1. Ch 9 Lecture 3 Constitutional Isomers and Structures
Constitutional Isomers = different ligands in coordination sphere
Hydrate Isomerism = Solvent Isomerism
Different members of inner sphere, but same overall formula
Different compounds with different characteristics
Example: CrCl3 • 6 H2O
[Cr(H2O)6]Cl3 = violet
[Cr(H2O)5Cl]Cl2 • H2O = blue-green
[Cr(H2O)4Cl2]Cl • 2 H2O = dark green
Ionization Isomers
Same formula, but different ions are produced in solution
Ligand/Counter ion changes places
Solvent Isomers are an example
Other Examples:
[Co(NH3)5SO4]NO3 vs. [Co(NH3)5NO3]SO4
[Co(NH3)4(NO3)Cl]Cl vs. [Co(NH3)4Cl2]NO3

2. Coordination Isomers = ratio of ligand:metal same, but ligands are attached to metal ions in different numbers
[Pt(NH3)2Cl2]
[Pt(NH3)3Cl][Pt(NH3)Cl3]
[Pt(NH3)4][PtCl4]
Linkage Isomers = depends on which atom of the ligand is attached to metal
SCN- = thiocyanato
Pb2+—SCN = soft/soft interaction
Fe3+--NCS = hard/hard interaction
NO2- = nitrito M—ONO vs. M—NO2

3. Coordination Number and Structure
Factors affecting the geometry of a coordination compound
Prediction can be difficult
VSEPR usually is a good first approximation; don’t count the d-electrons
Maximize the number of bonds (more bonds = more stable)
Occupancy of the d-orbitals (Chapter 10)
Steric interference by large ligands
Crystal packing interactions
Shape of the complex ion itself influences how it can be packed
Shape of solvent and/or counterions influences the packing
Low coordination number compounds
1-coordinate complexes
Cu(I) and Ag(I) complexes are
known in the solid state
Usually only see this in the
gas phase
The VO2+ species is seen, but
only transiently

4. 2-coordinate complexes
Cu(I) and Ag(I) complexes are known: [Ag(NH3)2]+
These metals are d10 and don’t require much more e- density
VSEPR geometry is linear
Sterically large ligands encourage this coordination number
Some d6 and d7 metal ions can also do this

5. 3-coordinate complexes
Cu(I) and Ag(I) d10 ions are again the prime examples
VSEPR geometry is trigonal planar
Large ligands are usually involved

6. 4-coordinate complexes
Tetrahedral Complexes
Metal ions with d0 and d10 {Cu(I), Zn(II), Ag(I)} configurations are most likely = “Inorganic Carbon”
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

7. Square Planar Complexes
Metal ions with d8 electron configuration are main examples = Ni(II), Pd(II), Pt(II)
Sometimes d9 Cu(II) complexes approach this geometry
Occupation of the d-orbitals causes the preference for this geometry

8. 5-coordinate complexes
Pentagonal Planar compounds are unknown due to steric crowding
Trigonal Bipyramidal and Square Pyramidal complexes are common
Little energy difference between the two arrangements of ligands
Often a distorted geometry between the two is found
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

9. 6-coordinate complexes
This is the most common coordination number for metal complexes
Allows for maximum e- donation to the cationic metal atom
Size of the transition metals allows about 6 molecules around it
All metals d0 to d10 exhibit this coordination number
Octahedral Complexes
The VSEPR predicted geometry is most common
Distortions are common
Elongation of trans bonds gives square planar
Compression of trans bonds is called tetragonal geometry

10. Trigonal Prism and Trigonal Antiprism Geometries
3) Many complexes that are 4-coordinate as an individual molecule are really 6-coordinate in the solid state

11. 7-coordinate complexes
Not common, but 3 different geometries are known
Pentagonal bipyramid
Capped trigonal prism
Capped octahedron
Capped = add another ligand at the center of one face of the basic geometry
Capped Trigonal Prism
Capped Octahedron
Pentagonal Bipyramid

12. 8-coordinate and 9-coordinate complexes
Uncommon except for Lanthanides and Actinides, which are large enough to allow for 8-9 molecules to surround them
Cube geometry is not found except in simple salts (NaCl)
Square Antiprism and Dodecahedron geometries known
Larger coordination numbers are special cases
Square
Antiprism
Dodecahedron
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+
Square
Antiprism