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How do we describe the bonding between transition metal (ions) and their ligands (like water, ammonia, CO etc) ? The Crystal Field Model gives a simple theory to explain electronic spectra.
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Bonding in Coordination Compounds
Dr.Christoph PU Sep.2011
Please contact me at office 2201 !
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Review: ionic vs. covalent bond
Review: electronic structure and periodic table
Crystal Field Theory
Spectrochemical Series
Ligand Field Theory
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Where are metals and non-metals in the periodic table ? Where are the transition metals ? Which elements form cations and which anions ?
How atoms form bonds ?
Watch tutorial videos on my website
http://myphayao.com “Chemical Bonding”
Atoms use their VALENCE ELECTRONS to exchange or share electrons to form bonds !
Examples: how many VE are in these elements ?
Na Ca Cl Ar C N S P
Which rule can we find for the main group elements ?
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8 electron rule (main group elements)
Each element in the main group tends to get 8 valence electrons.
2 ways to do this:
1. Exchange electrons between 2 atoms
2. Share electrons
Example: NaCl vs. HCl vs. PCl3
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Review: Oxidation Numbers of Transition Metal Ions
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In the ions, the energy of the 4s is higher than the 3d in the ions the 4s electrons are removed first !
+1 ONLY for Cu/Ag/Au ! (-1 for Au possible !) +2 possible for nearly ALL metals ( => remove the two s electrons)
Oxidation Numbers of Transition Metal Ions
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Write the d electron number for each metal in the highest common ox. state (highest big dot for each element)
0 d
18 electron rule Transition metals (TM)
TM have d-orbitals which should be filled together with s and p orbitals -> 8 + 10 electrons !
Examples: (write ox.nr. of the metal and then count metal plus ligand bonding electrons)
Fe(CO)5
Kx [Fe(CN)6]
Zr(Cp)2Cl2
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Excurs: Hückel's rule
“A planar ring system with 4n+2 π-electrons is aromatic” (n=0,1,2,…)
Are these molecules aromatic ?
Cyclobutadiene
Cyclopropene
Cyclopentadienyl (-) ion
Cyclooctatetraene
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Ionic Bonding
Two atoms exchange electrons => ionic bond
Combination Metal – Non-Metal: Fe 1.8 Cl 3.0 => ΔEN = 1.2
Na 0.9 Cl 3.2 => ΔEN = 2.3
High EN difference high ionic character (exceptions possible !)
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Covalent Bond
If the EN difference between two atoms is small (smaller than 1.7 as a rule), then the atoms SHARE unpaired electrons instead of exchanging them.
Number of valence electrons = number of bonds
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P Cl form PCl3 (PCl5 also possible)
Which kind of bond ?
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Depends on the EN difference (“END”) between 2 atoms:
Na(+) Cl(-) (δ+)H-Cl(δ-) Cl-Cl
What kind of bond ? And which oxidation numbers ?
• Calciumcarbide Ca2C
• Methyl Lithium LiCH3
• Boronhydride BH3
• Tungstencarbide WC
• Nickelchloride NiCl2
• Ironpentacarbonyl Fe(CO)5
• Xenonflouride XeF4
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Bonding Models
for
Transition metal compounds
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Christoph Sontag PhD, Phayao University Sept. 2011 e-mail: [email protected] Office SCI 2201
Transition metal complexes: Examples
GEMSTONES พลอย consist of SiO2 or Al2O3 crystals which contain traces ร่องรอย of transition metal โลหะการเปลี่ยนแปลง ions.
For example: RUBY Ruby is α-alumina (the most stable form of Al2O3) in which a small fraction เศษสว่น
of the aluminum3+ ions is replaced by chromium3+ ions.
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Examples of different Ni(2+) complexes
Hemoglobin
Iron(2+) ions are used to transport Oxygen in our bodies –
without Iron, no life !
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Chlorophyll
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Sun Protection Cream
… is made of Titanium Dioxide as very fine particles in a cream:
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Cancer Treatment
The growth of living cells can be blocked by a platinum complex:
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… and many more examples in our daily life ! Let’s look how these molecules are made !
Transition Metals have valence electrons
in d-orbitals !!
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How many groups of transition metals exist ? (compare to the 8 MAIN GROUP elements) And why ?
Energy Levels of d-Orbitals “Crystal Field Splitting”
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In an atom, the d-orbitals have the same energy (“degenerated”) BUT: if molecules with high electron density approach the atom, then the energy levels for these 2 d-orbitals go up:
el
el
el
el
el
el
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Look from the top down:
Energy Change in “crystal field”
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el
el
el
el x2-y2
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“Crystal Field Splitting Theory”
Combination of a metal ion (positive) and negative ligand molecules: 1. Electrostatic attraction 2. Metal-d electrons repulse ligand
electrons 3. Depending on the geometry of the
ligand sphere, metal d-orbitals are split in different energy levels
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Colour of complexes
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E = h * ν
Depends on the energy difference between the lower and higher metal d-orbital levels ! Visible light is absorbed and pushes electrons up => The higher Δ, the more “blue” is the light absorbed
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Green colour => red is absorbed Yellow colour => blue is absorbed
Conclusion: The energy absorbed by the green complex is lower than by the yellow complex.
