5.3.1 Transition Elements

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1 5.3.1 Transition Elements General properties of transition metals transition metal characteristics of elements Ti u arise from an incomplete d sub-level in ions these characteristics include formation of coloured ions, variable oxidation state catalytic activity. omplex ion formation, Sc 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 1 Ti 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 2 V 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 3 r 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 5 Mn 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 5 Fe 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 o 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 7 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 8 u 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 10 Zn 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 When forming ions lose 4s before 3d Sc 3 [Ar] 4s 0 3d 0 Ti 3 [Ar] 4s 0 3d 1 V 3 [Ar] 4s 0 3d 2 r 3 [Ar] 4s 0 3d 3 Mn [Ar] 4s 0 3d 5 Fe 3 [Ar] 4s 0 3d 5 o [Ar] 4s 0 3d 7 [Ar] 4s 0 3d 8 u [Ar] 4s 0 3d 9 Zn [Ar] 4s 0 3d 10 Why is Zn not a transition metal? Zn can only form a 2 ion. In this ion the Zn has a complete d orbital and so does not meet the criteria of having an incomplete d orbital in one of its ions. Why is Sc not a transition metal? Sc can only form a 3 ion. In this ion the Sc 3 has an empty d orbital and so does not meet the criteria of having an incomplete d orbital in one of its ions. Typical Properties of Transition metals The existence of more than one oxidation state for each element in its compounds For example iron commonly forms 2 and 3, chromium commonly forms 2,3 and 6 see page 5 of this guide for more detail The formation of coloured ions There are characteristic colours for each transition metal element and the colours can vary in the different oxidation state. See the rest of the guide for many examples The catalytic behaviour of the elements and their compounds and their importance in the manufacture of chemicals by industry Iron is used as a catalyst in the Haber process to produce ammonia. Manganese dioxide Mn 2 catalyses the decomposition of hydrogen peroxide. Vandandium pentoxide V 2 5 catalyses the contact process. See reaction rates for some more detail omplex formation complex :is a central metal ion surrounded by ligands. ligand.: An atom, ion or molecule which can donate a lone electron pair o-ordinate bonding is involved in complex formation. o-ordinate bonding is when the shared pair of electrons in the covalent bond come from only one of the bonding atoms. o-ordination number: The number of co-ordinate bonds formed to a central metal ion. H 2 H 2 H 2 u H 2 H 2 H 2 N Goalby chemrevise.org 1

2 ligands can be unidentate (e.g. H 2, and - ) which can form one coordinate bond per ligand or bidentate (e.g. and ethanedioate ion 2 4 ) which have two atoms with lone pairs and can form two coordinate bonds per ligand or multidentate (e.g. EDTA 4- which can form six coordinate bonds per ligand). H 2 H 2 r 3 A complex with bidentate ligands e.g. [r( ) 3 ] 3 There are 3 bidentate ligands in this complex each bonding in twice to the metal ion The EDTA 4- anion has the formula It has a coordination number of 6 - N N - - with six donor sites(4 and 2N) and forms a 1:1 complex with metal(ii) ions - r 3- A complex with bidentate ethanedioate ligands e.g. [r( 2 4 ) 3 ] 3- Equations to show formation of bidentate and mutidentate complexes u ) 6 3 [u( ) 3 ] 6H 2 u ) [u( 2 4 ) 3 ] 4-6H 2 u ) 6 EDTA 4- [u(edta)] 6H 2 Learn the two bidentate ligands mentioned above but it is not necessary to remember the structure of EDTA Shapes of complex ions transition metal ions commonly form octahedral complexes with small ligands (e.g. H 2 and ). transition metal ions commonly form tetrahedral complexes with larger ligands (e.g. - ). [o( ) 6 ] 2 [u ) 6 ] [o 4 ] Isomerism in complex ions u square planar complexes are also formed, e.g. cisplatin Ag commonly forms linear complexes e.g. [Ag( ) 2 ], [Ag(S 2 3 ) 2 ] 3- and [Ag(N) 2 ] - (all colourless). Ag omplexes can show two types of stereoisomerism: cis-trans isomerism and optical isomerism is-trans isomerism is-(nh3)22 trans-(nh3)22 N Goalby chemrevise.org 2

