Chemistry of Transition Metals & Coordination Compounds

Transition Metals (TMs)

  • Definition: Transition metals are characterized by the filling of d subshells.
  • Cation Formation: The s electrons are lost first when forming cations, resulting in many TM ions having partially filled d subshells.
  • Key Characteristics:
    • Often exhibit more than one stable oxidation state.
    • Many transition metal compounds display colors.
    • Exhibit interesting magnetic properties.

Coordination Compounds

  • Definition: A coordination compound is composed of a complex ion and its counterions. Examples include:
    • [Co(NH3)6]Cl3
    • K4[Fe(CN)6]
    • Pt(NH3)2Cl2
  • Roles:
    • Transition Metal (TM): Acts as a Lewis acid (electron pair acceptor).
    • Ligand: Acts as a Lewis base (electron pair donor).

Ligands

  • Types of Ligands:
    1. Monodentate:
    • A neutral molecule or ion with one pair of electrons to donate.
    1. Bidentate:
    • Can donate two electron pairs, must be on separate atoms far enough apart.
    • Examples:
      • Ethylenediamine (“en”)
      • Tetramethylethylenediamine (“TMEDA”)
    1. Polydentate:
    • Can attach through multiple points.
    • Example: EDTA (ethylenediaminetetraacetate ion).
  • Coordination Number (CN): The number of atomic attachments between the TM and ligands. CN varies depending on the TM, its oxidation state, and its size.

Common Shapes and Coordination Numbers

  • Linear:
    • CN = 2
    • Bond angle: 180exto180^ ext{o}
  • Square Planar:
    • CN = 4
    • Bond angle: 90exto90^ ext{o}
  • Tetrahedral:
    • CN = 4
    • Bond angle: 109.5exto109.5^ ext{o}
  • Octahedral:
    • CN = 6
    • Bond angle: 90exto90^ ext{o}

Isomerism

  • Definition of Isomers: Molecules or ions with the same number and type of atoms, different arrangements, and varying chemical/physical properties.
Types of Isomers

  1. Structural (Constitutional) Isomers:

    • Same atoms, differing bond arrangements.
    • Sub-types:
      • Coordination Isomers:
      • Complex ion composition differs (e.g., [Cr(NH3)5(OSO3)]Br vs. [Cr(NH3)5Br]SO4).
      • Linkage Isomers:
      • Different points of attachment for at least one ligand (e.g., [Co(NH3)4(NO2)Cl]Cl vs. [Co(NH3)4(ONO)Cl]Cl). Ligands that can cause linkage isomerism include SCN–, CN–, NO2–, OCN–.

  2. Stereoisomers:

    • Same atoms and bonds, but different spatial arrangements.
    • Sub-types:
      • Geometric Isomers (cis-trans isomers, diastereomers):
      • Example: Pt(NH3)2Cl2.
      • Optical Isomers (enantiomers):
      • Non-superimposable mirror images (e.g., [ZnFClBrI]2–).
      • Identical physical properties, differ chemically only in the presence of other optical isomers.
      • Common in tetrahedral complexes with 4 different substituents.
      • Also possible in octahedral complexes with multiple bidentate ligands.

Application: Platinum Anticancer Drugs

  • Example: Cisplatin
    • Developed unexpectedly while studying electric field effects on cell division using "inert" Pt electrodes and NH4Cl in gel.
    • Chemical structure: cis-Pt(NH3)2Cl2.
    • Cisplatin Family: Neutral Pt(II) or Pt(IV) compounds with two cis NH3 ligands.
    • Mechanism: Binds to adjacent guanine residues in DNA, blocking transcription, leading to cell death.
    • Noteworthy in testicular cancer treatment (e.g., cases involving Scott Hamilton, Lance Armstrong).