Transition Metal Complex Chemistry PDF

A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS All transition metals can be found in the “d” block of the periodic table as all have their outermost electron in a d-orbital. However, not all “d’ block elements are transition metals....

A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS All transition metals can be found in the “d” block of the periodic table as all have their outermost electron in a d-orbital. However, not all “d’ block elements are transition metals. s d p A transition metal is an element forms one or stable ion with a partially filled d- orbital. e.g. in Period 4 Outer Electronic Atom / Ion Explanation Structure Sc [Ar] 4s2 3d1 NOT a transition metal Atom has a partially filled d-orbital but Sc3+ [Ar] 4s0 3d0 its ion does not! They are empty Fe [Ar] 4s2 3d6 IS a transition metal Both the atom and all of its ions have Fe3+ [Ar] 4s0 3d5 partially filled d-orbitals Cu [Ar] 4s1 3d10 IS a transition metal Cu2+ has a partially filled d-orbital Cu2+ [Ar] 4s0 3d9 Zn [Ar] 4s2 3d10 NOT a transition metal Atom has a partially filled d-orbital but Zn2+ [Ar] 4s0 3d10 its ion does not! Reminder from Year 12! Remember, d block elements lose their 4s electrons before the 3d electrons (as shown above) AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.8 THERMODYNAMICS TRANSITION METAL CHARACTERISTICS All transition metals have the following distinct characteristics: 1. All have variable oxidation states All transition metals can form more than one stable ion / oxidation state. e.g. Fe can form Fe2+ & Fe3+, Cu can form Cu+ and Cu2+, Mn has 7 different oxidation states! By comparison, Sc and Zn in the d block can only form one stable ion that does not have a partially filled d-orbital so are not classed as transition metals. 2. All form Complex Ions What this means is that transition metal ions are able to coordinately bond to other atoms and molecules using the spaces their partially filled / empty orbitals. These arrange themselves around the transition metal ion to form a “complex”. More on these later! 3. All form Coloured Ions Transition metal complexes are all coloured e.g. You probably recognise copper sulfate as being blue. In actual fact it is the [Cu(H2O)6]2+ complex that is blue! More on how and why later! 4. All have Catalytic Properties As transition metals have partially filled d-orbitals, they can use them to facilitate (catalyse) other reactions. Again, more on how this works later! AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS LIGANDS AND COMPLEXES Transition metals can use their partially filled / empty orbitals to accept pairs of electrons from other species (atoms / molecules / ions), allowing them to form coordinate / dative covalent bonds with the ion. Once a species has coordinately bonded to a transition metal ions, it known as a “ligand”. The combination of the transition metal ion and its ligands is known as a “complex”. There are 3 types of ligand that you need to be aware of: Monodentate Ligands: These are species that use ONE electron pair to coordinately bond with a transition metal ion. e.g. H2O NH3 Cl- OH- : O N HO – : Cl – H H H H H Bidentate Ligands: These are species that use TWO electron pairs to coordinately bond with a transition metal ion. e.g. ethane-1,2-diamine ethanedioate ions O O H N CH2 CH2 N H C CH2 CH2 C H H – O O – Multidentate Ligands: These are species that use multiple electron pairs to coordinately bond with a transition metal ion. e.g. EDTA4- “Haem” as in Haemoglobin You do not need to know the structure of these, just know that one molecule has multiple lone pairs that it uses to form 6 coordinate bonds with a transition metal ion. