Transition Metals, Compounds and Complexes

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Presentation transcript:

Transition Metals, Compounds and Complexes Dr. E.R. Schofield Lecture 4: More Orgel Diagrams, d5 complexes and Selection Rules Orgel diagram for d2, d3, d7, d8 Orgel diagram for d5 ions Spin and Laporte Selection Rules Demo: Co oct to tet and Mn(II)

Ligand field strength (Dq) Orgel diagram for d2, d3, d7, d8 ions Energy A2 or A2g T1 or T1g T2 or T2g T1 or T1g P F d2, d7 tetrahedral d2, d7 octahedral d3, d8 octahedral d3, d8 tetrahedral Ligand field strength (Dq)

Energy diagram for oct d3, d8, tet d2, d7 15 B' 15 B 15 B > 15 B' 10 Dq 2 Dq 6 Dq A2(g) T1(g) T2(g) x P F T1(g) T2(g) A2(g) D = 10 Dq 10 Dq 2 Dq 6 Dq

d7 tetrahedral complex Calculating B' and x 4T1 n1 n2 n3 [CoCl4]2- 4T1 n1 n2 n3 n1 = 3 300 cm-1 IR region n2 = 5 800 cm-1 visible n3 = 15 000 cm-1 visible 10 Dq 2 Dq 6 Dq x 15 B' A 4T1 25 000 20 000 15 000 10 000 5 000 4T2 v / cm-1 4A2

Racah Parameters Free ion [Co2+]: B = 971 cm-1 d7 octahedral complex [Co(H2O)6]2+ [CoCl4]2- d7 octahedral complex 15 B' = 13 800 cm-1 B' = 920 cm-1 d7 tetrahedral complex 15 B' = 10 900 cm-1 B' = 727 cm-1 B' = 0.95 B B' = 0.75 B Nephelauxetic ratio, b b is a measure of the decrease in electron-electron repulsion on complexation

The Nephelauxetic Effect cloud expanding some covalency in M-L bonds – M and L share electrons effective size of metal orbitals increases electron-electron repulsion decreases Nephelauxetic series of ligands F- < H2O < NH3 < en < [oxalate]2- < [NCS]- < Cl- < Br- < I- Nephelauxetic series of metal ions Mn(II) < Ni(II) Co(II) < Mo(II) > Re (IV) < Fe(III) < Ir(III) < Co(III) < Mn(IV)

Orgel Diagram, d5 oct and tet 4E(g) 4T2(g) 4E(g), 4A1(g) 4T1(g) 6A1(g) 4A2(g) 4E(g) 4T2(g) 4E(g), 4A1(g) 4T1(g) 6A1(g) 4A2(g) 50 000 4G 4P 4D 4F 40 000 Energy (cm-1) 30 000 20 000 10 000 6S 500 1000 Ligand Field Strength, Dq (cm-1)

e Multiple absorption bands Very weak intensity d5 octahedral complex [Mn(H2O)6]2+ Transitions are forbidden e 4Eg (G) 4A1g (G) 0.01 0.02 0.03 4T2g (D) 4Eg (D) 4T1g(G) 4T2g (G) v / cm-1 20 000 25 000 30 000

Spin Selection Rule DS = 0 There must be no change in spin multiplicity during an electronic transition Laporte Selection Rule D l = ± 1 There must be a change in parity during an electronic transition g  u Selection rules determine the intensity of electronic transitions

e Spin allowed - Laporte forbidden Transition between d orbitals 2Eg E 0.01 0.02 0.03 [Ti(OH2)6]3+, d1, Oh field Spin allowed n / cm-1 - Laporte forbidden Transition between d orbitals 10 000 20 000 30 000 2Eg E 2D 2T2g Doct

e e Spin allowed; Laporte forbidden 4A2g 4T1g 4T2g 3T1 3T2 3A2 [V(H2O)6]3+, d2 Oh 3T1 3T2 3A2 [CoCl4]2-, d7 Td 600 10 4T1g 4T2g 4A2g 400 5 200 v / cm-1 n / cm-1 - 25 000 20 000 15 000 10 000 5 000 30 000 20 000 10 000 Spin allowed; Laporte forbidden F P Dq A2g T2g T1g T1 T2 A2 d7 tetrahedral d2 octahedral

Relaxation of the Laporte Selection Rule for Tetrahedral Complexes Octahedral complex Centrosymmetric Laporte rule applies Tetrahedral complex Non-centrosymmetric Laporte rule relaxed inversion centre Oh complex d  eg and t2g p  t1u Td complex d  e and t2 p  t2 Orbital mixing: In tet complexes, d-orbitals have some p character

Intenstity of transitions in d5 complexes Laporte forbidden 10 000 20 000 30 000 40 000 50 000 4G 4P 4D 4F Dq (cm-1) 500 1000 Energy (cm-1) 4E(g) 4T2(g) 4E(g), 4A1(g) 4T1(g) 6A1(g) 4A2(g) Spin forbidden Weak transitions occur due to: Unsymmetrical Vibrations (vibronic transitions) Spin-orbit Coupling