The direct answer
Basics & Werner Theory
Complex: [M(L)ₙ]. Central metal + ligands. Coordination number = number of ligand donor atoms. Werner: primary valency (ionizable) + secondary valency (coordination). [Co(NH₃)₆]Cl₃: 3 Cl⁻ are primary, 6 NH₃ are secondary.
Nomenclature
Cation before anion. Ligands: neutral (ammine, aqua, carbonyl), anionic (-o endings: chloro, cyano). Prefixes: di-, tri-, tetra- for identical ligands. Oxidation state in Roman numerals in parentheses. [Co(NH₃)₆]³⁺: hexaamminecobalt(III) ion.
Isomerism
Geometric (cis/trans): square planar and octahedral complexes. Optical: non-superimposable mirror images (d- and l-). Linkage: ambidentate ligands (NO₂⁻/ONO⁻). Ionization: [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br.
Crystal Field Theory
d-orbital splitting: octahedral (t₂g, e_g), tetrahedral (e, t₂), square planar. Δ₀ for octahedral. Strong field ligands (CN⁻, CO): low spin. Weak field ligands (I⁻, Br⁻): high spin. Spectrochemical series: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO.
Magnetic Properties
Unpaired electrons → paramagnetic. All paired → diamagnetic. [Fe(CN)₆]⁴⁻: low spin, diamagnetic. [Fe(H₂O)₆]²⁺: high spin, paramagnetic (4 unpaired). CFT splitting determines electron configuration.
Stability & Applications
Stability constant: K_f. Higher K_f = more stable complex. Chelate effect: polydentate ligands form more stable complexes. Applications: EDTA titration, cisplatin anticancer, hemoglobin oxygen transport.
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Frequently Asked Questions
How do I determine oxidation state of central metal?
Sum of oxidation state and ligand charges equals complex charge. For [Fe(CN)₆]³⁻: x + 6(-1) = -3, so x = +3. For neutral [Ni(CO)₄]: x + 4(0) = 0, so x = 0.
What is the difference between high spin and low spin?
High spin (weak field ligands): electrons fill singly before pairing, maximizing unpaired electrons. Low spin (strong field ligands): electrons pair in lower energy orbitals first. Applies mainly to d⁴-d⁷ in octahedral complexes.
Which is more stable: chelate or monodentate complex?
Chelate complexes (polydentate ligands like en, EDTA) are more stable due to the chelate effect. The entropy increase from releasing water molecules makes chelation thermodynamically favorable.
What is a ligand and what are the types?
A ligand is a molecule/ion that donates electron pair to the central metal atom. Types: monodentate (one donor atom - NH₃, H₂O, Cl⁻), bidentate (two - en, oxalate), polydentate (many - EDTA). Ambidentate: can bind through two different atoms (NO₂⁻/ONO⁻).
What is the coordination number?
The coordination number is the number of donor atoms surrounding the central metal. [Pt(NH₃)₂Cl₂]: CN = 4 (square planar). [Co(NH₃)₆]³⁺: CN = 6 (octahedral). [Ni(CO)₄]: CN = 4 (tetrahedral).
What is a chelate complex?
A chelate complex forms when polydentate ligands wrap around the metal forming ring structures. Example: [Co(en)₃]³⁺. Chelates are more stable due to the chelate effect (entropy gain from releasing water molecules).
What is the difference between coordination number and oxidation state?
Coordination number is the number of ligand donor atoms bonded to the metal. Oxidation state is the charge on the metal after removing all ligands with their electron pairs. [Fe(CN)₆]³⁻: CN=6, oxidation state = +3.
What is the spectrochemical series?
The spectrochemical series arranges ligands by field strength: I⁻ < Br⁻ < SCN⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NCS⁻ < NH₃ < en < NO₂⁻ < CN⁻ < CO. Strong field ligands cause large d-orbital splitting (low spin). Weak field: small splitting (high spin).
What is the difference between high spin and low spin complexes?
High spin: weak field ligands, electrons fill all d orbitals singly before pairing. Low spin: strong field ligands, electrons pair in lower energy orbitals first. Example: [Fe(H₂O)₆]²⁺ (high spin, 4 unpaired) vs [Fe(CN)₆]⁴⁻ (low spin, 0 unpaired).
How do I write the IUPAC name of a complex?
Rules: (1) Name cation first then anion, (2) Name ligands alphabetically before metal, (3) Use prefixes di-, tri- for identical ligands, (4) Add -o to anionic ligands (chloro, cyano), (5) Metal oxidation state in Roman numerals: [Co(NH₃)₆]Cl₃ = Hexaamminecobalt(III) chloride.
What are the types of isomerism in coordination compounds?
Stereoisomerism: geometric (cis/trans, fac/mer), optical (d/l). Structural: ionization ([Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br), linkage (NO₂⁻/ONO⁻), coordination ([Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆]).
What is the magnetic moment of a complex?
Magnetic moment μ = √(n(n+2)) BM where n is number of unpaired electrons. [Fe(CN)₆]³⁻: 1 unpaired → μ = √3 = 1.73 BM. [Fe(H₂O)₆]²⁺: 4 unpaired → μ = √24 = 4.9 BM.
What is the crystal field splitting energy?
Crystal field splitting energy (Δ₀) is the energy gap between t₂g and e_g orbitals in octahedral complexes. Δ₀ depends on: ligand field strength, oxidation state of metal, size of metal ion. Determining factor for color and magnetic properties.
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