Coordination Chemistry: Isomerism and Structure Coordination Chemistry: Isomerism and Structure Chapter 7 and 19 Chapter 7 and 19 1

1. Isomerism 2

A. Constitutional Isomers 3 I.Linkage (Ambidentate) Isomers  A ligand can bind in more than one way [Co(NH 3 ) 5 NO 2 ] 2+ Co-NO 2 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 4 II. Ionization Isomers  Difference in which ion is included as a ligand and which is present to balance the overall charge [Co(NH 3 ) 5 Br]SO 4 vs [Co(NH 3 ) 5 SO 4 ]Br III. Solvate (Hydrate) Isomers  The solvent can play the role of ligand or as an additional crystal occupant [CrCl(H 2 O) 5 ]Cl 2 · H 2 O vs [Cr(H 2 O) 6 ]Cl 3

A. Constitutional Isomers 5 IV. Coordination Isomers Same metal Formulation- 1Pt 2+ : 2NH 3 : 2 Cl - [Pt(NH 3 ) 2 Cl 2 ] [Pt(NH 3 ) 3 Cl][Pt(NH 3 )Cl 3 ] [Pt(NH 3 ) 4 ][PtCl 4 ] Same metal but different oxidation states Formulation- 1Pt 2+ : 1Pt 4+ : 4NH 3 : 6 Cl - [Pt(NH 3 ) 4 ][PtCl 6 ] [Pt(NH 3 ) 4 Cl 2 ][PtCl 4 ] Different Metals Formulation- 1Co 3+ : 1Cr 3+ : 6NH 3 : 6 CN - [Co(NH 3 ) 6 ][Cr(CN) 6 ] [Co(CN) 6 ][Cr(NH 3 ) 6 ]

6

B. Stereoisomers 7 I.Enantiomers  Optical isomers (chiral)  Non-superimposable mirror image  Recall from group theory, something is chiral if Has no improper rotation axis (S n )  Has no mirror plane (S 1 )  Has no inversion center (S 2 ) Square planar complex If it were tetrahedral, it would not be chiral.

B. Stereoisomers 8 II. Diastereomers a.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 cis (anticancer agent) trans

B. Stereoisomers 9 II. Diastereomers a.Geometric isomers  6-coordinate complexes Facial(fac) arrangement of ligands Meridional(mer) arrangement of ligands Two sets of ligands segregated to two different faces. Two sets of ligands segregated into two perpendicular planes.

B. Stereoisomers 10 II. Diastereomers a.Geometric isomers  6-coordinate complexes  Different arrangements of chelating ring

B. Stereoisomers 11 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 12 Conformational isomers  Ligands as propellers

B. Stereoisomers 13 Conformational isomers  Ligand symmetry can be changed by coordination. Coordination may make ligands chiral as exhibited by the four-coordinate nitrogens. Conformational isomers Geometric isomers

C. Separation of Isomers 14 I.Fractional crystallization can separate geometric isomers. a.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. II. Chiral isomers can be separated using a. Chiral counterions for crystallization b. Chiral magnets

D. Identification of Isomers 15 I.X-ray crystallography II.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 16 I.Common Structures Factors involved:  VSEPR fails for transition metal complexes  Occupancy of metal d orbitals  Sterics  Crystal packing effects d x 2 -y 2 d xz dz2dz2 d yz d xy

3. Coordination Numbers and Structures 17 a. Low coordination numbers  Making bonds makes things more stable. i.Coordination number = 1 Rare for complexes in condensed phases (solids and liquids). Often solvents will try to coordinate.

3. Coordination Numbers and Structures 18 ii. Coordination number = 2 Also rare Ag(NH 3 ) 2 + ; d 10 metal Linear geometry iii. Coordination number = 3 [Au(PPH 3 ) 3 ] + ; d 10 metal Trigonal planar geometry

3. Coordination Numbers and Structures 19 b. Coordination Number = 4  Avoid crowding large ligands around the metal i.Tetrahedral geometry is quite common Favored sterically Favored for L = Cl -, Br -, I - and M = noble gas or pseudo noble gas configuration Ones that don’t favor square planar geometry by ligand field stabilization energy ii.Square planar Ligands 90° apart d 8 metal ions; M(II) Smaller ligands, strong field ligands that π-bond well to compensate for no six- coordination Cis and trans isomers

3. Coordination Numbers and Structures 20 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 D 3h C 4v TBP Geometry favored by: d 1, d 2, d 3, d 4, d 8, d 9, d 10 metal ions Electronegative ligands prefer axial position Big ligands prefer equatorial position Sq Pyr Geometry favored by: d 6 (low spin) metal ions

3. Coordination Numbers and Structures 21 c. Coordination Number = 6 i.Mostly octahedral geometry (O h )  Favored by relatively small metals  Isomers ii. Distortions from O h  Tetragonal distortions: Elongations or compressions along Z axis Symmetry becomes D 4h

3. Coordination Numbers and Structures 22  Trigonal distortions (Elongation or compression along C 3 axis) Trigonal prism (D 3h ) Favored by chelates with small bite angles or specific types of ligands Trigonal antiprism (D 3d )  Rhombic distortions (Changes in two C 4 axes so that no two are equal; D 2h )

3. Coordination Numbers and Structures 23 c.Coordination Number = 7 Not common i.Pentagonal bipyramid ii.Capped octahedron  7 th ligand triangular face iii.Capped trigonal prism  7 th ligand rectangular face

3. Coordination Numbers and Structures 24 c.Coordination Number = 8 Not common i.Cube  CsCl ii. Trigonal dodecahedron iii. Square antiprism

3. Coordination Numbers and Structures 25 II. Rules of thumb Factors favoring low coordination numbers: a.Soft ligands and soft metals (low oxidation states) b.Large bulky ligands c.Counterions of low basicity  “Least coordinating anion” BArF

3. Coordination Numbers and Structures 26 II. Rules of thumb Factors favoring high coordination numbers: a.Hard ligands and hard metals (high oxidation states) b.Small ligands c.Large nonacidic cations

4. Bioinorganic Chemistry 27 Metal coordination in biology obeys coordination trends but expect distorted geometries. Classical example is hemoglobin for oxygen transport: 2+ Intermediate metal ion bound by intermediate ligand; stabilized by the reducing environment of blood cells. 2+

4. Bioinorganic Chemistry 28 In hemoglobin, a coordination site is made available to bind and transport O 2. The metal oxidation state of 2 + is important for this binding process.