Download Chemistry 1000 Lecture 23: Introduction to transition metal chemistry

Chemistry 1000 Lecture 23:
Introduction to transition metal chemistry
Marc R. Roussel
The transition metals
I
d and f blocks of periodic table
Sc
[Ar]3d1 4s2
Y
[Kr]4d1 5s2
Lu
[Xe]4f14 5d1 6s2
La
[Xe]5d1 6s2
Ac
[Rn]6d1 7s2
Ti
[Ar]3d2 4s2
Zr
[Kr]4d2 5s2
Hf
[Xe]4f14 5d2 6s2
Ce
[Xe]4f2 6s2
Th
[Rn]6d2 7s2
V
[Ar]3d3 4s2
Nb
[Kr]4d4 5s1
Ta
[Xe]4f14 5d3 6s2
Pr
[Xe]4f3 6s2
Pa
[Rn]5f2 6d1 7s2
Nd
[Xe]4f4 6s2
U
[Rn]5f3 6d1 7s2
Cr
[Ar]3d5 4s1
Mo
[Kr]4d5 5s1
W
[Xe]4f14 5d4 6s2
Pm
[Xe]4f5 6s2
Np
[Rn]5f4 6d1 7s2
Mn
[Ar]3d5 4s2
Tc
[Kr]4d5 5s2
Re
[Xe]4f14 5d5 6s2
Sm
[Xe]4f6 6s2
Pu
[Rn]5f6 7s2
Eu
[Xe]4f7 6s2
Am
[Rn]5f7 7s2
Fe
[Ar]3d6 4s2
Ru
[Kr]4d7 5s1
Os
[Xe]4f14 5d6 6s2
Gd
[Xe]4f7 5d1 6s2
Cm
[Rn]5f7 6d1 7s2
Co
[Ar]3d7 4s2
Rh
[Kr]4d8 5s1
Ir
[Xe]4f14 5d7 6s2
Tb
[Xe]4f9 6s2
Bk
[Rn]5f9 7s2
Ni
[Ar]3d8 4s2
Pd
[Kr]4d10
Pt
[Xe]4f14 5d9 6s1
Dy
[Xe]4f10 6s2
Cf
[Rn]5f10 7s2
Ho
[Xe]4f11 6s2
Es
[Rn]5f11 7s2
Cu
[Ar]3d10 4s1
Ag
[Kr]4d10 5s1
Au
[Xe]4f14 5d10 6s1
Er
[Xe]4f12 6s2
Fm
[Rn]5f12 7s2
Zn
[Ar]3d10 4s2
Cd
[Kr]4d10 5s2
Hg
[Xe]4f14 5d10 6s2
Tm
[Xe]4f13 6s2
Md
[Rn]5f13 7s2
Yb
[Xe]4f14 6s2
No
[Rn]5f14 7s2
Chemistry of the transition metals
I
Electronegativities range from 1.1 (La, Ac) to 2.4 (W, Au)
I
Many transition metal (TM) compounds are essentially
covalent
I
TM atoms/ions typically act as Lewis acids
I
Rough rule: TMs form ionic compounds in lower oxidation
states, covalent compounds in higher oxidation states
[Ar]3d104s2
[Ar]3d104s1
[Ar]3d8 4s2
[Ar]3d7 4s2
[Ar]3d6 4s2
[Ar]3d5 4s2
[Ar]3d5 4s1
[Ar]3d3 4s2
[Ar]3d2 4s2
[Ar]3d1 4s2
I
Common and less common oxidation states:
Sc Ti V Cr Mn Fe Co Ni Cu Zn
+1
+2
+3
+4
+5
+6
+7
Complexes and ligands
Coordinate bond: bond made between a metal ion and a Lewis
base
This can almost always be considered to be an
ordinary polar covalent bond. The special name only
serves to emphasize that coordinate bonds are
typically easier to rearrange than other covalent
bonds.
Coordination complex: complex molecule or ion consisting of a
central metal atom or ion acting as a Lewis acid with
coordinate bonds to one or (usually) several Lewis
bases
Ligand: one of the Lewis bases in a coordination complex
Complexes and ligands (continued)
Coordination number: number of coordinate bonds formed by a
metal centre
A given metal ion in a given oxidation state typically
has a preferred coordination number found in most of
its compounds.
I
Coordination complexes do not obey the VSEPR rules.
Rather, the shape is connected to the coordination number.
I
I
Coordination number 6 gives octahedral complexes.
Coordination number 4 gives either tetrahedral or square
planar complexes.
Example: Chloro-anions
I
Many metal ions form complex anions with chloride ions.
I
Some examples:
Complex
[CdCl4 ]2−
[HgCl4 ]2−
[PtCl4 ]2−
[PbCl3 ]−
[AgCl2 ]−
[CuCl3 ]2−
Metal oxidation state
2
2
2
2
1
1
Example: Ammine-cations
I
Many metal ions form complex cations with ammonia.
