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Ch 9 Lecture 3 Constitutional Isomers and Structures
I.
Constitutional Isomers = different ligands in coordination sphere
A.
Hydrate Isomerism = Solvent Isomerism
1) Different members of inner sphere, but same overall formula
2) Different compounds with different characteristics
3) Example: CrCl3 • 6 H2O
a) [Cr(H2O)6]Cl3 = violet
b) [Cr(H2O)5Cl]Cl2 • H2O = blue-green
c) [Cr(H2O)4Cl2]Cl • 2 H2O = dark green
B.
Ionization Isomers
1) Same formula, but different ions are produced in solution
2) Ligand/Counter ion changes places
3) Solvent Isomers are an example
4) Other Examples:
a) [Co(NH3)5SO4]NO3 vs. [Co(NH3)5NO3]SO4
b) [Co(NH3)4(NO3)Cl]Cl vs. [Co(NH3)4Cl2]NO3
C.
Coordination Isomers = ratio of ligand:metal same, but ligands are attached to
metal ions in different numbers
1) [Pt(NH3)2Cl2]
2) [Pt(NH3)3Cl][Pt(NH3)Cl3]
3) [Pt(NH3)4][PtCl4]
D.
Linkage Isomers = depends on which atom of the ligand is attached to metal
1) SCN- = thiocyanato
a) Pb2+—SCN = soft/soft interaction
b) Fe3+--NCS = hard/hard interaction
2) NO2- = nitrito M—ONO vs. M—NO2
II.
Coordination Number and Structure
A.
Factors affecting the geometry of a coordination compound
1) Prediction can be difficult
2) VSEPR usually is a good first approximation; don’t count the d-electrons
3) Maximize the number of bonds (more bonds = more stable)
4) Occupancy of the d-orbitals (Chapter 10)
5) Steric interference by large ligands
6) Crystal packing interactions
a) Shape of the complex ion itself influences how it can be packed
b) Shape of solvent and/or counterions influences the packing
B.
Low coordination number compounds
1) 1-coordinate complexes
a) Cu(I) and Ag(I) complexes are
known in the solid state
b) Usually only see this in the
gas phase
c) The VO2+ species is seen, but
only transiently
2)
2-coordinate complexes
a) Cu(I) and Ag(I) complexes are known: [Ag(NH3)2]+
b) These metals are d10 and don’t require much more e- density
c) VSEPR geometry is linear
d) Sterically large ligands encourage this coordination number
e) Some d6 and d7 metal ions can also do this
N
+
Cu
N
N
N
N
N
N
N
3)
3-coordinate complexes
a) Cu(I) and Ag(I) d10 ions are again the prime examples
b) VSEPR geometry is trigonal planar
c) Large ligands are usually involved
C.
4-coordinate complexes
1)
Tetrahedral Complexes
a) Metal ions with d0 and d10 {Cu(I), Zn(II), Ag(I)} configurations are
most likely = “Inorganic Carbon”
b)
c)
d)
Filled or empty d-orbital set has no preference for geometry
Low coordination number (4) VSEPR geometry is tetrahedral
Co(II) d7 is also well-known to have tetrahedral complexes
2)
Square Planar Complexes
a) Metal ions with d8 electron configuration are main examples =
Ni(II), Pd(II), Pt(II)
b) Sometimes d9 Cu(II) complexes approach this geometry
c) Occupation of the d-orbitals causes the preference for this geometry
D.
5-coordinate complexes
1) Pentagonal Planar compounds are unknown due to steric crowding
2) Trigonal Bipyramidal and Square Pyramidal complexes are common
a) Little energy difference between the two arrangements of ligands
b) Often a distorted geometry between the two is found
c) Fluxional behavior = geometry constantly switching between the two
Examples: Fe(CO)5 and PF5 give only one NMR peak each
Both geometries present would give 2 peaks
The NMR only sees the average structure
E.
6-coordinate complexes
1) This is the most common coordination number for metal complexes
a) Allows for maximum e- donation to the cationic metal atom
b) Size of the transition metals allows about 6 molecules around it
c) All metals d0 to d10 exhibit this coordination number
2)
Octahedral Complexes
a) The VSEPR predicted geometry is most common
b)
Distortions are common
i. Elongation of trans bonds gives square planar
ii. Compression of trans bonds is called tetragonal geometry
c)
3)
Trigonal Prism and Trigonal Antiprism Geometries
Many complexes that are 4-coordinate as an individual molecule are
really 6-coordinate in the solid state
Cl Cl Cl
Cl
Cl
Cu
Cl
Cu
Cl Cl Cl
Cl
Cu
Cl
Cl
F.
7-coordinate complexes
1) Not common, but 3 different geometries are known
a) Pentagonal bipyramid
b) Capped trigonal prism
c) Capped octahedron
2)
Capped = add another ligand at the center of one face of the basic geometry
Capped Trigonal Prism
Capped Octahedron
Pentagonal Bipyramid
G.
8-coordinate and 9-coordinate complexes
1) Uncommon except for Lanthanides and Actinides, which are large enough
to allow for 8-9 molecules to surround them
2) Cube geometry is not found except in simple salts (NaCl)
3) Square Antiprism and Dodecahedron geometries known
H.
Larger coordination numbers are special cases
Square
Antiprism
Dodecahedron
Square
Antiprism
12-Coordinate
6 bidentate
Nitrate ligands
[Ce(NO3)6]3-
Tri-Capped
Trigonal
Prism
[Re(H)9]2-
Capped
Square
Antiprism
[La(NH3)9]3+