Topic 15 - Transition Metals Flashcards
What are transition metals? What are some their characteristics?
D block elements that form one or more stable ions with incompletely filled d orbitals
Hard solids
High mpt/bpt
act as catalysts
form coloured ions/compounds
form ions with different oxidation numbers
form ions with incompletely filled d orbitals
Define a ligand and give a few examples
Species that uses a lone pair of electrons to form a dative bond with a metal ion
Examples: Water, hydroxide, ammonia, chloride
Describe process of ligands forming with transition metals, include definition of a complex
Relatively small size of transition metal ions enable them to attract electron rich species strongly like water molecules are attracted to the transition metal ions so strongly they form a specific number of bonds usually six. Water molecules can form dative bonds called ligands. The complete formula for ion and water is [Fe(H2O)6]2+ called a complex - species containing a metal ion joined to ligands. With charges they are complex ions. The total number of dative bonds is the coordination number
Explain why transition metals form coloured compounds in solution - aqueous solution containing copper(II) ions as an example
Aqueous Copper(II) sulfate is blue - Ions that have completely filled 3d energy levels and ions that have no electrons in their 3d level are not coloured. When water ligands are attrached to copper(II) ions the energy level splits into 2 levels with slightly different energies. The lower energy level contains six electrons and the higher energy level contains 3. If one in the lower energy level absorbs energy from spectrum, it can move to higher energy level. When electron moves to higher level, amount of energy absorbs depends on difference in energy between the two levels
How can you predict the shapes of complexes using electron pair repulsion theory?
A ligand bonds to the central metal ion by donating a pair of electrons to form a dative bond. You should ignore the 3d electrons in the transition metal ion and the overall charge on the complex - just count the no of electrons donated by the ligands.
number of ligands = six, electrons donated = 12, octahedral
Ligands = 4, electrons donated = 8, tetrahedral
Ligands = 2, electrons donated = 4, linear
Describe structure of octahedral and tetrahedral and linear complexes
Give examples, structure, formula, displayed formula, charges
Octahedral - water, ammonia and hydroxide ion
Ligands have different no of lone pairs of electrons, each ligand uses only one lone pair to form a coodrinate bond with transition metal ion. Contain 6 ligands, complexes described as six-fold coordination. Electron pair donor is element in Period 2. Solid/hatched bonds indicated shapes, overall charges all different, structures use square brackets, water ligands shown as H2O and OH2
Tetrahedral/linear = Rarer than octahedral, chloride ions act as ligands. Insufficient room around central metal ion for six chloride ions to act as ligands since chloride is in period 3. Only linear complex likely to find in Tollens reagent Ag+ ions has only 2 ligands but silver is transttion metal in period 5
What is a square planar molecules using XeF4 to explain? Give an example of a complex and the uses of this example
Atom of xenon has eight electrons in its outermost energy level, each fluorine atom uses one of its electron to form covalent bond. Outer energy shell now contains 12 electrons. Arranged in six pairs forming octahedral arrangement. Two of these pairs are lone pairs repel each other and therefore located opposite each other. 4 bonding pairs are in a plane with four fluorine atoms at the corners of a square.
Cis-Platin - has isomer trans-platin. These has a platinum(II) ion, two ammonia ligands and two chloride ion ligands. Cis indicates identical ligands next to each other.
Cis platin used as anti-cancer action = cells like cancer cells contain DNA, during cell division two strands of DNA separate from each other to form more DNA> Cis platin enables it to form a bond between two strands of DNA which prevents them from separating and so prevents the cancer cells from dividing.
What is denticity? Explain in terms of monodentate and bidentate and multidentate and give examples
Property of ligands - comes from word dentis meaning tooth. Monodentate means ligands uses one lone pair of electrons on one atom to form the dative bond. Bidentate has two atoms, each of which can use lone pair of electrons to form dative bond with metal ions. Multidentate a ligand with several atoms, uses a lone pair of electrons to form dative bond with metal ion.
Bidentate ligands e.g. NH2CH2CH2NH2 - 1,2 diaminoethane. When acts as bidentate ligand, uses lone pair of electrons on each nitrogen atom to attach to the metal ions.
Multidentate ions = Most likely to see an organic ion, consider structure of 1,2 diaminoethane - 4 hydrogen atoms on two nitrogens replaced by -CH2COOH - 4 ethanoic acids used to form structure form EDTA - final step consider ion formed when each ethanoic acid loses its H+ ion and gives ion with 4 negative charges so hexadentate ligand called EDTA4-
It shows the 6 lone pairs of electrons - two pairs on nitrogen and 4 pairs on Oxygen to form dative bond when ion acts as a ligand
Discuss the stablility of complexes?