I
Some examples:
Complex
[Co(NH3 )6 ]3+
[Co(NH3 )6 ]2+
[Ni(NH3 )6 ]2+
[Cd(NH3 )4 ]2+
[Cu(NH3 )4 ]2+
Metal oxidation state
3
2
2
2
2
Square bracket notation
I
In the preceding examples, you may have noticed that
coordination complexes are shown in square brackets.
I
This is important since it makes the connectivity clear: The
metal atom/ion and its ligands are shown in square brackets
together.
Complex salts
A complex ion is a coordination complex with a net charge.
A complex salt is a compound in which at least one of the ions is
a complex ion.
Examples with simple counterions:
I Na [CdCl ]
2
4
I [Co(NH ) ]Cl
3 6
3
Note that we list the cation first, then the anion.
The following pairs of compounds are different:
I [Co(NH ) Br]SO and [Co(NH ) SO ]Br
3 5
4
3 5
4
I [Co(NH ) ][Cr(CN) ] and [Cr(NH ) ][Co(CN) ].
3 6
6
3 6
6
Monodentate ligands
Monodentate: literally, having one ‘tooth’
I These are ligands that coordinate only once to a
metal.
I
Examples of monodentate ligands:
F− , Cl− , Br− , I− , OH− , H2 O, NH3 , CO
I
Note that these are all Lewis bases, but not necessarily
Brønsted bases.
For example, Cl− is a Lewis base, but it is not a Brønsted
base.
How do we know that Cl− is not a Brønsted base?
Polydentate ligands
Bidentate: coordinates twice to a metal, i.e. can donate lone
pairs from two different atoms
Polydentate: coordinates more than once to a metal
Denticity: number of donor atoms in a ligand through which it
coordinates
Polydentate ligands (continued)
Structure
..
:O:
..
O
..
Denticity
Abbreviation
2−
C
1 or 2
CO2−
3
2
ox2−
2
en
2
phen
:O:
..
:O: 2−
:O:
C
C
:O:
..
:O:
..
H 2C
CH2
H2 N
..
N
..H2
N
..
..N
C
CH2
..
N
..
:O
..
C
C H2
:O:
4−
:O:
:O:
..
:O
..
C H2
CH 2
CH 2
C
..
O:
..
CH 2
C
..
O:
..
..
N
:O:
6
EDTA4−
Chelates
Chelate: a complex in which a bidentate or polydentate ligand
forms a closed ring with a metal atom by forming
two or more coordinate bonds
Chelating agent: a ligand that can form chelates
Examples of chelates:
I
Chelates tend to be much more stable than similar complexes
containing monodentate ligands (chelate effect).
Example:
2+
Cu2+
(aq) + 4NH3(aq) [Cu(NH3 )4 ](aq)
2+
Cu2+
(aq) + 2en(aq) [Cu(en)2 ](aq)
K = 1.1 × 1013
K = 1.0 × 1020
I
Chelation makes ions unavailable in solution.
Applications:
Added to detergents to reduce water hardness
(EDTA)
Food additive to prevent catalysis of oxidation by
metal ions (EDTA)
Chelation therapy for metal poisoning, esp.
hypercalcemia, mercury or lead
poisoning (dimercaptosuccinate)
..
:S
:O:
..
:O
..
C
..
:O:
CH
CH
C
:S
..
:O:
H
2−
H
Isomerism
Isomers are compounds with the same chemical formula
(same atoms), but arranged differently.
Structural isomers differ in what is bonded or coordinated to what.
Stereoisomers have identical chemical bonds, but are arranged
differently in space.
Linkage isomerism: an example of structural isomerism
Linkage isomers contain a ligand coordinated to the metal centre
through different donor atoms.
Example:
O
O
2+
N
N
H3N
Co
H3N
O
O
NH3
NH3
NH3
yellow-orange
H3N
and
Co
H3N
NH3
NH3
NH3
red-orange
2+
Some types of stereoisomerism
Geometrical isomers have identical bonds, but the distances
between some nonbonded atoms are different due to
a different arrangement of the bonds in space.
I
In square planar complexes with chemical formula MX2 Y2 or
in octahedral complexes with formula MX2 Y4 , the two X
ligands can be adjacent (cis) or opposed (trans).
Example:
NH3
Cl
Cl
Pt
NH3
NH3
cis
(cisplatin)
and
Cl
Pt
Cl
NH3
trans
(transplatin)
Example:
+
NH3
H 3N
Co
Cl
Cl
H 3N
NH3
cis
+
NH3
Cl
NH3
Co
and
Cl
H 3N
NH3
trans
I
In octahedral complexes with formula MX3 Y3 , the three X
ligands can be in the same face of the octahedron (fac) or
along a meridian (mer):
Example:
NH3
H 3N
NH3
O 2N
NH3
Co
O 2N
NH3
Co
NO 2
NO 2
fac
and
O 2N
NO 2
NH3
mer