions occur from a singlet GS to a singlet ES DS = 0 CT transitions are therefore much more intense than d-d transitions

LMCT Transitions spin-allowed; Laporte allowed [MnO4]-, dark purple LMCT = ligand to metal charge transfer e- rich ligand O2-, Cl-, Br-, I- e- poor metal (electropositive), high charge Cr(III), d3 ion, Mn(VII), d0 ion

MO diagram of MnO4- L(t1)  M(e) 17 700 cm-1 L(t1)  M(t2*) 29 500 cm-1 L(t2)  M(e) 30 300 cm-1 L(t2)  M(t2*) 44 400 cm-1 M ML4 4L a1 t2 t1 e t2* a1* t* t2 (n+1)p a1 (n+1)s Dt e, t2 nd t1 ,t2 p a1 ,t2 s

spin-allowed; Laporte allowed LMCT Transitions [CrCl(NH3)5]2+, Cr(III), d3 LMCT 4 3 log(e/L mol-1 cm-1) 2 d-d d-d 1 200 (50 000 cm-1) 400 (25 000 cm-1) 600 (17 000 cm-1) l / nm Identifying charge transfer transitions Intensity Solvatochromism - variation in absorption wavelength with solvent

e Charge-Transfer Transitions: MLCT spin-allowed; Laporte allowed l / nm 400 500 600 300 100 200 lmax = 458 nm [Cu(phen)2]+, dark orange MLCT = metal to ligand charge transfer e- rich metal, low charge, lower OS Cu(I), d10 ion p-acceptor ligand with low-lying p* orbitals 1,10-phenanthroline

e Charge-Transfer Transitions: MLCT spin-allowed; Laporte allowed p-p* lmax = 452 nm MLCT [Ru(bpy)3]2+, bright orange 200 300 400 500 l / nm MLCT = metal to ligand charge transfer e- rich metal, low charge, lower OS Ru(II), d6 ion, low spin p-acceptor ligand with low-lying p* orbitals 2,2'-bipyridine

Interelectron repulsion Limits of LFT – existence of electronic states Russel-Saunders coupling Collecting microstates into terms Using Hund's rules  GS and ES with same multiplicity Effect of LF on free ion terms Orgel diagram for d1, d4, d6, d9 ions [Ti(OH2)6]3+ Orgel diagram for d2, d3, d7, d8 ions [Ni(OH2)6]2+ Calculating D, x and B' [Co(OH2)6]2+, [CoCl4]2- Racah parameters in free ions and complexes The Nephelauxetic effect Orgel diagram for d5 ions [Mn(OH2)6]2+ Spin and Laporte Selection Rules Selection rules for Oh and Td complexes Lifting selection rules Origin of band broadening [Ni(OH2)6]2+, [Mn(OH2)6]2+, [Ti(OH2)6]3+ Tanabe-Sugano diagrams Low spin complexes [Mn(CN)6]4- Calculating E, B' and D [Cr(OH2)6]3+, [V(OH2)6]3+ CT transitions MLCT, LMCT [Cu(phen)2]+, [Ru(bpy)3]2+, [MnO4]-