Bonding in Transition Metals and Coordination Complexes
Jan 16, 2016
Bonding in Transition Metals and Coordination Complexes
Bonding in Transition Metals and Coordination Complexes
Chemistry of the Transition metals
Properties
Atomic Radius : lanthanide contraction – unusual contraction of lanthanide ions.
Binding energy: higher – more unpaired electrons i.e.) m.p. -- higher in the middle of the row : W ( 3410oC), Hg (-39oC)
Oxidation states: higher oxidation state– more covalent bond character lower oxidation state – more ionic bond character
Mn(OH)2, Mn(OH)3, H2MnO3, H2MnO4, HMnO4
basic acidic
Chemistry of the Transition metals
Coordination complex ; coordination chemistry 배위화학
CuSO4 : greenish white
Coordination complex : Cu(H2O)42+
CuSO4.4H2O : bluev.s.
Mn+ + mL M Lmn+
coordinationLigand
Lewis baseLewis acid
Ag+ + 2NH3 (Ag(NH3)2+
Au+ + 2CN- (Au(CN)2-
Coordination number : total number of metal-to-ligand bondUsually 2 ~ 6
Ligands
Br-
Cl-
N3-
CN-
OH-
NH3
H2O
CO
NO2-
O2-
Bromo
Chloro
Azido
Cyano
Hydroxo
Ammine
Aqua(o)
Carbonyl
Nitro
Oxo
H-
ONO-
SCN-
NCS-
NO+
CO32-
Hydrido
Nitrito
Thiocyanato
Isothiocyanato
Nitrosyl
Carbonato
OxalateO
-O
O
O-
H2N NH2Ethylenediamine (En)
bidentateligand
chelates
Nomenclature
NH4[Cr(NH3)2(NCS)4] Reinecke’s salt
Systematic naming
[Co(NH3)5Cl]Cl2 Purpureocobaltic chloride
[Co(NH3)5Cl]Cl2 Pentaamminechlorocobalt(III) chloride
K4[Fe(CN)6] Potassium Hexacyanoferrate(II)
Making coordination complex
charge of a complex = sum of charges of metals and ligandscharge of a complex + charges of counter ions = 0
coordination number = numbers of donor atoms
Rules of Nomenclature
1. Cation Anion b
2. In the complex : names of ligands come first and then name of metal among ligands : alphabetical order
3. Names of ligands : anion – change the last letter to o neutral – same as the original ones
4. Counting number of ligands : di, tri, tetra, penta, hexa, hepta….. if the ligand contains these names in it, use : bis, tris, tetrakis, pentakis……
5. If the compex is an anion : at the end of the name put ate
6. Oxidation number of metal : in parenthesis with roman letter - (IV)
Pentaamminechlorocobalt(III) chloride Potassium Hexacyanoferrate(II)
Influence of Coordination
1. Color
Pale yellow
[Fe(H2O)6]3+ + SCN- [Fe(H2O)5SCN]2+ + H2O
orange
2. Reduction potential
Ag+ + e- Ag Eo = +0.799V
[Ag(CN)2]+ + e- Ag Eo = -0.31V+ 2CN-
3. Chemical reactivity
Structure of coordination complexes
CoCl3.6NH3
CoCl3.5NH3
CoCl3.4NH3
CoCl3.3NH3
Chemical formular(19thC.) color
orange-yellow
pruple
green
green
[Co(NH3)6]3+Cl-3
Chemical formular (Werner)
[Co(NH3)5Cl]2+Cl-2
[Co(NH3)4Cl2]+Cl-
[Co(NH3)3Cl3]
structure
octahedral
[Co(NH3)6]3+Cl-3
[Co(NH3)5Cl]2+Cl-2
[Co(NH3)4Cl2]+Cl-
Cl +2
[Co(NH3)3Cl3]
[Co(En)2Cl2]+Cl-
cis transGeometrical isomers
Chiral structures
[Co(NH3)2(H2O)2Cl2]+
[Pt(En)3]4+
Structure of coordination complexes
Linear[Ag(NH3)2]+
[Zn(NH3)4]2+
Atomic orbitalof metal
coordinationnumber
2
structure
Tetrahedral4
[Pt(NH3)4]2+ Square Planar4
[Co(NH3)6]3+ Octahedral6
d10
d9
d8
d6
Super chelating ligand
EDTA ( ethylenediaminetetraacetate)
Strong affinity to certain metal ionsSolubilize metal ions
[Ni(H2O)6]2+ + 6NH3 [Ni(NH3)6]2+ + 6H2O
Kf = 4 x 108
[Ni(H2O)6]2+ + 3en [Ni(en)3]2+ + 6H2O
Kf = 2 x 1018
Entropy factor : bigger S
Transition Metals
Partially filled d orbitals
Octet rule in transition metal chemistry : 18 electron rule
Coordination complexStructural variety
Low-lying unoccupied orbitals color
Unpaired electrons Magnetic property
Many oxidation states Catalysts, new reactions
Ligands Donates electron pairs
coordination Changes color, reactivity, reduction potential
number of electrons in 4s + 3d + 2 x number of ligands = 18
18-electron rule for transition metal complexes
Octet rule : Lewis structure
consider a transition metal : Cr
Chromium: [Ar] (4s)2(3d)4 6 valence electrons
Chromium need 18 electrons in its most outer shell. 18-electron rule
Therefore the complex of Cr with CO will look like
i.e. CO provides 2 x 6 = 12 electrons Cr provides 6 electrons
Total 18 electrons
Using the 18-electron rule
Given that H2Fe(CO)x exists, what does x equal?
