Download Transition Metal Chemistry

APPLIED INORGANIC CHEMISTRY FOR CHEMICAL ENGINEERS
Transition Metal Chemistry
CHEM261HC/SS1/01
Periodic table
q Elements are divided into four categories
Main-group
elements
Transition metals
1.Main-group elements
2.Transition metals
3.Lanthanides
4.Actinides
Main-group
element s
Lanthanides
Actinides
CHEM261HC/SS1/02
Transition metals vs. Main -group elements
Transition
metals
Main-group
elements
v Metals
• malleable and ductile
• conduct heat and electricity
• form positive ions
v Transition metals
• more electronegative than the main
group metals
• more likely to form covalent
compounds
• easily form complexes
• form stable compounds with neutral
molecules
q There is some controversy about the classification of the elements
i.e. Zinc (Zn), Cadmium (Cd) and Mercury (Hg)
CHEM261HC/SS1/03
Electron configuration of Transition-metal ions
§ The relationship between the electron configurations of
transition-metal elements and their ions is complex.
Example
§ Consider the chemistry of cobalt which forms complexes that
contain either Co 2+ or Co 3+ ions.
Co: [Ar] 4s2 3d7
Co 2+: [Ar] 3d7
Co 3+: [Ar] 3d6
v In general, electrons are removed from the valence shell s orbitals
before they are removed from valence d orbitals when transition
metals are ionized.
CHEM261HC/SS1/04
q How do we determine the electronic configuration of the central
metal ion in any complex?
•
Try to recognise all the entities making up the complex
•
Need knowing whether the ligands are neutral or anionic
•
Then you can determine the oxidation state of the metal ion.
A simple procedure exists for the M(II) case.
22
23
24
25
26
27
28
29
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
4
5
6
7
8
9
Cross off the first 2,
2
3
CHEM261HC/SS1/05
EXAMPLES
Elements
Configuration
Sc
4s23d1
V
4s23d3
Cr
4s13d5
Fe
4s23d6
Ni
4s23d8
Cu
4s13d10
Zn
4s23d10
Evaluating the oxidation state
[CoCl(NO 2)(NH 3)4]+
+1
Neutral
zero charge
x - 2 = +1
x = +3
Co3+
CHEM261HC/SS1/06
Oxidation states and their relative stabilities
v Why do these elements exhibit a variety of oxidation states?
ü Because of the closeness of the 3d and 4s energy states
Sc
+3
Ti
+1
+2
+3
+4
V
+1
+2
+3
+4
+5
Cr
+1
+2
+3
+4
+5
+6
Mn +1
+2
+3
+4
+5
+6
Fe
+1
+2
+3
+4
+5
+6
Co
+1
+2
+3
+4
+5
Ni
+1
+2
+3
+4
Cu
+1
+2
+3
Zn
+7
+2
§ The most prevalent oxidation numbers are shown in green.
CHEM261HC/SS1/07
§ An increase in the No. of oxidation states from Sc to Mn.
§ All seven oxidation states are exhibited by Mn.
§ There is a decrease in the No. of oxidation states from Mn to Zn.
WHY?
ü Because the pairing of d-electrons occurs after Mn (Hund's rule)
which in turn decreases the number of available unpaired
electrons and hence, the number of oxidation states.
§ The stability of higher oxidation states decreases in moving from
Sc to Zn.
§ Mn(VII) and Fe(VI) are powerful oxidizing agents and the higher
oxidation states of Co, Ni and Zn are unknown.
CHEM261HC/SS1/09
§ The relative stability of +2 state with respect to higher
oxidation states increases in moving from left to right. On the
other hand +3 state becomes less stable from left to right.
§ This is justifiable since it will be increasingly difficult to
remove the third electron from the d-orbital.
Example
22
23
24
25
26
27
28
29
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
M = [Ar]4s23dx
M+2 = [Ar]3dx
M+3 = [Ar]3dx-1
loss of the two s electrons
more difficult
CHEM261HC/SS1/10
Chromium
• Oxidized by HCl or H2SO4 to form blue Cr2+ ion
• Cr2+ oxidized by O2 in air to form green Cr3+
Assignment 1
ü Write down balance equations that show the
two reactions
• Cr also found in +6 state as in CrO42− and
Cr2O72− are strong oxidizer
Assignment 2
ü Use balanced equations to show that
and Cr2O72− are strong oxidizing agents
CrO42−
Assignment 1
Solution
2 Cr(S) + H2SO4(aq)
2 Cr(s) + 4 HCl (aq)
CrCl 2(aq) + 2O 2(g)
Cr2SO4)(aq)
+
H2(g)
2 CrCl 2(aq) + 2H 2(g)
CrOCl (aq)
Iron
•
Exists in solution in +2 or +3 state
•
Elemental iron reacts with non-oxidizing acids
to form Fe2+, which oxidizes in air to Fe3+
•
Brown water running from a faucet is caused by
insoluble Fe2O3
•
Fe3+ soluble in acidic solution, but forms a
hydrated oxide as red-brown gel in basic
solution
Assignment 3
ü Complete and balance the following equation
Fe2O3
+ HCl
Coordination Chemistry
q A coordination compound (complex), contains a central metal atom
(or ion) surrounded by a number of oppositely charged ions or neutral
molecules (possessing lone pairs of electrons) which are known as
ligands.
q If a ligand is capable of forming more than
one bond with the central metal atom or ion,
then ring structures are produced which are
known as metal chelates
§ the ring forming groups are described as
chelating agents or polydentate ligands.
q The coordination number of the central metal atom or ion is the total
number of sites occupied by ligands.