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS MONODENTATE COMPLEXES : L L : L : :L M M L: L: :L : L L : : L Octahedral Tetrahedral o Bond Angle: 90 Bond Angle: 109.5o Coordinate bonds: 6 Coordinate bonds: 4 Coordination Number: 6 Coordination Number: 4 No lone pairs on central ion Only 4 ligands bond if they are Most common shape large ions e.g. Cl- L: L: L: :L : M M : L: L : Linear Bond Angle: 180o Square Planar Coordinate bonds: 4 Bond Angle: 90o Coordination Number: 4 Coordinate bonds: 4 No pairs on central metal ion Coordination Number: 4 e.g. Ammoniacal Silver (nitrate) 2 lone pairs on central metal ion [NH3:Ag:NH3]+ e.g. cisplatin Used in Tollen’s Reagent. A test [Pt(Cl2)(NH3)2] for carbonyl compounds. M = Transition metal ion L = Ligand AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS BIDENTATE COMPLEXES All examples of these that you will see are octahedral and have a coordination number of 6. 3 ligands for two coordinate bonds each.The carbon chains in these are shown as skeletal formula to simplify the diagram. e.g. Ethane-1,2-diamine NH2 : H N CH2 CH2 N H H2N : :NH2 H H Number of Ligands: 3 M Coordinate bonds: 6 Coordination Number: 6 H2N: : NH2 An ethane molecule with an amine group on each carbon. The lone pair on each nitrogen : NH2 forms a coordinate bond. O Ethanedioate Ion O – O O O : – – C CH2 CH2 C O : :O – O O – O M Number of Ligands: 3 Coordinate bonds: 6 O–: : O– Coordination Number: 6 O Also known as the oxalate ion (C2O42-) : –O It’s the ion of ethanedioic acid. A lone pair on each O- forms a O coordinate bond. O AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS MULTIDENTATE COMPLEXES There are only two examples of these that you need to know. You do not have to be able to draw them. Just know that each multidentate ligand has 6 atoms with lone pairs, that form 6 coordinate bonds with the central metal ion. EDTA4- : Number of Ligands: 1 : : Coordinate bonds: 6 Coordination Number: 6 The EDTA4- molecule surrounds the M central metal ion creating 6 : : coordinate bonds. : “Haem” Number of Ligands: 3 : Coordinate bonds: 6 : : Coordination Number: 6 This is a ring-shaped molecule that Fe bonds around an Fe ion, forming the haemoglobin molecule. : : Fe coordinately bonds to oxygen to transport it around the body. : However, if CO (carbon monoxide) bonds to this molecule, it is permanent. Hence why CO is toxic. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS CHARGES ON COMPLEXES The central metal ion will always have a positive charge (usually 2+ or 3+). Ligands may be neutral or have a negative charge. We must take both of these charges into account when deducing the overall charge on a complex ion as they will cancel out. e.g. Charge on metal ion: 2+ H2O 2+ Charge on ligand: 0 Overall charge: 2+ : H2O : : 2 OH Shorthand: [Cu(H2O)6]2+ This shows that the single Cu copper ion has 6 H2O ligands bonded around it. The square H2O : :OH2 brackets represent the “complex” and the charge is placed outside it. : H2O O 4- O Charge on metal ion: 2+ – Charge on ligand: 3 x 2- = 6- O : Overall charge: 4- – – O : :O Shorthand: [Ni(C2O4)3]4- O This shows that the single nickel Ni ion has 3 C2O4 ligands bonded around it. The square brackets O–: : O– represent the “complex” and the O charge is placed outside it. : –O O O AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS LIGAND SUBSTITUTION REACTIONS In (aq) solution, transition metal ions form complexes using H2O molecules as ligands. e.g. [Cu(H2O)6]2+ [Fe(H2O)6]3+ However, when other ligands are added to the solution, they can substitute into the complex for the H2O ligands. This can change the colour and the shape of the complex itself. The next few pages detail the monodentate ligand substitution reactions that you need to know including: formulae of the complexes reagents needed colours of the complexes shapes of the complexes In truth, there are two reaction types here that you need to be aware of: 1. Ligand Substitution (L.S.) Where one ligand is literally exchanged for another 2. Acid-Base (A-B) These look like ligand substitution reactions but they are not. Both OH- and NH3 are bases. When they are added drop by drop to a a complex that has 6 H2O ligands around it (e.g. [Cu(H2O)6]2+) they accept a proton (H+) from water molecules, leaving