Download Transition Metal Chemistry

Transition Metal Chemistry
Transition metals exhibit:
1. Multiple oxidation states
2. Metal complex formation
3. Colours
4. Catalytic properties
5. Magnetic properties
1. Multiple oxidation states
Sc
Ti
V
Cr
Mn
Fe
Co
+3
+2
+3
+4
+2
+3
+4
+5
+2
+3
+2
+3
+4
+6
+7
+2
+3
+6
+2
+3
+6
Ni
+1
+2
+3
Cu
Zn
+2
+2
 +2 oxidation state due to 4s2 electrons being lost.
 As you go from left to right it is easier to remove e- from the 4s orbital vs. the 3d
orbitals
L
R
decreasing stability of maximum oxidation state
Increasing stability of +2 oxidation state
4s higher
Free neutral metal:
4s filled, then 3d
Ion formation
gap L to R
due to Zeff
3d
Why? Increasing Zeff from L to R pulls 3d orbitals closer. Therefore the gap between
4s & 3d becomes larger and more difficult to remove d e-.
Mn atom
B/c p+ > e- energy levels pulled in closer
Mn 2+ ion
Multiple oxidation states attainable via ionization energies. In order to have higher
oxidation states, there needs to be greater packets of (single) energy.
Note: removal of successive e-'s
Zeff, therefore higher amounts of energy required.
2. Complex formation
Metal complexes are also called coordination compounds
- consist of a metal bonded to electron rich molecules or ions.
Eg. [Ag(NH3)2]+
diamine silver (I) ion complex
Or
[Co(NH3)5Cl]Cl2
Metals due to their small size and positive charge, will attract –ve species and form
dative bonds (coordinate) with species that are electron rich (ligands).
Lewis acid – electron pair acceptor (M+)
Lewis base – electron pair donator (L)
Alfred Werner – known as the father of coordinate chemistry proposed an explanation
of how stoichiometric quantities of a small neutral molecule is incorporated into metal
compounds.
-Depending on the types and number of molecules bonded to a metal ion, different
shapes of molecules will be created.
M+ +
6L
ML6+
octahedral
M+ +
4L
ML4+
tetrahedral
square planar (Pt group)
M+ +
2L
ML2+
linear
The coordination #: # of lone pairs (ligands) bonded to a metal
or
# of ligands inside the brackets in a formula
-ranges between 2 to 9, but 4 or 6 most common
Eg.
[Cu(NH3)4]2+
[Fe(CN)6]4[Co(NH3)5Cl]Cl2
coord. # =
coord. # =
coord. # =
Note: Complexes can have a negative or positive charge (ion complexes) or have no
charge (neutral).
How can the overall charge of a complex be determined?
Charge of complex = sum of charge + ligands on central atom
Examples:
Complex
Shape
Coordination #
[Cu(NH3)4]2+
[Co(NH3)5Cl](NO3)2
[Pt(NH3)2]Br2]
[Ag(NH3)2]1+
Types of ligands:
1. monodentate (one-toothed): NH3, Cl-occupy only one site on coordination sphere.
2. Polydentate (many-toothed): b/c they appear to clutch the metal like a claw
Therefore called a chelating agent
-occupy 2 or more coordination sites
Ex.
NH3
F1OH2
CN1SCN1-
- monodentate
- monodentate
- monodentate
- monodentate
- monodentate, but can form bonds via S or N
[ S = C = N ]
Ethylenediamine (en) -bidentate
Oxalate ion (C2O4)2-
EDTA4- (ethylenediaminetetraacetate ion) - food additive
Chelating agents – prevent rxns that may interfere by chelating to unwanted metals
Eg. Na2[Ca(EDTA)] used in lead poisoning.
Porphyrins: porphine + Fe = heme
Chelate effect: chelating agents form more stable complex than just monodentates
Naming – brief overview
1. cation first and 2nd anion
2. ligands in alpha order ending in "o" before metals
3. anion ligands end in "ate"
[Co(NH3)5Cl]Cl2
- pentaminechlorocobalt (III) chloride
Na2[MoOCl4]
- sodium tetrachlorooxomolybdate (IV)
-due to the various numbers of ligands bonded to the central metal – isomerism is also
exhibited in complexes
Isomers
Structural isomers
Linkage
Coordination sphere
Stereoisomers
Geometric
Optical
Linkage isomerism (rare) – same lewis bases linked but not same atom
- ligand capable of coordinating to a metal in 2 different ways
Eg. NO2
Nitro (NO2)-
nitrito (ONO)-
Yellow
red
Coordination sphere isomers – ligands directly bonded to metal as opposed to being
outside coordination sphere in solid lattice.
[Cr(H2O)6]Cl3
[Cr(H2O)5Cl]Cl2 . H2O
[Cr(H2O)4Cl2]Cl . 2H2O
violet
green
green
or
[Co(NH3)4Cl2]Br
[Co(NH3)4ClBr]Cl
Geometric isomers
Cis-platin
Also,
Trans-platin
Cis
MA4B2 – cis/trans in octahedral
MA3B3 – mer/fac in octahedral
fac – facial
mer- meridinal
Trans
Optical Isomers – chiral complexes that are non-superimposable (mirror images)
Ex. [Co(en)3]3+
Enantiomers
-affect polarized light by rotating it to the left (lerorotary) or the right (dextrorotary)
-regular rxns produce racemic mixture of the isomers. Therefore, D & L made is 50:50
3. Colours
What causes colour?
a. type of transition metal (d-orbital splitting energies)
b. oxidation state of transition metal
c. type of ligand
4. Catalytic Properties
- speed up rxn rate without being consumed (an alternative pathway for rxn)
- Lower Ea
Heterogeneous Catalysts – catalyst in a different phase from reactants and products
Ex. Nickel metal and ethane
Homogeneous – same phase (reactants and products)
5. Magnetism
-compounds with one or more unpaired e- are paramagnetic (attracted to a pole
of a magnet)
-force of attraction between complex & magnet of unpaired e- in complex