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What is electronic spectroscopy? Absorption Absorption of radiation leading to electronic transitions within a molecule or complex UV=higher energy transitions-

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Presentation on theme: "What is electronic spectroscopy? Absorption Absorption of radiation leading to electronic transitions within a molecule or complex UV=higher energy transitions-"— Presentation transcript:

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2 What is electronic spectroscopy? Absorption Absorption of radiation leading to electronic transitions within a molecule or complex UV=higher energy transitions- between ligand orbitals visible=lower energy transitions- between d-orbitals of transition metals - between metal and ligand orbitals UV 400  nm (wavelength) visible Absorption ~ UVvisible  cm -1 (frequency)  [Ru(bpy) 3 ] 2+ [Ni(H 2 O) 6 ]

3 Absorption maxima in a visible spectrum have three important characteristics 1.number (how many there are) This depends on the electron configuration of the metal centre 2.position (what wavelength/energy) This depends on the ligand field splitting parameter,  oct or  tet and on the degree of inter-electron repulsion 3.intensity This depends on the "allowedness" of the transitions which is described by two selection rules

4 [Ti(OH 2 ) 6 ] 3+ = d 1 ion, octahedral complex white light nm blue: nm yellow yellow-green: nm red: nm 3+ Ti A / nm This complex is has a light purple colour in solution because it absorbs green light max = 510 nm Absorption of light

5 egeg t 2g oo h d-d transition [Ti(OH 2 ) 6 ] 3+ max = 510 nm  o is  243 kJ mol cm -1 The energy of the absorption by [Ti(OH 2 ) 6 ] 3+ is the ligand-field splitting,  o An electron changes orbital; the ion changes energy state complex in electronic Ground State (GS) complex in electronic excited state (ES) GS ES GS ES egeg t 2g

6 Electron-electron repulsiond 2 ion egeg t 2g xyxzyz z2z2 x 2 -y 2 egeg t 2g xyxzyz z2z2 x 2 -y 2 xz + z 2 xy + z 2 lobes overlap, large electron repulsionlobes far apart, small electron repulsion x z x z y y These two electron configurations do not have the same energy

7 Selection Rules Transition  complexes Spin forbidden10 -3 – 1Many d 5 O h complexes Laporte forbidden [Mn(OH 2 ) 6 ] 2+ Spin allowed Laporte forbidden1 – 10Many O h complexes [Ni(OH 2 ) 6 ] – 100 Some square planar complexes [PdCl 4 ] – coordinate complexes of low symmetry, many square planar complexes particularly with organic ligands Spin allowed10 2 – 10 3 Some MLCT bands in cxs with unsaturated ligands Laporte allowed 10 2 – 10 4 Acentric complexes with ligands such as acac, or with P donor atoms 10 3 – 10 6 Many CT bands, transitions in organic species

8 Tanabe-Sugano diagram for d 2 ions E/B  /B [V(H 2 O) 6 ] 3+ : Three spin allowed transitions 1 = cm -1 visible 2 = cm -1 visible 3 = obscured by CT transition in UV   cm -1  =  /B=32 3 = = 2.1 x  3 = cm -1 = 32

9 Magnetism

10 N S macroscopic world « traditional, classical » magnets

11 A pioneering experiment by M. Faraday « Farady lines of forces » about magnetic flux N S macroscopic world

12 « traditional » magnets N S N S attraction N S N S

13 macroscopic world « traditional » magnets N S repulsion N S N S N S

14 macroscopic world looking closer to the magnetic domains S N many sets of domains   many sets of  atomic magnetic moments

15 The magnetic moments order at Curie temperature … Paramagnetic solid : thermal agitation (kT) larger than the interaction (J) between molecules Solid, Magnetically Ordered thermal agitation (kT) weaker than the interaction (J) between molecules A set of molecules / atoms : kT << JkT >> J T C kT ≈ J Magnetic Order Temperature or Curie Temperature

16 Magnetic Order : ferro-, antiferro- and ferri-magnetism + = Ferromagnetism : Magnetic moments are identical and parallel + = 0 Antiferromagnetism : Magnetic moments are identical and anti parallel + = Ferrimagnetism (Néel) : Magnetic moments are different and anti parallel

17 Origin of Magnetism … the electron I am an electron rest mass m e, charge e -, magnetic moment µ B everything, tiny, elementary

18 Origin of Magnetism e-e- « Orbital » magnetic moment« Intrinsic » magnetic moment due to the spin µ spin = g s x µ B x s ≈ µ B s = ± 1/2 µ orbital = g l x µ B x µ total = µ orbital + µ spin µ orbital µ spin

19 Dirac Equation Nobel Prize 1933 The Principles of Quantum Mechanics, 1930

20 l = 0123 s p d angular representation Electron : particle and wave Wave function or « orbital »  n, l, m l …

21 Empty Doubly occupied Orbitals Energy Singly occupied Electron : also an energy level

22 Singly occupied Electron : also a spin ! Up Down Doubly occupied « Paramagnetic » S = ± 1/2 « Diamagnetic » S = 0

23 Molecules are most often regarded as isolated, non magnetic Dihydrogen diamagnetic Spin S = 0

24 the dioxygen that we continuously breath is a magnetic molecule paramagnetic, spin S =1 Two of its electrons have parallel magnetic moments that shapes aerobic life and allows our existence as human beings orthogonal π molecular orbitals

25 Transition Elements

26 Mononuclear complex ML 6 E Splitting of the energy levels

27 How large is the splitting ? High spin L = H 2 O [C 2 O 4 ] 2- Low spin L = CN- Weak FieldStrong FieldIntermediate Field Temperature Dependent Spin Cross-Over

28 The system « remembers » its thermal past ! Room Temperature O. Kahn, C. Jay and ICMC Bordeaux Red 3 0

29 Understanding … why the spins of two neighbouring electrons (S = 1/2) become : to get magnetic compounds … antiparallel ? S=O or parallel ? S=1

30 Interaction Models between Localized Electrons

31 Energy levels

32 J = 2 k + 4ßS if S = 0 Orthogonality if S≠0;|ßS|>>k Overlap >0 <0 H2H2 Aufbau O2O2 Hund

33 Exchange interactions can be very weak … order of magnitude : cm -1 or Kelvins … ≈ order of magnitude : >> 150 kJ mol -1 … « Chemical » bonds Robust ! Exchange interactions Energy

34 ≈ 5 Å Negligible Interaction ! How to create the interaction … ? Problem : Cu(II)

35 Orbital Interaction … ≈ 5 Å Cu(II) The ligand ! Solution : Ligand

36 A B Examples with the ligand Cyanide Ligand

37 Cyanide Ligand Friendly ligand : small, dissymetric, forms stable complexes Warning : dangerous, in acid medium gives HCN, lethal CN-CN-

38 Dinuclear µ -cyano homometallic complexes

39 “Models” Compounds Cu(II)-CN-Cu(II) Compounds exp [Cu 2 (tren) 2 CN] 3+ [Cu 2 (tmpa) 2 CN] J/cm -1 Rodríguez-Fortea et al. Inorg. Chem. 2001, 40, 5868 Overlap : antiferromatic coupling …


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