Title: Lesson 6 Complex Ions Learning Objectives: Explain and use the terms ligand/complex/complex ion and ligand substitutions. Describe the formation of complexes in terms of dative (coordinate) bonding and shapes of complexes.

Transition metal ions in solution When a transition metal compound such as cobalt chloride is dissolved in water it becomes Co 3+ and Cl - ions. Transition metal ions have a high charge density. The Co 3+ ions attract water molecules around them to form something we call a complex. The bonds between the water molecules and the positive ion is called a COORDINATE BOND (Dative Bond).

Charged or not?

How do we write complexes down?

AfL – using whiteboards Write the symbol formula for:

Ligands and bonding

AfL – test your understanding What is the ligand and coordination number in each case:

Coordinate (dative) bonding

Shapes of complexes

Square planar

Linear

Polydentate ligands Some ligands can bond to the metal more than once. Each molecule of ethandioate bonds to the metal twice. Each molecule of EDTA 4- bonds to the metal six times.

Polydentate ligands Why can they bond to a metal more than once? More than one lone pairs EDTA 4- has 6 atoms with lone pairs (2 nitrogen and 4 oxygen)

Chelating Agents Because EDTA 4- can bond 6 times it is described as a hexadentate (six toothed) ligand. It can grip the central ion in a six-pronged claw called a CHELATE. Chelates are important in foods and biological systems. EDTA 4- can remove transition metals from solutions.

Ligand Exchange In aqueous solution, water molecules generally act as ligands but these can be replaced in a process known as ligand exchange. Complexes often have distinctive colours so can be used in qualitative analysis. Example: Test for iron (III) ion Left – [Fe(H 2 O) 6 ] 3+ - Orange colour Centre – Potassium thiocyanate solution (SCN - ion) Right – Complex formed when reactants on the left and centre react together. One of the water ligands has been replaced with the thiocyanate ion. (SCN - )  FORMING BLOOD RED [Fe(SCN)(H 2 0) 5 ] 2+

Lability Ligands can be exchanged in substitution reactions because they are labile (can attach on and off).

Other substitution reactions

Main Menu Ligands  A ligand is a species with a lone pair  Often negative ions  Common ligands include:  Water, H 2 O  Ammonia, NH 3  Chloride, Cl -  Hydroxide, OH -  Cyanide, CN -  Thiocyanate, SCN -

Main Menu Practical - Making transition metal complexes  Add potassium thiocyanate solution to a solution of iron (III)  Add conc. HCl (fume hood!) to 1 cm 3 of a strong solution of cobalt (II). Repeat but use conc. NH 3 instead (fume hood!).  Add dilute NH 3 to a copper (II) solution until no further change occurs  Record all observations  Suggest possible structures for the complexes you have formed and possible reaction equations

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Transition metals and their ions – Important Catalysts  Transition elements and their compounds/complexes can act as catalysts. (They allow chemical processes to proceed at an economic rate)  Two forms: Heterogenous (different state to the reactants) Homogenous (same state to the reactants)  Transitional metals have varying oxidation numbers and are able to co- ordinate to other molecules/ions to form complex ions

Main Menu The magnetic properties of Transition Metals and their compounds  Every spinning electron in an atom or molecule can behave as a tiny magnet.  Electrons with opposite spins have no net magnetic effect.  Unpaired electrons found in the d block of transitions metals can be aligned to lead to magnetic effects.

Main Menu Responses to externally applied magnetic fields  Diamagnetism – a property of all materials and produce a very week opposition to an applied magnet  Paramagnetism – only occurs with substances with unpaired electrons. Stronger than diamagnetism. Magnetisation is proportional to the applied field and in the same direction.  Ferromagnetism – largest effect. Can produce magnetisation greater than the applied field

Main Menu Iron, Cobalt and Nickel are ferromagnetic  The unpaired d electrons in large numbers of atoms line up with parallel spins in regions called domains.  Domains are generally randomly orientated but will be ordered if exposed to an external magnetic effect.  After external magnet is removed, domains remain aligned due to the long range interaction between unpaired electrons in different atoms.

Main Menu Transition metals and their complexes show paramagnetic properties  Transition metals with unpaired electrons will be pulled into the magnetic field.  Non Transition metals will moved out of the magnetic field in the opposite direction.  Paramagnetism increases with the number of unpaired electrons (max at Chromium)

Main Menu A Challenge  Working in small groups, complete the following activity on the structure of some cobalt complexes.following activity

Main Menu Key Points  Transition metals form ions with partially filled d-orbitals  Ligands are species with lone pairs  Ligands will form dative covalent bonds to transition metals forming ‘complex ions’