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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

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ch 9 lecture 3 constitutional isomers and structures
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
slide2
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
slide3
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

slide4
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
slide5
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
slide6
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
slide7
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
slide8
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

slide9
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
slide10
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

slide11
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

slide12
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