§ Note: a bidentate ligand uses two sites, a tridentate three sites etc.
CHEM261HC/SS1/13
Ligands
molecular
formula
Lewis
base/ligand
Lewis
acid
donor
atom
coordination
number
[Zn(CN)4]2-
CN-
Zn2+
C
4
[PtCl6]2-
Cl-
Pt4+
Cl
6
[Ni(NH3)6]2+
NH 3
Ni2+
N
6
CHEM261HC/SS1/14
Mono-dentate
Multidentate ligands
Abbreviation
en
ox2-
EDTA4-
Name
Formula
Ethylenediamine
Oxalato
Ethylenediaminetetraacetato
CHEM261HC/SS1/15
q Chelating ligands bond to metal
§
forms rings – chelate rings
Ø Five or six atoms rings are common
(i.e. including metal)
q Coordination numbers and geometries
Linear
Square planar
Tetrahedral
Octahedral
CHEM261HC/SS1/16
Nomenclature of Coordination Compounds
—
The basic protocol in coordination nomenclature is to name the
ligands attached to the metal as prefixes before the metal
name.
—
Some common ligands and their names are listed above.
—
As is the case with ionic compounds, the name of the cation
appears first; the anion is named last.
—
Ligands are listed alphabetically before the metal. Prefixes
denoting the number of a particular ligand are ignored when
alphabetizing.
—
The names of anionic ligands end in “o”; the endings of
the names of neutral ligands are not changed.
—
Prefixes tell the number of a type of ligand in the complex.
If the name of the ligand itself has such a prefix,
alternatives like bis-, tris-, etc., are used.
—
If the complex is an anion, its ending is changed to -ate.
—
The oxidation number of the metal is listed as a roman numeral
in parentheses immediately after the name of the metal.
Exercise 1
Name the following coordination complexes:
(i) Cr(NH 3)Cl3
(ii) Pt(en)Cl2
(iii) [Pt(ox)2]2-
Exercise 2
Give the structures of the following coordination complexes:
(i) Tris(acetylacetanato)iron(III)
(ii) Hexabromoplatinate(2-)
(iii) Potassium diamminetetrabromocobaltate(III)
Solutions
(i) Cr(NH3)Cl3
chromium (III)
ammine
tri chloro
Amminetrichlorochromium(III)
(ii) Pt(en)Cl2
Platinum (II) ethylenediammine dichloro
Dichloroethylenediammineplat inum(II)
(iii) [Pt(ox)2]2Platinate (II)
Dioxalatoplatinate(II)
dioxalato
Solutions
(i) Tris(acetylacetanato)iron(III)
Fe(acac) 3
Fe3+
(acac)3
(ii) Hexabromoplatinate(2-)
[PtBr6]2-
Pt
Br6
[
]2-
(ii) Potassium diamminetetrabromocobaltate(III)
K
K[Co(NH3)2Br4]
(NH3)2
Br4
3+
Co
Isomers
q Primarily in coordination numbers 4 and 6.
q Arrangement of ligands in space and also the ligands themselves.
Types
Ionization isomers
§ Isomers can produce different ions in solution
e.g.
[PtCl2(NH 3)4]Br2
D
[PtBr 2(NH 3)4]Cl2
Polymerization isomers
§ Same stoichiometry, different arrangement in space.
§ Different compounds with similar formula
e.g.
[Co(NH 3)3 (NO 2)3 ]°
[MX x B b ] n
( n = 1)
[Co(NH 3)4 (NO 2)2 ] + [Co(NH 3)2 (NO 2 )4] − ( n = 2)
[Co(NH 3)6 ]3+ [Co(NO 2)6 ] 3−
( n = 2)
CHEM261HC/SS1/17
Hydration isomers
§ Hydration isomers exist for crystals of complexes containing water
molecules
e.g.
CrCl3·6H2O
v exist in three different crystalline
forms, in which the number of
water molecules directly attached
to the Cr 3+ ion differs
[Cr(H2O) 4 Cl2]Cl·2H2O
dark green
[Cr(H2O) 5 Cl]Cl2·H2O
light green
[Cr(H2O) 6 ]Cl3
gray-blue
§ In each case, the coordination number of the chromium cation is 6
Coordination isomers
§ In compounds, both cation and anion are complex, the distribution
of ligands can vary, giving rise to isomers.
