Coordination compounds

What is a coordination compound?

Coordination compounds have stable complex ions in their lattice and their solutions. These ions are formed from metal ion surrounded by anions or neutral molecules held together with coordinate covalent bond.

What is a complex ion?

A central metal ion surrounded by ligands

What is a ligand?

An atom, ion, or molecule which can donate a lone electron pair

Coordinate bond

Shared pair of electrons between 2 atoms where both electrons came from the SAME atom

Coordination number

Total number of coordinate bonds formed between the ligands and central metal ion

What are the types of ligands?

• Monodentate - Form one coordinate bond, donate one lone pair of electrons e.g water, ammonia, chloride

• Bidentate - Form 2 coordinate bonds, donate two lone pairs e.g ethanedioate (C₂O₄^2-)

• Multidentate

What are the types of coordination compounds? (3)

Depending on the nature of ligands

Depending on electrical charge of the complex

Depending on the types of the ligands

Nature of ligands

• CC can have inorganic ligands - K[Ag(CN)₂] [Co(NH₃)₆]Cl₃

• CC can have organic molecules as ligands (chelates) - haemoglobin, chlorophyll etc.

Electrical charge of complex

Can be cations (+ve) [Ag(NH₃)₂]⁺Cl^-, anions (-ve) K₃⁺[Fe(CN)₆]^3-, neutral [Co(NO₃)₃(NH₃)₃] 

Type of ligands

Complexes having amines (R-NH₂) as ligands

Complexes  of  the NH₃

Complexes with H₂O

How are CC named depending on electrical charge

• Anions - add the ending ‘-o’ (Cl^- - chloro, CN^- cyano, OH⁻ → Hydroxo)

• Neutral - names of neutral molecules, no change

• Name begins with the cation

• NB! Special names - H₂O aqua, NH₃  ammine, NО nitrozyl, CO → Carbonyl

Naming rules

• Begin with +ve ion, could be the complex OR counter ion

• Identify the ligands and name them alphabetically. (numerical prefixes do not affect)

• Use prefixes (di-, tri-, etc.) for multiple ligands.

• Name the metal, using "-ate" if the complex is an anion.

• Specify the oxidation state of the metal in Roman numerals.

• Include counterions if present.

Names of metals in anions for : copper, gold, iron, lead, silver, tin

cuprate, aurate, ferrate, plumbate, argentate, stannate

Shapes & coordination number

• 4 - square planar, cisplatin Pt(NH₃)Cl₂

• 4 - tetrahedral

• 6 - octahedral

[Cr(H₂O)₄Cl₂]Cl - list 6 things from this compound

1. Name - dichlorotetraaquacromium (III) chloride

2. Central metal - Cr³⁺

3. Ligands - Н₂О, Cl₂

4. Counterions - Cl^-

5. Complex ion - [Cr(H₂O)₄Cl₂]⁺

   H₂O (water): Neutral charge = 0.

   Cl⁻ (chloride ligands inside the complex): Each has a charge of -1, and there are 2 Cl⁻, so total = -2.

   x+(−2)=+1 so Cr is +3

6. Coordination number - 6

Common metal forming complexes - Fe, Cu, Cr, Ag, Zn, Al, Ca, Ba

Fe²⁺, Fe³⁺, Cu²⁺, Cr³⁺, Ag⁺ , Zn²⁺, Al³⁺, Ca²⁺, Ba²⁺ 

Determine whether these complexes are neutral or ions :

• Fe(CO)₅

• K₃[Fe(CN)₆] 

• K₂[PtCl₆]

• [Cr(H₂O)₄Cl₂]Cl

1. Neutral

2. (-1×6) + 3 = -3 → [Fe(CN)₆]^3- 

3. (-1×6) + 4 = -2 → [PtCl₆]^2-

4. (-1×2) + 3 + 4 = +1 → [Cr(H₂O)₄Cl₂]¹⁺

Complex ions exist as stable units in the lattice of coordination compounds and in their solutions

Counterbalancing  ions

these are the ions with opposite electrical charge of the complex ion → form external coordination sphere

What are the counterbalancing ions :

• K₃[Fe(CN)₆]^3-

• K₂[PtCl₆]^2-

• [Cr(H₂O)₄Cl₂]¹⁺Cl

• [Cu(NH₃)₄](OH)₂

1. K⁺

2. K⁺

3. Cl^-

4. OH^-

Stability complex (K_f or Kst)

Mⁿ⁺ + xL^m- ⇌ [ML_x] ^p+

K_st = [ [MLx]^p+]

[Mⁿ⁺].[L^m-]^x

What does the stability complex tell you about the ion?

Quantifies how strongly the ligand(s) bind to the central metal ion in solution. A higher stability constant indicates a more stable complex, meaning the equilibrium favours complex formation.

Factors affecting K_f (3)

1. charge and size of metal ion - smaller, highly charged metal ions form more stable complexes

2. nature of ligands - strong > weak ligands, higher K_f

3. chelation effect - multidentate ligands form more stable complexes

What is the Kst of : K₄[Fe(CN)₆]

1. 4K⁺ + [Fe(CN)₆]^4-

2. Fe²⁺ + 6CN^- ⇌ [Fe(CN)₆]^4-

3. К_st = [[Fe(CN)₆]^4-]

   [Fe²⁺].[CN^-]⁶

What is the structure of this compound : K₄[Fe(CN)₆]

• Central ion - Fe⁺²

• Ligands - CN^-

• Complex ion - [Fe(CN)₆]^4-

• Counter ions - K⁺

• Coordination number - 6

• Charge of central ion - +2

Bonding in coordination compounds

In complex : the central metal ion and ligands are held together by coordinate bond.

The complex ion and counter ions are linked via IONIC INTERACTIONS

What is a chelate?

Coordination compounds with organic molecules as ligands - usually polydentate

Depending on the type of organic molecule chelates are formed with (7)

1. amino acids, peptides & proteins

2. polyols

3. salt of tartaric acid

4. phenol & salicylic acid

5. EDTA

6. protoporphyrin

7. polypeptide chain of the enzymes

Reaction - chelate with amino acids as ligands

Reaction - chelate with peptides/proteins

• biuret test - detects peptide bond, blue →purple

Reaction - chelate with polyols

The reaction is used to prove the presence of -OH groups attached to adjacent carbon atoms

Reaction - chelate with salt and tartaric acid

EDTA forms chelates with…

• Ca²⁺, Mg²⁺,Pb²⁺, Cu²⁺,Cd²⁺

• Tests hardness of water, treatment of heavy metal poisoning, enlarges root canals

What part of haemoglobin is a chelate?

Haem group (non-protein part). Central metal ion - Fe²⁺, ligand - protoporphyrin

Methaemoglobin contains Fe³⁺ a stable oxidized form of haemoglobin that is unable to release oxygen to the tissues.

What is vitamin B₁₂?

• collection of cobalt & corrin ring

• vitamin B₁₂ is important for the normal functioning of the brain and nervous system

What are cytochromes?

• Generally membrane bound haemoproteins that contain haem groups, and carry out electron transport

• Metal ion is iron, interconverts between Fe²⁺ (reduced) and Fe³⁺ (oxidized) states