Ch 9 Lecture 2 Isomers of Coordination Compounds

1. Ch 9 Lecture 2 Isomers of Coordination Compounds • Coordination Chemistry Isomers = same ligands arranged differently • Stereoisomers = a subset of all isomer types • Hierarchy of isomers

2. II. Stereoisomers • Four-Coordinate Complexes • Tetrahedral Complexes • Just like carbon compounds in organic chemistry • If four different ligands are bound, the complex is chiral • No cis- or trans- isomers possible • Square Planar Complexes • cis- or trans- isomers possible for MA2B2 • Chelate rings dictate cis isomer • No chiral isomers are present if the molecule has a mirror plane • Ignore minor changes in ligand when looking for a plane • Rotation of substituents is ignored • Conformational changes are generally ignored • Bending of bonds is ignored • Special square planar chiral complexes have been synthesized

3. Stereoisomers of 6-coordinate complexes • Monodentate Ligands Only • Ma3b3 complexes • Fac-isomer has three of the same ligands on one face (facial) • Mer-isomer has three of the same ligand on one meridian • Present in ML2 complexes if L = tridentate chelate

4. b) M<ab><cd><ef> complexes where <xy> = a trans pair of ligands i. There are only two possible stereoisomers ii. The positions are numbered as below c) Mabcdef complexes i. 15 diastereomers possible Each diastereomer is chiral, so 30 total stereoisomers possible Method for finding all the isomers Hold one trans pair constant (ab) Hold one of the pair constant in the next row (c-) Continue changing second ligand until done (cd), then (ce), then (cf) Other possibilities are listed below this first row Example: Find all Ma2b2c2 stereoisomers

6. Chelate Complexes • Similar methods work as for only monodentate ligands • There are usually less isomers because chelate ligands are restricted to cis binding of the metal ion • Example: M(AA)(BB)cd has only 5 isomers possible • Chirality and Chelate Rings • Right-handed Propeller = If you blow on it, it will turn counter-clockwise • Left-handed Propeller = If you blow on it, it will turn clockwise

7. M(AA)3 complexes, such as [Co(en)3]3+, are “3-propeller” systems • The right-handed propeller isomer is called D • The left-handed propeller isomer is called L • Procedure for Determining Handedness • Rotate molecule to put one chelate ring in back horizontal • Imagine both triangular faces superimposed • How would the front triangular face move to take its original place? • If rotation was counterclockwise = L • If rotation was clockwise = D • A designation is needed for each pair of “skew rings“

10. Ligand Ring Conformation • Many chelate rings are not planar • Use a similar process as above to designate them as d or l • Example: trans-[Co(trien)X2]+

11. Experimental Separation and Identification of Isomers • Separation • Fractional Crystallization • Often, different isomers precipitate under different conditions • Sometimes, both isomers crystallize • Solubility and Crystal Packing influence precipitation • Addition of Chiral Anions • A chiral anion such as S-tartrate may precipitate a single isomer • The resulting salt will be diastereomeric (D,S instead of L,S) • The added anion must generally be removed in another step • Chiral chromatography • Chiral solid phases have been developed • These generally separate chiral isomers • Identification • X-Ray Crystallography • Optical Activity