[Co(NH 3)6]3+ [Cr(CN) 6]-3
and
[Cr(NH 3)6]+3 [Co(CN) 6]-3
Linkage isomers
e.g. Nitro and nitito
N or O coordination
possible
Yellow
(a) [Co(NO 2)(NH3)5]2+
(b) [Co(ONO)(NH 3)5
]2+
Red
CHEM261HC/SS1/18
Geometric isomers
§ Formula is the same but the
arrangement in 3-D space is
different.
e.g. square
planar molecules give
cis and trans isomers.
CHEM261HC/SS1/19
For hexacoordinate systems
CHEM261HC/SS1/20
For M(X)3(Y)3 systems there is
facial and meridian
CHEM261HC/SS1/21
Stereoisomer
§ Are “stereoisomers” also possible?
§ An analogy to organic chirality.
§ Molecules which can rotate light.
§ Enantiomers
– non-superimposable mirror images
CHEM261HC/SS1/22
Complex Stabilities
§ Generally in aqueous solution, for a given metal and
ligand, complexes where the metal oxidation state is +3
are more stable than +2
§ Generally the stabilities of complexes of the first row of
transition metals vary in reverse of their cationic radii
MnII < FeII < CoII < NiII > CuII > ZnII
§ Hard and soft Lewis acid-base theory
v Hard acids and bases
tend to have:
•
small atomic/ionic radius
•
high oxidation state
•
low polarizabilty
•
high electronegativity
•
hard bases - energy low-lying HOMO
•
hard acids - energy high-lying LUMO
CHEM261HC/SS1/23
§ Chelate effect - is the additional stability of a complex
containing a chelating ligand, relative to that of a complex
containing monodentate ligands with the same type and number
of donors as in the chelate.
[Cu(H 2O)4(NH3)2]2+ + en
[Cu(H 2O)4(en)] 2+ + 2 NH 3
CHEM261HC/SS1/24
§ Mainly an entropy effect.
Cu(H2O) 4(NH3)2]2+ + en = [Cu(H2O) 4(en)]2+ + 2 NH3
§ When ammonia molecule dissociates - swept off in solution
and the probability of returning is remote.
§ When one amine group of en dissociates from complex
ligand retained by end still attached so the nitrogen atom
cannot move away – swings back and attach to metal again.
§ Therefore the complex
dissociating.
has a smaller
probability of
CHEM261HC/SS1/25
CHEM261HC/SS1/26
Metal carbonyl
§ Compounds that have the metal bonded to the carbon
monoxide, giving a general formula of M(CO) n
M + CO
M
C
O
M(CO)n
∏-orbitals in CO are very empty
Molecular orbital diagram (CO)
Molecular orbital diagram (CO)
Therefore the bond order is:
4–1=3
Bond order:
No. of e- pairs in the bonding orbital — No. of e- pairs in
the anti-bonding orbital
Back-bonding (back donation)
q Formation of ∏-bonding as a result of the overlap of metal d ∏orbitals and the ligand, CO, ∏* orbitals
Effects:
§ It enhances the bonding strength between the metal and the ligand.
§ The metal-ligand bond is shortened (M
§ The C
O
CO)
becomes longer, weaker and the bond order decreases
Evidence and extent
§ IR spectra – Vibration frequency
– The greater the extent of back bonding the lower the C
stretching frequency (bond order decreases)
Free C
O
≈ 2143 cm -1
M
O
CO ≈ 1900 - 2125 cm -1
Effect of replacing the CO ligands
Non- ∏ accepting ligands (donor ligands)
Cr(CO)6
2100 cm-1
2000 cm-1
1985 cm-1
§
Trien
Cr(triens)(CO)3
1900 cm-1
1760 cm-1
Replacement of the 3 x (CO) groups with donor ligands (trien) increases ∏acidity of the remaining ligands (CO) so as to counter the accumulation of
the negative charge on the metal centre
Effect of introducing a positive charge on metal complex
V(C O) 6 1860 cm -1
§
1 proton
V(CO )6
1 proton
2000 cm -1
V(CO) 6 +
2090 cm -1
Introducing a +ve charge on the metal inhibits shift of electrons from metal
to empty ∏*- orbital of the CO ligands
– This weakens ∏-bonding or decrease stretching frequencies of M-C
O increases. (wave number or frequenc y increases)
while the C
Thought
Ø V(CO) - and Cr(CO) are isoelectronic yet
stretching frequencies of CO in V(CO) 6
is lower than that of CO in Cr(CO) 6 ?
The origin of colour - absorption
CHEM261HC/SS1/27
Colours on coordination compounds
The colour can change depending on a number of factors
e.g.
Ø Metal charge
Ø Ligand
Physical phenomenon
CHEM261HC/SS1/29
Are there any simple theories to explain the colours in transition
metal complexes?
§ There is a simple electrostatic model used by chemists to
rationalize the observed results
This theory is called Crystal Field Theory
§ It is not a rigorous bonding theory but merely a simplistic
approach to understanding the possible origins of photoand electrochemical properties of the transition metal
complexes
CHEM261HC/SS1/30