Study Notes on Coordination Compounds

COORDINATION COMPOUNDS

Introduction to Coordination Compounds

  • You have previously learned about:

    1. Transition Elements
    • General characteristics
    • Electron configurations of first-row elements
    • Variable oxidation states of transition elements

  • This guide will expand on compounds formed by transition elements, specifically coordination compounds.

    • Topics to be covered:
      • Definitions and terminology
      • Nomenclature
      • Isomerism

Definitions and Terminology

Coordination Complex

  • A coordination complex is a compound that results from a coordinate bond formation between a ligand and a transition metal ion.

    • Ligands typically possess a lone pair of electrons.
    • Transition Elements have vacant orbitals.

  • A coordinate bond forms when a ligand donates an electron pair to the central metal ion.

Donor Atom
  • The atom in the ligand that donates an electron pair is known as the donor atom or coordinating atom.
Complex Ion
  • A complex ion is an electrically charged ion consisting of a central metal atom/ion surrounded by a group of ions or neutral molecules (ligands).
    • Example: In the compound [Ni(NH3)6]Cl2,
    • Central Ion: Ni²⁺
    • Ligand: NH3,
    • Complex Ion: [Ni(NH3)6]²⁺
    • Example: For Na3[Fe(CN)6], identify:
    • Complex Ion: [Fe(CN)6]³⁻, Central ion: Fe³⁺, Ligands: CN⁻
    • In [Cu(NH3)4]SO4, the complex ion is:
    • Complex Ion: [Cu(NH3)4]²⁺
Coordination Sphere
  • The coordination sphere consists of the central atom along with the molecules or ions coordinated to it.
  • When writing the formula for complex ions, the central ion and its coordination sphere are written within square brackets.
Coordination Number
  • The coordination number refers to the total number of ligands attached to a central ion.
    • Examples include:
      • Silver (Ag) in [Ag(CN)2]⁻?
      • Coordination number: 2
      • Copper (Cu) in [Cu(NH3)4]²⁺?
      • Coordination number: 4
      • Cobalt (Co) in [CoCl3(NH3)4]?
      • Coordination number: 6
Charge on Complex Ion
  • The charge on a complex ion is the sum of the charges on the central atom and the ligands associated with it.
    • Example:
      • [Cu(NH3)4]²⁺ has a net charge of +2
      • [Fe(CN)6]⁴⁻ has a net charge of -4
Types of Ligands
  • Ligands are classified based on the number of donor atoms:
    1. Unidentate (Monodentate)
      • Donate one pair of electrons.
      • Examples: CN⁻, H2O, Cl⁻, Br⁻, NH3, OH⁻, NO2⁻, CO
    2. Bidentate
      • Coordinate via two donor atoms.
    3. Polydentate
      • Coordinate via more than two donor atoms, e.g.
        • Tridentate, Tetradentate, Pentadentate, Hexadentate
Examples of Ligands

  • Anionic Ligands:

    • Br⁻: bromo
    • CN⁻: cyano
    • OH⁻: hydroxo
    • CO32⁻: carbonato
    • C2O42⁻: oxalato

  • Neutral Ligands:

    • H2O: aqua
    • NH3: ammine
    • CO: carbonyl

  • Cationic Ligands:

    • NH2NH3⁺: hydrazinium
    • NO⁺: nitrosonium
Chelating Ligands
  • A chelation ligand forms a ring structure by coordinating to a metal center.
  • The resulting complex is referred to as a chelate.

Calculating Oxidation Number

  • To find the oxidation number of the metal in given complexes:
    1. [Co(NH3)6]Cl3
    2. [CoSO4(NH3)4]NO3
    3. [Cd(SCN)4]²⁺
    4. [Cr(en)3]Cl3
Example Calculation
  • For [Cd(SCN)4]²⁻
    • The complex ion is anionic
    • Charge: -2
    • Coordination number: 4
    • Coordinate bond forms between Cd and S
    • Calculation:
    • Cd + 4*(-1) = -2
    • Cd = +2
    • Result: Cd²⁺

Nomenclature of Coordination Compounds

  • Writing the IUPAC formula from the name and vice versa requires following established rules.
Writing the IUPAC Formula
  1. Central atom listed first, followed by anionic ligands, then neutral ligands.
    • Example: For pentaamminechloroplatinum(IV) bromide, the formula is [PtCl(NH3)5]Br3
  2. Ligands are listed in alphabetical order based on the first letter of their formula
  3. Parentheses are used for polyatomic ligands.
  4. Use standard abbreviations for complicated ligands.
  5. The entire formula for the coordination sphere is enclosed in square brackets.
Naming Examples
  • Example: [Ag(NH3)2]Cl is named Diamminesilver(I) chloride
  • Example: [Co(CO3)(NH3)4]Cl is named Pentaamminecarbonatocobalt(III) chloride
  • Example: K4[Fe(CN)6] is named Potassium hexacyanoferrate(II)
Rules for Naming Coordination Compounds
  1. Ligands named first, then central metal ion.
  2. Names of negative ligands end in -o, positive ligands in -um.
  3. The order of naming ligands is in alphabetical order.
  4. Prefixes indicate the number of each ligand type (di-, tri-, tetra-).
  5. Anionic complexes have names that reflect the charge (ate added).
  6. Oxidation state of central ion is designated by Roman numeral.
  7. Bridging groups represented by the Greek symbol µ.
  8. Isomers are named using terms cis or trans.
  9. Abbreviations are used for complex names.

Class Exercise

  • Example: For [Cr(NH3)3(H2O)3]Cl3
    • The complex ion is cationic.
    • Ligands: ammine before aqua.
    • Charge on Complex Ion: +3 (compared to Cl⁻).
    • Determine oxidation number of Chromium: +3.
    • Final Answer: triamminetriaquachromium(III) chloride
  • Similar examples and exercises follow with a focus on practicing naming and recognizing coordination compounds.