Coordination Chemistry: Isomerism and Structure

Review of the Previous Lecture Acid and Base Theories Lewis Definition: Includes adduct formation reactions Hard and Soft Acids and Bases: Helps identify the “why” behind the affinity of species 2. Introduction to Coordination Chemistry Metals as Lewis Acids and Ligands as Lewis Bases Alfred Werner began the field with his “Werner Cobalt Complexes” Nomenclature rules 3. Thermodynamics of metal ligand interactions Distinguished between affinity constants for one step metal-ligand binding versus cumulative steps Factors that drive metal ligand complex formation HSAB Theory Chelate Affect Ligand binding can finetune metal redox properties C

1. Isomerism

A. Constitutional Isomers Linkage (Ambidentate) Isomers A ligand can bind in more than one way [Co(NH3)5(NO2)]2+ Co-NO2 Nitro isomer; yellow compound Co-ONO Nitrito isomer; red compound The binding at different atoms can be due to the hard/soft-ness of the metal ions SCN- Hard metal ions bind to the N Soft metal ions bind to the S

A. Constitutional Isomers II. Ionization Isomers Difference in which ion is included as a ligand and which is present to balance the overall charge [Co(NH3)5Br]SO4 vs [Co(NH3)5SO4]Br III. Solvate (Hydrate) Isomers The solvent can play the role of ligand or as an additional crystal occupant [CrCl(H2O)5]Cl2· H2O vs [Cr(H2O)6]Cl3

A. Constitutional Isomers IV. Coordination Isomers Same metal Formulation- 1Pt2+ : 2NH3 : 2 Cl- [Pt(NH3)2Cl2] [Pt(NH3)3Cl][Pt(NH3)Cl3] [Pt(NH3)4][PtCl4] Same metal but different oxidation states Formulation- 1Pt2+ : 1Pt4+ : 4NH3 : 6 Cl- [Pt(NH3)4][PtCl6] +2 +4 [Pt(NH3)4Cl2][PtCl4] +4 +2 Different Metals Formulation- 1Co3+ : 1Cr3+ : 6NH3 : 6 CN- [Co(NH3)6][Cr(CN)6] [Co(CN)6][Cr(NH3)6]

B. Stereoisomers Square planar complex Enantiomers Optical isomers (chiral) Non-superimposable mirror image Recall from group theory, something is chiral if Has no improper rotation axis (Sn) Has no mirror plane (S1) Has no inversion center (S2) Square planar complex If it were tetrahedral, it would not be chiral.

B. Stereoisomers cis (anticancer agent) trans II. Diastereomers Geometric isomers 4-coordinate complexes Cis and trans isomers of square-planar complexes (cis/transplatin) Chelate rings can enforce a cis structure if the chelating ligand is too small to span the trans positions (constrained bite angle) cis (anticancer agent) trans

B. Stereoisomers II. Diastereomers Geometric isomers 6-coordinate complexes Facial(fac) arrangement of ligands Meridional(mer) arrangement of ligands Two sets of ligands segregated into two perpendicular planes. Two sets of ligands segregated to two different faces.

B. Stereoisomers II. Diastereomers Geometric isomers 6-coordinate complexes Different arrangements of chelating ring

B. Stereoisomers Coordination may make ligands chiral as exhibited by the four-coordinate nitrogens. Optical isomers Optical isomers Geometric isomers

B. Stereoisomers III. Conformational isomers Because many chelate rings are not planar, they can have different conformations in different molecules, even in otherwise identical molecules.

