Download Coordination complexes

Coordination Chemistry
• General aspects (Ch. 9)
• Bonding (Ch. 10)
• Electronic spectra (Ch. 11)
• Reaction mechanisms (Ch. 12)
Acids and bases (the Lewis concept)
A base is an electron-pair donor
An acid is an electron-pair acceptor
acid
adduct
base
Lewis acid-base adducts involving metal ions
are called coordination compounds (or complexes)
Coordination complexes
Coordinated ligands
Central metal atom
Solv
+n
L
L
L
L
M
L
[A-]n
Solv
L
L
L
Inner coordination sphere counteranion
L
Solv
Solv
M
L
Solv
Inner coordination sphere
The metal cation is the Lewis acid, the ligands are the Lewis bases
Naming coordination complexes
General nomenclature rules in coordination chemistry
1. Cation first, then anion (as for simple salts) (K3[Fe(CN)6], potassium hexacyanoferrate)
2. Inner coordination sphere in square brackets in formula. Ligands named before the metal
Hexaaminecobalt(III) chloride: [Co(NH3)6]Cl3
3. Number of ligand indicated by prefix (di,tri,tetra or bis, tris, tetrakis if ligand in parenthesis)
tris(bipyridine)iron(II) chloride: [Fe(bipy)3]Cl2
1. Ligands named in alphabetical order ignoring prefix
2. Anionic ligands are given the suffix -o (chloro-, sulfato-, nitrato-)
while neutral ligands retain name (except aqua for H2O and ammine for NH3)
6. Metal named after ligands with oxidation state in roman numerals or give overall charge of
coordination sphere
Fe(III), tetrachloroplatinate(-2)
7. Cis (adjacent)-trans (opposite) or fac (C3v) –mer (C2v) isomers are indicated with prefix
8. Bridging ligands are indicated with m (greek mu)
m-oxo for M-O-M
7. If complex is anionic, use ending “-ate”
-cobaltate, ruthenate, but note ferrate for Fe, argentate for Ag, plumbate for Pb,
stannate for Sn and aurate for Au
Isomerism
• Stereoisomers (enantiomers, diastereomers,
cis/trans, mer/fac, conformational) have
same metal ligand bonds but different 3D
arrangement.
• Hydrate (solvate) isomers, ionization,
linkage, coodination isomers have different
metal-ligand bonds.
Examples of Four Coordinate
Stereoisomers
planar
NH3
NH3
Cl
Pt
Cl
Cl
NH3
Cl
NH3
trans
Pt
cis
Tetrahedral, chirality now possible.
Four different monodentate ligands.
stereoisomers
Chirality in tetrahedral complexes
Very common
L4
L1
L1
M
M
L2
L3
L2
L3
(2 enantiomers if all ligands different)
L4
Examples of Six coordinate
Stereoisomers
How many stereoisomers are there of formula Mabcdef?
For the six sites in the octahedron there are 6! = 6 * 5 * 4 * 3 * 2 * 1
ways of positioning the ligands.
However some of these ways are the same structure; simply rotated.
An octahedron has many rotations which simply interchange ligands: 8
C3, 6 C2, 6 C4 and 3 C2. Thus there are 23 rotated structures to be
generated from an original structure. 6!/(23+1) = 30 stereoisomers.
For some complexes with multidentate ligands there are geometry
constraints which reduce the number of isomers.
Chirality in octahedral complexes
a
c
b
a
a
a
b
b
c
c
b
b
c
c
b
c
a
c
a
b
a
a
c
b
a
a
a
a
b
c
c
b
b
b
b
a
c
c
a
a
c
c
b
b
c
non-chiral
b
a
c
chiral
Multidentate ligands and isomer
count.
Let AA be a multidentate ligand which must bond cis.
For octahederal complex MAAbcde how many stereoisomers?
Permutation count is not 6! but
6 * 4 *4!
# stereoisomers = 6 * 4 *4!/(24*2)
Only four spots
for the second
A to enter.