an OH- behind. This is why both NaOH(aq) and NH3(aq) drop by drop both produce the same product with each of the complexes you will see on the next few pages. How To Write Equations for Ligand Substitution Reactions AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS THE CHELATE EFFECT It is also possible for bidentate and multidentate ligands to substitute into the complex in the place of monodentate ligands. You do not need to know any specific complex or colours here, but you do need to explain why these substitution reactions occur so readily. [Cu(H2O)6]2+(aq) + 3 C2O42-(aq) → [Cu(C2O4)3]2+(aq) + 6H2O(l) In thermochemistry we learned that Gibb’s Free Energy is a measure of how likely a reaction is to occur. When bidentate or multidentate ligands substitute for monodentate ligands, this reaction is highly feasible, as ΔG is always negative. ΔG = ΔH - TΔS ΔH for this reaction is always ΔS for these reactions is always positive. negligible (≅0). The amount of i.e. there is an increase in entropy. energy required to break the This is because the reaction produces a coordinate bonds between the greater number of moles of products water ligands and the transition than there are reactants as we are metal ion is very similar to the replacing 6 monodentate ligands with amount of energy released fewer bidentate or multidentate ligands. when the new coordinate bonds e.g. in the equation above, there are 4 form. So there is no significant moles of reactants and 7 moles of overall enthalpy change (ΔH). products. ΔG = ΔH - TΔS 0 - a positive value So, if ΔH is 0 (zero), T is always positive (as it is in Kelvin) and ΔS is always positive, ΔG must always be less than 0 (zero) at any temperature. Hence these ligand substitution reacts are feasible at any temperature. This is known as the chelate effect. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS ISOMERISM IN MONODENTATE COMPLEX IONS Octahedral monodentate complex ions can show geometrical / cis-trans isomerism. This occurs when there its a 4 : 2 split in the types of ligands coordinately bonded. i.e. 4 of one type and 2 of another. To identify these as cis or trans, focus on the position of the two ligands. e.g. [Fe(H2O)4(OH)2] or [Cu(NH3)4(H2O)2]2+ OH OH : : H2O : :OH2 H2O : : OH Cu Cu : : H2O: OH2 H2O: OH2 :OH : OH2 This is a cis isomer, as the two ligands This is a trans isomer, as the two (OH) are next to each other. These ligands (OH) are opposite each other. can be in any position, so long as they These can be in any position, so long are not directly opposite each other. as the central metal ion is directly between them. This can also occur when there are 2 monodentate ligands and 2 bidentate ligands in an octahedral complex! e.g. Trans[Cu(C2O4)2(H2O)2]2+ OH2 2- Remember, if the two ligands : are opposite each other, it's a O : :O “trans" isomer. O O Cu Anywhere else, it’s a “cis" isomer. : O: O O O : OH2 AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS Square planar monodentate complex ions can also show this when there are two pairs of different monodentate ligands coordinately bonded. e.g. Cisplatin and Transplatin [Pt(NH3)2(Cl)2] H3N : :NH3 Cl : :NH3 Pt Pt Cl : : H3N : : Cl Cl Cisplatin Transplatin AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS ISOMERISM IN BIDENTATE COMPLEX IONS Octahedral bidentate complex ions can show optical isomerism. This occurs in any octahedral bidentate complex where 3 bidentate ligands are coordinately bonded to the central metal ion. e.g. [Cu(C2O4)3]4- O O 2- 2- O O O O : : O : :O O : :O O O Cu Cu : : O: O: O O O O : : O O O O O O Isomer A Isomer B Mirror Image As in organic chemistry, these optical isomers (enantiomers) are mirror images of each other. They are non-superimposable. They rotate plane, polarised light in opposite directions. If there is an equimolar (racemic) mixture of the two isomers, plane, polarised light will not be rotated as they cancel each other out. AQA www.chemistrycoach.co.uk © scidekick ltd 2024