B. Stereoisomers Conformational isomers Ligands as propellers

B. Stereoisomers Conformational isomers Ligands as propellers

C. Separation of Isomers Fractional crystallization can separate geometric isomers. Strategy assumes isomers have different solubilities in a specific solvent mixture and will not co-crystallize. b. Ionic compounds are least soluble when the positive and negative ions have the same size and magnitude of charge. Large cations will crystallize best with large anions of the same charge. Chiral isomers can be separated using a. Chiral counterions for crystallization b. Chiral magnets

D. Identification of Isomers X-ray crystallography Spectroscopic methods In general, crystals of different handedness rotate light differently. a. Optical rotatory dispersion (ORD): Caused by a difference in the refractive indices of the right and left circularly polarized light resulting from plane-polarized light passing through a chiral substance. b. Circular dichroism (CD): Caused by a difference in the absorption of right-and left-circularly polarized light.

3. Coordination Numbers and Structures Common Structures Factors involved: VSEPR fails for transition metal complexes Occupancy of metal d orbitals Sterics Crystal packing effects dx2-y2 dxz dz2 dyz dxy

3. Coordination Numbers and Structures a. Low coordination numbers Making bonds makes things more stable. Coordination number = 1 Rare for complexes in condensed phases (solids and liquids) but found in gaseous phase. In solution, often solvents will try to coordinate. Bulky ligands can play a big role here.

3. Coordination Numbers and Structures ii. Coordination number = 2 Also rare Ag(NH3)2+; d10 metal Soft metal ion Linear geometry iii. Coordination number = 3 [Au(PPH3)3]+; d10 metal Trigonal planar geometry

3. Coordination Numbers and Structures b. Coordination Number = 4 Avoid crowding large ligands around the metal Tetrahedral geometry is quite common Favored sterically Favored for L = Cl-, Br-, I- and M = pseudo noble gas configuration (d10); intermediate or hard metal Ones that don’t favor square planar geometry by ligand field stabilization energy Square planar Ligands 90° apart d8 metal ions; M(II) Smaller ligands, strong field ligands that π-bond well to compensate for lack of two additional ligands that would give a coordination number six Cis and trans isomers

3. Coordination Numbers and Structures c. Coordination Number = 5 Trigonal bipyramidal vs square pyramidal Can be highly fluxional in that they interconvert Isolated complexes tend to be a distorted form of one or the other D3h C4v TBP Geometry favored by: d1, d2, d3, d4, d8, d9, d10 metal ions Electronegative ligands prefer axial position Big ligands prefer equatorial position Sq Pyr Geometry favored by: d6 (low spin) metal ions

3. Coordination Numbers and Structures c. Coordination Number = 6 Mostly octahedral geometry (Oh) Favored by relatively small metals Numerous Isomer Types possible ii. Distortions from Oh Tetragonal distortions: Elongations or compressions along Z axis Symmetry becomes D4h

3. Coordination Numbers and Structures Trigonal distortions (Elongation or compression along C3 axis) Trigonal prism (D3h) Favored by chelates with small bite angles or specific types of ligands Trigonal antiprism (D3d) Rhombic distortions (Changes in two C4 axes so that no two are equal; D2h)

3. Coordination Numbers and Structures Not common Pentagonal bipyramid Capped octahedron 7th ligand added @ triangular face Capped trigonal prism 7th ligand added @ rectangular face

3. Coordination Numbers and Structures Not common Cube CsCl ii. Trigonal dodecahedron iii. Square antiprism

3. Coordination Numbers and Structures II. Rules of thumb Factors favoring low coordination numbers: Soft ligands and soft metals (low oxidation states) Large bulky ligands Counterions of low basicity “Least coordinating anion” BArF

3. Coordination Numbers and Structures II. Rules of thumb Factors favoring high coordination numbers: Hard ligands and hard metals (high oxidation states) Small ligands Large nonacidic cations to not compete with the metal for ligand interaction

4. Bioinorganic Chemistry Metal coordination in biology obeys coordination trends but expect distorted geometries. Classical example is hemoglobin for oxygen transport: 2+ 2+ Intermediate metal ion bound by intermediate ligand; stabilized by the reducing environment of blood cells.

4. Bioinorganic Chemistry In hemoglobin, a coordination site is made available to bind and transport O2 . The metal oxidation state of 2+ is important for this binding process.