Ch 9 Lecture 3 Constitutional Isomers and Structures

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