3 ptical isomerism H 2 omplexes with 3 bidentate ligands can form two optical isomers (non-superimposable mirror images). H 2 N NH H 2 2 H2 H 2 H 2 - isplatin The (II) complex cisplatin is used as an anticancer drug. The cisplatin version only works as two chloride ions are displaced and the molecule joins on to the DNA. In doing this it stops the replication of cancerous cells. In the body one ligand is subsituted by a water molecule ( ) 2 2 H 2 [( ) 2 )] Be able to apply your knowledge of bonding to given information in the question to explain how it bonds to DNA molecule- generally a combination of dative covalent bonding and hydrogen bonding Platin binds to DNA of cancer cells and stops cancer cells dividing. cisplatin transplatin It can also prevent the replication of healthy cells by bonding on to healthy DNA which may lead to unwanted side effects like hair loss. Ligand substitution Reaction with excess With excess ligand substitution reactions occur with several transition aqueous ions. [u ) 6 ] 4 [u( ) 2 ] 4H 2 (l) blue solution deep blue solution Be able to write equations for other metal ions given information about the complex formed. No need to learn colours for these other ions. [o ) 6 ] 6 [o( ) 6 ] 6H 2 (l) Note: This substitution in the case with u is incomplete as not all the water molecules are substituted. The ligands and H 2 are similar in size and are uncharged. Ligand exchange occurs without change of co-ordination number for o and r Reactions with hloride ions Addition of a high concentration of chloride ions (from conc H or saturated Na) to an aqueous ion leads to a ligand substitution reaction. Addition of conc H to aqueous ions of u and o lead to a change in coordination number from 6 to 4. The - ligand is larger than the uncharged H 2 and ligands so therefore ligand exchange can involve a change of co-ordination number. Be careful: If solid copper chloride (or any other metal )is dissolved in water it forms the aqueous [u ) 6 ] complex and not the chloride [u 4 ] complex N Goalby chemrevise.org [u ) 6 ] 4 - [u 4 ] 6H 2 blue solution yellow/ [o ) 6 ] 4 - [o 4 ] 6H 2 pink solution blue solution 3

4 Biological omplexes Fe(II) in haemoglobin enables oxygen to be transported in the blood. Haem is an iron(ii) complex with a multidentate ligand. 2 bonds to Fe ions in the Haemoglobin and when required the 2 is released. is toxic to humans as can from a strong coordinate bond with haemoglobin. This is a stronger bond than that made with oxygen and so it prevents the oxygen attaching to the haemoglobin. With, the stability constant is greater than with complex in 2. Precipitation Reactions with sodium hydroxide and ammonia The bases H - and ammonia when in limited amounts form the same hydroxide precipitates. These reactions are classed as precipitation reactions [u ) 6 ] 2H - u (H) 2 (s) 2H 2 (l) Blue solution blue ppt [Mn ) 6 ] 2H - Mn (H) 2 (s) 2H 2 (l) Very pale pink solution Pale brown ppt [Fe ) 6 ] 2H - Fe (H) 2 (s) 2H 2 (l) [Fe ) 6 ] 3 3H - Fe ) 3 (H) 3 (s) 3H 2 (l) brown ppt Yellow/brown solution [r ) 6 ] 3 3H - r ) 3 (H) 3 (s) 3H 2 (l) u 2H - u(h) 2 (s) Blue solution blue ppt Mn 2H - Mn(H) 2 (s) Very pale pink solution Pale brown ppt Fe 2H - Fe(H) 2 (s) Fe 3 3H - Fe(H) 3 (s) Yellow/brown solution brown ppt r 3 3H - r(h) 3 (s) With ammonia when added in limited amounts the same hydroxide precipitates form. The ammonia acts as a base, removes a proton from the aqueous complex and becomes the ammonium ion [Mn ) 6 ] 2 Mn (H) 2 (s) 2NH 4 [Fe ) 6 ] 3 3 Fe ) 3 (H) 3 (s) 3NH 4 Reaction with excess H - With excess NaH, the r hydroxide dissolves. r becomes [r(h) 6 ] 3- This hydroxides is classed as amphoteric because it can react with alkali to give a solution and react with acid to form the aqueous salt r ) 3 (H) 3 (s) 3H - (aq ) [r(h) 6 ] 3-3H 2 (l) r ) 3 (H) 3 (s) 3H (aq ) [r ) 6 ] 3 Reaction with excess With excess ligand exchange reactions occur with u and r, and their hydroxide precipitates dissolve in excess ammonia The ligands and H 2 are similar in size and are uncharged. Ligand exchange occurs without change of co-ordination number for r r becomes [r( ) 6 ] 3 purple solution This substitution may, however, be incomplete as in the case with u u becomes [u( ) 2 ] deep blue solution r(h) 3 ) 3(s) 6 [r( ) 6 ] 3 3H 2 (l) 3H - u(h) 2 (s) 4 [u( ) 2 ] 2H 2 (l) 2H - N Goalby chemrevise.org 4

