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Physical Methods in Inorganic Chemistry. or How do we know what we made and does it have interesting properties?. -. n / cm -1 (frequency). What is electronic spectroscopy?. Absorption of radiation leading to electronic transitions within a molecule or complex. Absorption.
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Physical Methods in Inorganic Chemistry or How do we know what we made and does it have interesting properties?
- n / cm-1 (frequency) What is electronic spectroscopy? Absorption of radiation leading to electronic transitions within a molecule or complex Absorption Absorption [Ru(bpy)3]2+ [Ni(H2O)6]2+ 104 10 ~14 000 25 000 50 000 200 400 700 visible UV UV visible l / nm (wavelength) UV = higher energy transitions - between ligand orbitals visible = lower energy transitions - between d-orbitals of transition metals - between metal and ligand orbitals
Absorption maxima in a visible spectrum have three important characteristics 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, Doct or Dtet and on the degree of inter-electron repulsion intensity This depends on the "allowedness" of the transitions which is described by two selection rules
3+ Ti Absorption of light [Ti(OH2)6]3+ = d1 ion, octahedral complex white light 400-800 nm blue: 400-490 nm yellow-green: 490-580 nm red: 580-700 nm A This complex is has a light purple colour in solution because it absorbs green light l / nm lmax = 510 nm
The energy of the absorption by [Ti(OH2)6]3+ is the ligand-field splitting, Do ES ES eg eg hn Do GS GS t2g t2g d-d transition complex in electronic excited state (ES) complex in electronic Ground State (GS) [Ti(OH2)6]3+lmax = 510 nm Do is 243 kJ mol-1 20 300 cm-1 An electron changes orbital; the ion changes energy state
d2 ion Electron-electron repulsion eg eg x2-y2 x2-y2 z2 z2 t2g t2g xy xz yz xy xz yz xy + z2 xz + z2 z z y y x x lobes overlap, large electron repulsion lobes far apart, small electron repulsion These two electron configurations do not have thesameenergy
Selection Rules Transition e complexes Spin forbidden 10-3 – 1 Many d5 Oh complexes Laporte forbidden [Mn(OH2)6]2+ Spin allowed Laporte forbidden 1 – 10 Many Oh complexes [Ni(OH2)6]2+ 10 – 100 Some square planar complexes [PdCl4]2- 100 – 1000 6-coordinate complexes of low symmetry, many square planar complexes particularly with organic ligands Spin allowed 102 – 103 Some MLCT bands in cxs with unsaturated ligands Laporte allowed 102 – 104 Acentric complexes with ligands such as acac, or with P donor atoms 103 – 106 Many CT bands, transitions in organic species
10 e 5 30 000 20 000 10 000 D/B = 32 - n / cm-1 n3 = 2.1n1 = 2.1 x 17 800 n3 = 37 000 cm-1 = 32 Tanabe-Sugano diagram for d2 ions [V(H2O)6]3+: Three spin allowed transitions E/B n1 = 17 800 cm-1 visible n2 = 25 700 cm-1 visible n3 = obscured by CT transition in UV 25 700 = 1.44 17 800 D/B
N S macroscopic world « traditional, classical » magnets
N S macroscopic world A pioneering experiment by M. Faraday « Farady lines of forces » about magnetic flux
N N N N attraction S S S S macroscopic world « traditional » magnets
N N S S N N repulsion S S macroscopic world « traditional » magnets
N S macroscopic world looking closer to the magnetic domains many sets of domains many sets of atomic magnetic moments
kT ≈ J T C Solid, Magnetically Ordered thermal agitation (kT) weaker than the interaction (J) between molecules … Paramagnetic solid : thermal agitation (kT) larger than the interaction (J) between molecules The magnetic moments order at Curie temperature A set of molecules / atoms : Magnetic Order Temperature or Curie Temperature kT << J kT >> J
Ferromagnetism : Magnetic moments are identical and parallel = + Ferrimagnetism (Néel) : Magnetic moments are different and anti parallel Antiferromagnetism : Magnetic moments are identical and anti parallel = + = 0 + Magnetic Order : ferro-, antiferro- and ferri-magnetism
Origin of Magnetism … the electron I am an electron • rest mass me, • charge e-, • magnetic moment µB everything, tiny, elementary
µorbital = gl x µB x Origin of Magnetism « Orbital » magnetic moment « Intrinsic » magnetic moment µorbital due to the spin s = ± 1/2 µspin e- µspin = gs x µB x s ≈ µB µtotal = µorbital + µspin
Dirac Equation The Principles of Quantum Mechanics, 1930 Nobel Prize 1933 1905 1928 http://www-history.mcs.st-and.ac.uk/history/PictDisplay/Dirac.html
Electron : particle and wave Wave function or « orbital »n, l, ml … l = 0 1 2 3 s p d angular representation
Energy Electron : also an energy level Orbitals Empty Singly occupied Doubly occupied
Electron : also a spin ! Up Singly occupied Doubly occupied Down « Paramagnetic » S = ± 1/2 « Diamagnetic » S = 0
Molecules are most often regarded as isolated, non magnetic Dihydrogen diamagnetic Spin S = 0
the dioxygen that we continuously breath is a magnetic molecule orthogonal π molecular orbitals paramagnetic, spin S =1 Two of its electrons have parallel magnetic moments that shapes aerobic life and allows our existence as human beings
Mononuclear complex ML6 Splitting of the energy levels E
How large is the splitting ? Weak Field Intermediate Field Temperature Dependent Spin Cross-Over Strong Field High spin Low spin L = H2O [C2O4]2- L = CN-
Spin Cross-Over Room Temperature 3 Red 0 The system « remembers » its thermal past ! O. Kahn, C. Jay and ICMC Bordeaux
or parallel ? antiparallel ? S=O S=1 to get magnetic compounds … Understanding … why the spins of two neighbouring electrons (S = 1/2) become :
O2 H2 Aufbau Hund J = 2 k + 4ßS <0 >0 if S = 0 Orthogonality if S≠0;|ßS|>>k Overlap
≈ Exchange interactions can be very weak … Energy Exchange interactions order of magnitude : cm-1 or Kelvins … « Chemical » bonds Robust ! order of magnitude : >> 150 kJ mol-1 …
Cu(II) Cu(II) ≈ 5 Å Negligible Interaction ! Problem : How to create the interaction … ?
Ligand Cu(II) Cu(II) ≈ 5 Å Solution : The ligand ! Orbital Interaction …
A B Ligand Examples with the ligand • Cyanide
CN- Cyanide Ligand Friendly ligand : small, dissymetric, forms stable complexes Warning : dangerous, in acid medium gives HCN, lethal
Dinuclear µ-cyano homometallic complexes
“Models” Compounds Cu(II)-CN-Cu(II) J/cm-1 Compounds exp [Cu2(tren)2CN]3+ [Cu2(tmpa)2CN]3+ -160 -100 Overlap : antiferromatic coupling … Rodríguez-Fortea et al. Inorg. Chem. 2001, 40, 5868