Coordination Compounds: Formulas and Ligands

• A coordination compound consists of a central metal atom or ion surrounded by atoms or molecules.

Coordination Compound Formula

• General formula: [M(L)n]
• M represents the metal ion, and L represents the ligand.

Central Metal Ion

• A central metal ion is a Lewis acid, accepting electron pairs.

Ligand

• Ligands are electron pair donors or Lewis bases.

Types of Ligands Based on Density or Electron Donor Atoms

• Monodentate Ligand: Has a single donor atom attached to the central metal atom.
• Polydentate Ligand: Has two or more donor atoms attached to the central metal atom.
• Ambidentate Ligand: Has two donor atoms but uses only one at a time for donation.
• Bidentate Ligands: (bi=2) Ethylene diamine, Oxalate
• Tridentate Ligands: (tri=3)
• Hexadentate Ligands: (Hexa=6) EDTA

Coordination Sphere

• The central metal atom and its attached ligands enclosed in square brackets.

Counter Ions

• Ionisable groups that exist outside the coordination sphere.

Charge Number

• The net charge residing on the complex ion.

Oxidation State or Number

• The formal charge carried by the metal ion.

Coordination Number

• The number of liquid donor atoms directly attached to the metal atom.
• For [M(L)2], the coordination number is 2.
• For [M(L)6], the coordination number is 6 where L is a monodentate ligand.

Double Salt vs. Coordination Complex

• Double Salt: Dissociates into simple ions when dissolved in water.
• Coordination Complex: Forms complex ions when dissolved in water.

Werner's Theory of Coordination Complexes

• Metals in complexes exhibit two types of valencies: primary and secondary.
• Primary valency is ionisable and satisfied by anions.
• Secondary valency is non-ionisable and satisfied by anions or neutral ligands.
• Secondary valency dictates the fixed arrangement of ligands around the central metal, determining the complex's geometry.

Geometry and Coordination Number

• Geometry is tetrahedral when CN=4
• Square planar when CN=4
• Geometry is octahedral when CN=6

Classification of Complexes

• Classification is based on the types of ligands or the charge on the coordination sphere.
• Homoleptic Complex: The metal is attached to only one type of ligand.
• Hetroleptic Complex: The metal is attached to more than one type of ligand.
• Cationic Sphere: The coordination sphere carries a positive charge.
• Anionic Sphere: The coordination sphere carries a negative charge.
• Neutral Sphere: The coordination sphere has no charge.

IUPAC Nomenclature Rules For Coordination Compounds

• [M(L)n]: Ligand name - metal name - (oxidation number).
• [M(L)n]X (X=counter anion): Ligand name - metal name - (oxidation number) - counter anion name.
• Y[M(L)n] (Y = counter cation): Counter cation name - ligand name - metal name - (oxidation number).
• [M(L)n]b+ (b=charge): Ligand name- metal name - (oxidation number) - ion.
• [M(L)n]b- (b=charge): Ligand name + o - metal name + ate- (oxidation number) + ion.
• Never write di, tri, or tetra in front of the names of counter ions.
• Always write di, tri, or tetra in front of the name of a directly attached to the to metal in the coordinate area.

Effective Atomic Number (EAN)

• The atomic number + 2 x #ligands - oxidation state.

Isomers

• Different compounds with the same molecular formula

Isomers Types

• Structural isomers differ in the linkages among constituent atoms
• Stereoisomers have the same linkages but differ in the spatial arrangement of atoms
• Geometric isomers (diastereomers), which are non-superimposable mirror images.
• Optical isomers (enantiomers), which are non-superimposable chiral molecules also exist

Constitutional Isomers

• Linkage Isomers: Occur when a ligand can coordinate through different atoms..
• Ionization Isomers: Exchange of ligands between the coordination sphere and ionization sphere..
• Coordination Isomers: Exchange of ligands between complex cation and anion..
• Solvate/Hydrate Isomers: Differ by the presence of water molecules either as ligands or as free molecules within the crystal lattice.

Geometric Isomers

• Cis and trans isomers possible in square planner complex (CN =4).
• Cis and trans isomers in octahedral complex (CN=6).
• Dextrorotatory (d) rotates light to the right.
• Levorotatory (l) rotates light to the left.

Stability depends on

• Stability constant value (K): Higher K = higher stability.
• Depends on two factors like charge (higher ratio of small metal ion is more stable) and ligand (stronger more stable).

Valence Bond Theory (VBT)

• The number of vacant hybrid orbital=Number of ligand donor atom.
• Stronger ligand pairing occurs and creates inner orbital complexes of metals.
• Weaker do not pair outer orbital structures are formed.

Octahedral Complexes

• : [Co(NH3)6] Low spin
• Oxidation state of Cobalt: 3+
• Valence shell electronic configuration of Co3.
• Six orbitals available for hybridisation are two 3d, one 4s, three 4p orbitals.
• Electronic configuration after complex formation.
• Complex is diamagnetic when all electrons paired.

Square Planar

• Square planar so Ni Ion us dsp hybrid orbitals are created.
• 30 Number of CN liganda =4

Limitations of VBT

• VBT is unable to explain High spin or low spin natures of the ligands and complexes.
  • It also does not give reasons for colour of coordination compounds.
  • The Crystal field theory explains issues with VBT.

Crystal Field Theory assumptions

• Ligands treated as point charges interacting purely electrostatically with the metal ion
• In an isolated gaseous atom, dxy, dyz, dzx, dz2, dx2-y2 have same energy
• Ligands create field, around metal ion and d-orbitals split as dx2-y2, dz²=eg and dxy, dyz, dzx=t₂g.
• Splitting gives two sets of orbitals includes t₂g and eg . eg are orbitals t2g and eg as crystal field splitting

Crystal Field Splitting Parameter (Δ)

• ( Δ Strength) is proportional to ligand strength
• Strong-field ligands involve donor atoms C, N, and P. Examples: CN, NC, CO, NH₃, EDTA, and en (ethylene diamine).
• Weak-field ligands have donor atom such as X, O, and S. Examples: Halogens, SCN, and C₂O₄²⁻.
• In tetrahedral complex , Aor high spin or low spin complexes. d orbital electronic configuration

Factors affecting Crystal Field splitting parameter (Δo)

• The magnitude of crystal field strength is affected by spectrochemical series and
• A metal that possesses high oxidation state draw ligands closer as well as affects splitting.

Colour of Octahedral Complexes

• An octahedral complex can be induced to form with splitting of d-orbitals in the the t2g and eg states.
• A, which can be measured from experiments is proportional to corresponds certain frequency of electromagnetic radiation usually in the visible region which complements adsorbed frequency.

Applications of Coordination Compounds

• Coordination complexes used greatly in multiple things such as metal complexes where number of process of occur as well as biology.
• Examples of metal complex would be Mg(present in chlorophyll) as well as hemoglobin(present in blood).
• Pt complex cisplatin is used in the treatment of cancer. EDTA used in treatment of lead poisoning. Stable coordination complexes on dissolution dissociate to small extent and furnish a controlled supply of metal ions where tend form a protective layer on the certain cyanide complexes.