Isomerism In Coordination Chemistry

Question 1

What is geometric isomerism in transition complexes, including the number of isomers with different ligand denticities

Answer 1

1. Geometric isomerism does not exist in tetrahedral geometry, only in square planar due to the bond inflexibility
   Monodenate ligands:
   In Ma2b2 there are two isomers - one trans and one cis form
   In Ma2bc there are two isomers - one trans and one cis form
   In Mabcd there are three isomers - (Three trans ab, ac, ad)

Bidentate ligands sit on cis/adjacent sites:
Ma2b2 where a is bidentate there are no isomers
Ma2bc where a is bidentate there are no isomers

Monodentate in octahedral arrangement form mer and fac isomers
Ma2b2c2 has 5 geometric isomers: all cis, all trans, 3 x (one trans the rest cis)

Octahedral with bidentate ligands
Ma2b4 with a bidentate has no isomers
M(a2)2b2 has trans and cis isomerism

Question 2

When does optical isomerism arise

Answer 2

1. Tetrahedral complexes
2. Octahedral all cis isomer has enantiomers - but not possible if more than 2 of any type of ligand
3. Bidentate octahedral have a left handed and right handed helix isomer.

Question 3

Polarimetry equation

Answer 3

Specific rotation for each enantiomer = 100 x angle of rotation / pathlength (dm) x concentration g/100ml

Written as [a]^20 subcript wavelength of incident light +89 (c 5.0, H20)

Question 4

What is coordination isomerism and under what conditions/when does it occur

Answer 4

Two pairs of complex ions that contain the same number of ligands but different distributions of these ligands that lead to a different overall charge on each complex

Requires both cationic and anionic complex ions that must be DIFFERENT e.g. Cr(NH3)6 3+ and Cr(CN)6 3- VS Cr(CN)(NH3)5 2+ and Cr(CN)5(NH3) 2-

Question 5

What is hydrate (solvate) isomerism

Answer 5

Hydrate isomerism occurs when there is a varying arrangement of water ligands in or out of the coordination sphere

E.g. three forms of CrCl3.6H20:
[Cr(OH2)6]Cl3
[CrCl(OH2)5]Cl2 .H20
[CrCl2(Oh2)4]Cl .2H20

Question 6

How to distinguish between different hydrate isomers

Quick fact about ammonia

Answer 6

1. Reacting with AgCl will lead to the formation of a Ag salt alongside conductivity measurements

Ammonia is too volatile to appear outside the coordination sphere so always a ligand

Question 7

Explain what linkage isomerism is and when it occur

Answer 7

Linkage isomerism occurs when there are ambidentate ligands e.g. nitro NO2, nitrito ONO, thiocyanate SCN, isothiocyanate NCS.

Stick to one type of ambidentate per complex not mixing and matching SCNs and NCSs

Question 8

What is ionisation isomerism

Answer 8

Involves the swapping of ligands and counter-ions into and outside the coordination sphere to form different complexes

E.g. [PtCl2(NH3)4]Br2 AND [PtBr2(NH3)4]Cl2 AND [PTBr(Cl)NH3)4]Cl.Br

Question 9

What is ligand isomerism and when does it occur

Answer 9

If a ligand has structural isomers of itself e.g. NH2 attached to large chains with different substituted carbons

Question 10

Neutral ligands:
OH2
N2
O2
NH3
CO

Anionic ligands:
CL-
Br-
SCN-
OH-
CN-
NO2-

Answer 10

Aqua, dinitrogen, dioxygen, ammine, carbonyl

Chloro, bromo, thiocyanato/isothyiocyanato, hydroxo, cyano, nitro/nitrito