Iron: [Ar] (4s)2(3d)6 8 valence electrons 8
n = 4
Total : 10 + 2n = 18 electrons
hydrogen: 1s1 1 valence electrons x 2 = 2
CO: 2 valence electrons x n = 2n
H2Fe(CO)4
Understanding of metal-ligand binding mode
1. Color : only for partially filled d orbitals i.e. d0, d10 : colorless
facts
2. magnetism: paramagetic v.s. diamagnetic unpaired electrons
[Co(NH3)6]3+ diamagnetic – no unpaired electrons
[CoF6]3- paramagnetic – 4 unpaired electrons
[CrF6]3- [Cr(H2O)6]3+ [Cr(NH3)6]3+
[Cr(CN)6]3-
green violet yellow yellow
3. tetrahedral or square planar
[NiCl4]2- [Ni(CN)4]2-
tetrahedral square planar
Crystal Field Theory
Color, magnetic properties, and choice of tetrahedral, square planar & octahedralHow to explain
Crystal field theory : ionic description of the metal-ligand bonds
Consider only the energy changes of d orbitals of metal during coordination
Consider only electrostatic interaction with ligands : charge-charge, charge-dipole
Begin with octahedral geometry
Low spin complex : when o is large
High spin complex : when o is small
magnetism
d1 ~ d5 : always paramagnetic
d7 ~ d9 : always paramagnetic
d10 : always diamagnetic
d6 : depending on the ligands
Square planar & tetrahedral complexes
Tetrahedral Reversal of octahedral !
M
Square planar & tetrahedral complexes
Tetrahedral
Reversal of octahedral !
Square planar & tetrahedral complexes
Square planar Removal of axial ligands from octahedral
Square planar Removal of axial ligands from octahedral
I- < Br- < Cl- < F-, OH- < H2O < NCS- < NH3 < en < CO, CN-
Spectrochemical Series
Color of complexes and magnetic properties are determined by o
o can be determined by ligands
Weak field Ligands Strong field Ligands
small olarge o
High spin complex Low spin complex
[CoF6]3- [Co(CN)6]3-v.s.
Weak point of crystal field theory
1. Coordination is not fully ionic.
2. Spectrochemical series is all empirical. Ligand field theory
Ligand Field Theory
Consider ionic interaction onlyCrystal field theory :
Modification : addition of covalent aspect of coordination.
How? : construction of molecular orbitals.
Using 4s, 4p, 3d orbitals of metals & coordinating orbitals of ligands
Ligand field theory :
For an octahedral complex
1. Orbital overlap is 0 for dxy, dyz, dzx
become nonboinding orbitals
2. Varying overlapping ligand orbitals
for Cl- : p NH3 : sp3
3. MO’s can be formed from 6 ligand orbitals and 6 metal orbitals (4s, 4p, 3d)
For [CoF6]3-
I- < Br- < Cl- < F-, OH- < H2O < NCS- < NH3 < en < CO, CN-
Now, we can explain Spectrochemical Series
Weak field Ligands Strong field Ligands
small olarge o
Interaction between dxy of metal and py of halide :
ionic ---- increases energy level of t2g
Makes smaller o for I- and less smaller one for F-
back-bonding
bonding of ligand can overlap with dxy orbital
Lowers the energy level of t2g
Makes larger o for CO, CN-
For [CoF6]3-
Organometallic compounds and Catalysis
Catalytic converter : Pt catalyst
CO + O2 CO2
Pt ( cat.)
Haber process
N2 + 3H2 NH3
Fe ( cat.)
Olefin metathesis reaction
+Ru ( cat.)X
Y
Z
W
+
ZX
WY“Grubbs catalyst”
2005 Nobel Prize in Chemistry
Coordination complexes and Life
We need transition elements for life : V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo…….
Porphine structure Important for oxygen transfer, detoxification,
photosynthesis, nitrogen fixation
숙제18 장 : 6, 12, 22, 26, 34, 42, 46
제출일 : 10 월 19 일