5 Variable xidation States Transition elements show variable oxidation states When transition metals form ions they lose the 4s electrons before the 3d Iron (II) xidation Fe () can be easily oxidised to Fe 3 (brown solution) by various oxidising agents. We commonly use potassium manganate (VII), although oxygen in the air will bring about the change Mn - 4 8H 5Fe Mn 4H 2 (l) 5Fe 3 Purple colourless Iron (III) Reduction Fe 3 (brown solution) can be reduced to Fe () by various reducing agents. We commonly use potassium iodide solution. The brown colour of the iodine formed can mask the colour change of the Iron. 2Fe 3 2I - I 2 2Fe Reducing hromium r 3 (green) and then r (blue) are formed by reduction of r 2 7 (orange) by the strong reducing agent zinc in (H) acid solution. Fe is a less strong reducing agent and will only reduce the dichromate to r 3 r H 6Fe 2r 3 7H 2 6 Fe 3 range green The Fe and r 2 7 in acid solution reaction can be used as a quantitative redox titration. This does not need an indicator xidising hromium When transition metals in low oxidation states are in alkaline solution they are more easily oxidised than when in acidic solution It is easier to remove an electron from a negatively charged ion [r ) 6 ] 3 Acidified Not easy to oxidise excessnah [r(h) 6 ] 3- alkaline easier to oxidise Alkaline chromium(iii) can be oxidised by using oxidising agents such as hydrogen peroxide to the (yellow solution) chromate ion r(h) 6 3- Green solution H 2 2 r 4 yellow solution Reduction :H 2 2 2e - 2H - xidation: [r(h) 6 ] 3-2H - r 4 3e - 4H 2 2 [r(h) 6 ] 3-3H 2 2 2r 4 2H - 8H 2 Reduction of u to u u (blue solution) can be reduced to u by various reducing agents. We commonly use potassium iodide solution. 2u 4I - I 2 2uI (s) Brown White solution precipitate Disproportionation of copper(i) ions opper(i) ions when reacting with sulphuric acid will disproportionate to u and u metal 2u u u u e u(s) E o = 0.52 V u e u E o = 0.15 V So E o cell = = 0.37 V As E o u /u > E o u /u and Ecell has a positive value of 0.37V, u disproportionates from 1 oxidation state to 0 in u and 2 in u N Goalby chemrevise.org 5

H H O - : O O. Complex formation. N Goalby chemrevise.org H 2 O OH 2

H H O - : O O. Complex formation. N Goalby chemrevise.org H 2 O OH 2 omplex formation complex :is a central metal ion surrounded by ligands. ligand.: An atom, ion or molecule which can donate a lone electron pair. H 2 H 2 H 2 u H 2 H 2 H 2 o-ordinate bonding is involved