M
A
For a complex MAABCde
Due to
rotations
Since both
ends of the
AA are the
same.
Rotation factor
Due to A-A
symmetric ligand
For a complex MAABCde
Number of stereoisomers = 6 * 4 * (2 *2 * 2! + 2*3 *2!)/(24 * 2) = 10 stereoisomers
B
Assign first A and
second A in cis position
A
M
A
A
M
A
B
Chirality in octahedral complexes with chelating ligands
Cl
N
N
non-chiral
Co
N
N
Cl
N
N
N
Cl
Cl
Cl
Cl
Co
Co
N
N
N
N
N
chiral
Several chelate rings and chirality
N
N
N
N
N
M
N
N
M
N
N
N
N
N
D isomer
L isomer
Right hand screw
Left hand screw
Conformational Isomers
The chelate rings can have alternative
conformations.
Constitutional Isomers
• Hydrate Isomers: in crysal structure is water
part of the first ligand shell or a hydrate
–
–
–
–
[Cr(H2O)6]Cl3, violet
[CrCl(H2O)5]Cl2.H2O, blue-green
[CrCl2(H2O)4]Cl.2H2O, dark green
[CrCl3(H2O)3].3H2O, yellow green
3+
OH2
H2 O
Cr
2+
Cl
OH2
H2 O
OH2
H2 O
violet
3Cl-
H2 O
Cr
H2 O
OH2
OH2
H2 O
green
+
Cl
-
2Cl
H2 O
Cr
H2 O
OH2
OH2
Cl
green
Cl-
Constitutional Isomers
• Hydrate Isomers: in crysal structure is water
part of the first ligand shell or a hydrate
–
–
–
–
[Cr(H2O)6]Cl3, violet
[CrCl(H2O)5]Cl2.H2O, blue-green
[CrCl2(H2O)4]Cl.2H2O, dark green
[CrCl3(H2O)3].3H2O, yellow green
• Ionization isomerization: different ions produced
in solution
– [Co(NH3)5SO4]NO3 & [Co(NH3)5NO3] SO4
• Coordination Isomers: More than one ratio
of ligand can exist but maintaining overall
ratio
– [Pt(NH3)2Cl2]
– [Pt(NH3)3Cl] [Pt(NH3)Cl3]
• Linkage (ambidentate) isomerism
– Thiocyanate, SCN-, can bind through either
the N (to hard acids) or through S (to soft
acids).
– Nitrite, NO2-, can bond through either the N or
the O
Typical coordination numbers and structures
of coordination complexes
and isomerism
Coordination number 1
Very rare, bulky ligands, linear structures, no possible isomers
Coordination number 2
Also rare, typical of d10, linear structures, no possible isomers
Coordination number 3
Also typical of d10, trigonal planar structures (rarely T-shaped), no possible isomers
Coordination number 4
Very common
L1
L2
M
L1
L1
L2
cis
M
L4
L2
L1
L3
L2
M
L2
Tetrahedral
L1
trans
(2 enantiomers if all ligands different)
Square planar
(2 geometrical isomer
for two types of ligands)
typical of d8
Tetrahedral
Square planar
Coordination number 5
La
La
Le
Le
M
Le
Lb
Lb
M
Lb
Lb
La
Trigonal bipyramidal (tbp)
Square-based pyramidal sbp)
Very similar energies, they may easily interconvert in solution (fluxionality)
Coordination number 6
M
M
Octahedral
most common
Trigonal prism
less common
Some possible isomers in octahedral complexes
A
A
B
A
B
B
M
M
B
B
B
B
A
B
cis-MA2B4
trans-MA2B4
A
A
A
B
M
M
B
B
B
A
B
A
B
A
fac-MA3B3
mer-MA3B3
Some examples of trigonal prismatic structures
Coordination number 7
M
M
M
Pentagonal bipyramidal
Capped octahedral
Capped
trigonal prismatic
Examples of coordination number 7