23.2 Complex formation and shape of complex ions

. Co-ordinate bonds: When one atom in a covalent bond provides both the electrons. By accepting electron pairs from other ions or molecules. So the bonds that form are dative/coordinate bonds.

A ion or molecule with a lone pair of electrons, that forms a co-ordinate bond with a transition metal. H20: :NH3 :Cl- :CN-

When 2, 4 or 6 ligands bond to a transition metal ion. Its coordination number is the number of co-ordinate bonds to ligands that surround the d block metal ion

Octahedral shape 90 degrees They have six as a coordination number. Eg [Co(NH2)6]3+

. Tetrahedral in most cases Eg Co(Cl4)2- Angles are 109.5 degrees However with the metals nickel and platinum, they form square planar Eg [NiCN4]2- Angles are 90 degrees

Linear shape 180 degrees Coordination number is 2 This occurs in Ag+ complexes

. If you dissolve the salt of a transition metal in water, eg copper sulfate, the positively charged metal ion becomes surrounded by water molecules acting as ligands. Normally there are 6 water molecules in an octahedral arrangement. The oxygen from the water bonds to the metal ion

. A ligand (particle with a lone pair of electrons that bonds to metals by dative bond) that forms one co-ordinate bond to a metal ion.

• Ligands that form two or more co-ordinate bonds to a metal ion. They have more than one atom with a lone pair of electrons which can bond to a transition metal ion.

. Each nitrogen has a lone pair of electrons which can form a co-ordinate bond to the metal ion, there are 2 nitrogen’s on each ligand. . So overall there are 3 ligands that can bind to the chromium ion as each can form 2 coordinate bonds H2N: - CH2 - CH2 - :NH2 It is a neutral ligand and the chromium ion has a 3+ charge so the complex ion has an overall 3+ charge

(C2O4)2- so the ligand has a 2- charge [Cr(C2O4)3] 3- It is overall 3- because each ligand brings the overall charge of the 3+ down to 3-

Benzene 1,2 diol is a neutral ligand also Has two oxygens on it, each with a lone pair to form co-ordinate bond C6H6O2 Is the formula of it But it is shaped as a hexagon ring

. It forms 6 bonds And it acts as a hexadentate ligand. It uses lone pairs on 4 oxygen and both nitrogen atoms

Called chalates Can be used to effectively remove d block metal ions from a solution.

. When a hexadentate ligand eg EDTA is added to a solution of a transition metal salt, the EDTA will replace all six water ligands in the aqua ion [Cu(H20)6]2+(aq) + EDTA4-(aq) This makes [CuEDTA]2-(aq) + 6H20(l) In this equation, two species are replaced by seven, as there are 7 products. This increase in particles causes a significant increase in entropy which drives the reaction to the right. For this reason, chelate complexes with multidentate ligands are favoured over complexes with monodentate ligands

Haemoglobin carries oxygen around body from the lungs. Molecule consists of Fe2+ ion with a coordinate number 6. Four of the coordination sites are taken up by a ring system celled a porphyrin which acts as a tetradentate ligand. This complex is called a haem.

Fe2+ ion binds to 4 :N , a globin, and an O2 or H2O It first binds to 4 :N but below the plane of this ring is a fifth nitrogen atom acting as a ligand, and it is part of a complex called globin. The Fe2+ to O2 bond is weak as :O2 is not a very good ligand, allowing the molecule to easily give up to its cells. However, when a better ligand than oxygen binds, it binds irreversably to the fe2+ so destroys the oxygen carrying site. This explains the poisonous effect of carbon monoxide

:Cl- ions Eg in [CoCl4]2- ion, with 4 ligands it is a tetrahedral .

• [Co(NH3)6]3+ ion has 6 ligands and is an octahedral shape. An octahedron has six points but eight faces. Metal ion Co3+ has a charge of +3 and the ligands are all neutral. So complex ion has an overall charge of +3 In [CoCl4]2- ion, the metal ion is Co2+ so has a 2+ charge, and there are four 1- ions, the overall charge is -2. It is a tetrahedral shape, and so is different from other cobalt complex. The :Cl- ions are larger ligands than the ammonia molecules so fewer ligands can fit around the central metal ion.

Linear complex The solution containing it is called Tollens’ reagent. It is used in organic chemistry to distinguish aldehydes from ketones. The aldehydes reduce the [Ag(NH3)2]+ to Ag (metallic silver) while ketones do not. The silver forms a mirror on the surface of the test tube, giving the name of the test. Called the silver mirror test

Compounds with the same molecular formula but with different arrangements of their atoms in space. Geometrical isomers Optical isomers

Cis-Trans Occurs for monodentate ligands Occurs in octahedral and square planar complexes Yes, they will eg cisplatin is an anti cancer drug which is a cis isomer, whilst the trans isomer of it does nothing

Formula [Pt(Cl)2(NH3)2] It is the square planar complex of platinum Anti cancer drug It is chiral so only the cis isomer works to cure disease, the trans one doesn’t affect cancer

Eg in an octahedral complex ion, the same ligands are next to each other Same with a square planar complex