Calculate the absorbed light energy – example red light absorbed (700 nm wavelength) E = h * c / λ = 6.6*10^(-34) * 3*10^8 m/s / 700 * 10^-9 m = 2.82 * 10^(-19) J or 1.76 eV Blue light absorbed (400 nm wavelength) E =
Spectrochemical Series
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The splitting energy depends on: 1. metal ion:
high charge => high splitting : Ni(3+) > Ni(2+) high period => high splitting : Pd > Ni Pt4+ > Ir3+ > Rh3+ > Co3+ > Cr3+ > Fe3+ > Fe2+ > Co2+ > Ni2+ > Mn2+
2. ligands: empirical order by measurements = “spectrochemical series”
Energy Calculations
UV/VIS spectra: scala in cm-1 = 1/λ = ν “wavenumber”
BECAUSE: the wavenumber ~ energy (h* ν)
10.000 cm-1 ≈ 120 kJ/mol ≈ 1.24 eV ≈
Example: Fe(H2O)6 2+ = d ?
Compare the energy of low and high spin ! (Δ = 10.400 cm-1 / P = 17.600 cm-1)
Is the complex dia- or paramagnetic ?
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End of Part 1 Summary: • Transition metal ions have d-electrons which can interact with electron
rich small molecules like ammonia, water, chloride ... (we call them “ligands”)
• The electronic field of the ligands around the d-orbitals cause a splitting in their energy -> for ML6 the d-orbitals split into 3 low- and 2 high-energy orbitals
• The amount of splitting increases with higher charge on the metal and with higher electron density on the ligands
• Electrons in the lower d-orbitals can jump to the high energy levels by absorbing light energy => the complex has a colour !
THANKS FOR WATCHING … SEE YOU LATER !
Coordination Numbers
How many ligands will a metal ion have ?
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Tetrahedral Complexes ML4
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In a tetrahedron, the ligands have the maximum distance to each other !
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Which set of d-orbitals comes in closer contact to the ligands (and is therefore de-stabilized) ?
(draw typical orbitals into this picture and estimate the distances)
Energy Level Splitting
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How do we expect the d-energy levels to split up in a tetrahedral field ?
How many d-electrons are OK for tetrahedral ?
Tetrahedral complexes
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Write ox.number of the metal and no. of d electrons: [MnO4]- (permanganate) [CrO4]2- (chromate) [FeCl4]2- [CoCl4]2- [NiCl4]2- [ZnCl4]2- Ni(CO)4
Which 2 factors determine the geometry ??
Square planar from Octahedral
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How would the d-orbitals split from an octahedral complex ?
How many d-electrons are OK for square-planar ?
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d-orbital energy split according to the ligand coordination:
Identify the d-orbitals for each coordination !
Trigonal bipyramidal ML5
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For example (refer to VSEPR Theory !)
Example: Ru(CO)3(PPh3)2
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Which possible structures can we draw from this compound and which one is most likely to form ?
Popular example: Iron Pentacarbonyl
Organic reactions on Cyclobutadiene:
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Coordination 2 and 3
• Occurs rarely
• Most for Cu(I), Ag(I), Au(I) and Hg(II) which are all d__ (?)
Why are these complexes variable in their coordination geometry ? example: Hg(CN)2 linear – HgI3(-) trigonal planar
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Compare d10 and d5
Both kinds of electron configuration result in zero CFSE.
What is the difference ?
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Draw the energy level diagram for both answers and state if high- or lowspin
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End of Part 2 Summary: • Two factors determine the geometry of a compound:
• CFSE • Steric influences
• Most common compounds are octahedral ML6 (middle of
the TM block) and trigonal-bipyramidal • Tetrahedral ML4 are common for the first row TM’s • Square-planar ML4 are typical for 2nd and 3rd row TM’s • d10 ions arrange ligands according to their ox.numbers and
max. distance of the ligands to each other THANKS FOR WATCHING … SEE YOU LATER !
UV/VIS Spectroscopy
LIght absorption by electron transfers
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Visible Colours
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How to convert between cm-1 and nm ? Why do we use wavenumber ν instead of wavelength λ ?
Questions • Which colour will the complex have ?
• How many electron transitions are happening and why ?
• How will the colour change if we add ammonia to the aqueous solution ? And why ?
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Compare 2 Ni-compounds
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Ni(H2O)6 (2+) Colour:
Ni(NH3)6 (2+): Colour:
Which compound absorbs light at higher energy ?
http://jchemed.chem.wisc.edu/JCEWWW/Articles/JCENi/JCENi.html
Jahn-Teller Effect
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The Jahn-Teller effect is generally only important for odd number occupancy of the eg level. The effect of Jahn-Teller distortions is best documented for Cu(II) complexes (with 3 electrons in the eg level) where the result is that most complexes are found to have elongation along the z-axis.
Student’s Projects Short Presentations (ca. 2 mins)
• Why is gold “gold” ? (explain from electron configuration)
• Gold cyanidation (Use of gold complex to extract gold from minerals)
• Tungsten as gold copy ? (find similarities and differences between the metals)
• Molybdenum disulfide as lubricant (how to produce, where to use ?)
• Permanganate as oxidizing agent (what ox.numbers are possible, where and how to use ?)
• Platinum as catalyst for exhaust gases in cars (how to produce and how does it work ?)
• Silver as antibacterial agent, for example in Samsung washing machines (use of colloidial silver as antibiotic – how it works ?)
• Highly purified nickel from nickel carbonyl (production, properties and use of nickel carbonyl)
• Heavy metal compounds as anti-knocking agents in fuels (tetraethyl Lead vs. alternatives and vs. alcohols)
• Changes in octahedral complexes by Jahn-Teller Effect (why and how is the high symmetry of octahedral complexes distorted with examples)
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MO Theory
“Ligand Field Theory” for Coordination Compounds
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