Electronic Spectroscopy of Transition Metal Ions (continued)

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1 Electronic Spectroscopy of Transition Metal Ions (continued) What about the spectroscopy! First some selection rules are found to apply: 1) Spin selection rule: S = 0 theory: transitions can only occur between states of the same spin (and therefore the same spin multiplicity) eg. 2 T 2g 2 E g is allowed but 2 T 2g 1 E g is not practice: spin forbidden transition seldom occur and if they do ε<1 L mol -1 cm -1 so we can effectively assume that this rule is not violated 2) Laporte (symmetry) selection rule: in centrosymmetric environments, transitions can only occur if there IS a change in parity theory: this means g g or u u transitions are not allowed in an Oh field since an octahedron has a centre of symmetry which effectively means no d d (or f f) transitions are allowed since all d derived orbitals have g parity practice: molecular vibrations can lower symmetry so this selection rule is partially relaxed and d d transitions do occur, albeit with low intensity, eg. ε~10 L mol -1 cm -1

2 Other implications: tetrahedra are NOT centrosymmetric so the Laporte rule is relaxed and ε of 100-1,000 L mol -1 cm -1 are common CT bands are not subject to either selection rule so they are usually very intense with ε of 10, ,000 L mol -1 cm -1 (eg. MnO 4 - ) Orgel diagrams: a correlation diagram that shows how ligand field strength affects the energy of the crystal field terms. Only terms with the same multiplicity as the GS are shown because only these can be involved in transitions from the GS (spin selection rule) from: C. Jones, d- and f-block Chemistry, Wiley, 2002.

3 All of this allows us to predict and understand the number of UV- Vis bands observed for ions with various d counts: e - count GS term Oh CF terms Higher E terms = multiplicity d 1 (d 9 ) d 4 (d 6 ) 2 D 5 D 2 T 2g, 2 E g none 5 T 2g, 2 E g none Therefore only bands expected are from 2S+1 T 2g 2S+1 E g (d 1, d 6 ) or from 2S+1 E g 2S+1 T 2g (d 4, d 9 ) = 1 band in either case (Note: a d 1 state is equivalent and a d 9 state if we consider a hole replacing and e - but this reverses the order of the CF states) [NOTE: SEE FIGURE ON FOLLOWING PAGE] e - count GS term Oh CF terms Higher E terms = multiplicity d 2 (d 8 ) 3 F 3 T 1g, 3 T 2g, 3 A 2g 3 P ( 3 T 1g ) d 3 (d 7 ) 4 F 4 T 1g, 4 T 2g, 4 A 2g 4 P ( 4 T 1g ) Therefore, in this case there will be three bands total: 2S+1 T 1g 2S+1 T 2g 2S+1 T 1g 2S+1 A 2g 2S+1 T 1g 2S+1 T 1g (d 2, d 7 ) 2S+1 A 2g 2S+1 T 2g 2S+1 A 2g 2S+1 T 1g 2S+1 A 2g 2S+1 T 1g (d 3, d 8 )

4 e - count GS term Oh CF terms Higher E terms = multiplicity d 5 6 S 6 A 1g none Thus NO bands are expected for a d 5 ion and in fact, a series of very weak and sharp bands is all that is usually observed so most d 5 ions are very faintly coloured or colourless.

5 A few other points: If one ignores the spin multiplicity, it is clear that the Orgel diagrams fall into three related groups shown above: [d 1,d 4,d 6,d 10 ]; [d 2,d 3,d 7,d 8 ] and [d 5 ] oct can be extracted directly from the spectrum by inspection from frequency of the lone d d transition in the [d 1,d 4,d 6,d 10 ] case only; in the [d 2,d 3,d 7,d 8 ] case it can be obtained from a difference in frequencies between the lowest and highest energy d d transitions. For the [d 5 ] case it cannot be obtained directly from the spectra. It can be shown (but not done here) that the Td and Oh fields essentially cause an inverted set of crystal field states so a general Orgel diagram that reflects this can be constructed:

6 Charge transfer complexes LMCT: an electron is excited from an orbital that is primarily ligand based to an orbital that is primarily metal based egs. O lone pair e - excited into an empty M orbital MnO 4 - (528 nm) TcO 4 - (286 nm) ReO 4 - (227 nm) metal harder to reduce increases relative energy of acceptor orbital halide (X) lone pair excited into an empty M orbital [FeBr 4 ] 2- (244 nm) [FeCl 4 ] 2- (220 nm) halide harder to oxidize decreases relative energy of donor orbital [OsCl 6 ] 2- Os(IV) (370 nm) [OsCl 6 ] 3- Os(III) (282 nm) lower oxidation state harder to reduce increases relative energy of acceptor orbital

7 MLCT: an electron is excited from an orbital that is primarily metal based to an orbital that is primarily ligand based requires an acceptor orbital on the ligand that is capable of taking electron density: egs. CO, PR 3, low energy π* especially of heterocycles like py metal that is relatively easy to oxidize (high lying metal orbitals): common in low oxidation states Not a d metal but an example I am intimately familiar with... Yb(η 5 -C 5 Me 5 ) 2 Yb(II) 4f 14 intensely coloured Yb(η 5 -C 5 H 5 ) 3 Yb(III) 4f 13 pale yellow or colourless Yb 2+ (aq) Yb 3+ (aq) ε 1/2 (SCE) = +1.5V