Most complexes stable - do not decompose readily - stability doesnt refer to complexes containing transition metal ions with unstable oidation numbers. Refers to comparison of stabilities of two complexes in which number of ligands has changed. Consider ligand exchange reaction in which monodentate ligand replaced by bidentate lingand
[Cu(H2O)6]2+ + 3en —> [Cu(en)3]2+ + 6H2O
Six water ligands replaced by three 1,2 diaminoethane ligands so total number of species increased from four to seven so system more disordered and there is increase in DeltaS of system. Ligand exchange reactions of this sort lead to increase in stability of products to reactants so formation of product favoured
When monodentate repalced by multidentate ligand - increase in stability is even greater
[Cu(H2O)6]2+ + EDTA4- –> [Cu(EDTA)]2- + 6H2O
Six water ligands replaced by one EDTA4- ligand so total number of species goes from 2 to 7
Discuss heamoglobin and oxygen transport as well as haemoglobin and carbon monoxide in regards to ligands and structures
Haemoglobin consists of 3 parts - largest is protein. Within protein there are four haem groups made up mostly of C and H atoms. Inside each haem group, 4 nitrogen atoms that hold Fe2+ ion by forming dative bond in square planar structure. 5th dative bond from protein to Fe2+ ion. When blood passes through lungs, haemoglobin collects O molecules and transports them to cells where its released. When haemoglobin collects O, O molecule acts as ligand by using one of lone pairs of electrons to form dative bond with Fe2+ ion
CO molecule has lone pair of electrons on its C atom that enables it to act as ligand. Strength of dative between O and haemoglobin not very strong but advantage of this is O molecule easily released when needed. Much strong dative bond between CO and haemoglobin so any CO breathed in likely to replace O already bound to haemoglobin - ligand substitution reaction. Once CO formed carboxylhaemoglobin - dative bond so strong doesnt break easily. If enough carboxylhaemoglobin formed, too little oxygen transported to sypport life
Name types of reactions and expected results in transitional metal topic
Redox - oxidation number of transition metal ion changes
Acid-base reaction - one or more of ligands gains or loses H ion
Ligand exchange - one or more of ligands around transition metal ion replaced by different ligand
Coordination number change - number of ligands changes
Any of these reactions can cuase change in colour of the complex
Discuss change in oxidation number and colour change for transition metal for example aqueous Fe2+ exposed in air
An aq solution containing Fe2+ ions is pale green but exposed to air turns yellow or brown - oxidation no to +3. Type and number of ligands remain unchanged in this oxidation reaction so formulae of two complexes are [Fe(H2O)6]2+ to [Fe(H2O)6]3+
Discuss the formation of [Cu(NH3)4(H2O)2]2+ an acid-base and ligand exchange reaction
Consider when aqueous sodiun hydroxide is added to copper(II) sulfate solution
When added - pale blue solution froms blue ppte.
[Cu(H2O)6]2+ +2OH- —> [Cu(H2O)4(OH)2] + 2H2O
2 hydroxide ions replaced two water molecules - acid-base reaction because two hydroxide ions removed hydrogen ions from 2 of the water ligands and converted them into water molecules. The two water molecules that have lost hydrogen ions are now hydroxide ligands - term amphoteric behaviour describes this acid-base reaction because of its reversible nature - when acid reacts with hydroide ion ligand, converts them back to water molecule
Same obs made druing addition of aq ammonia instead
[Cu(H2O)6]2+ + 2NH3 —> [Cu(H2O)4(OH)2] + 2NH4+
Two of water ligands transfer hydrogen ion to the ammonia molecules - when aq ammonia is added again - blue ppte formed, dissolves to form deep blue solution
[Cu(H2O)4(OH)2] + 4NH3 —> [Cu(NH3)4(H2O)2]2+ + 2H2O + 2OH-
Ligand exchange reaction - 4 ammonia molecules replace 2 water molecules and 2 hydroxide ions
Solutions:
- pale blue solution contains [Cu(H2O)6]2+
- pale blue ppte contains [Cu(H2O)4(OH)2]
- deep blue solution cotains [Cu(NH3)4(H2O)2]2+
Describe the formation of [CuCl4]2- = change in coordination number
Example copper(II) sulfate solution and conc HCl
When acid slowly and continuously added, colour changes from blue to green and finally yellow
[Cu(H2O)6]2+ + 4Cl- —> [CuCl4]2- + 6H2O
All six water ligands substituted for 4 chloride ions. Reation change in coordiantion number - from 6 to 4. Charge on complex changed from 2+ to 2-, no change in oxidation number
Arrows show reaction reversible, helps to explaine colour change. Hexaaquacopper(II) ion is blue and tetrachlorocuprate(II) ion yellow so green is mixture of the two complex ions
Describe reactions involving cobalt complexes with…
1) alkalis
2) Concentrated HCl
1) Aq sodium hydorxide added to solution with hexaaquacobalt(II) ion until no further change seen. Pink solution forms blue ppte.
[Co(H2O)6]2+ + 2OH- —-> [Co(H2O)4(OH)2] + 2H2O
With copper, acid-base reaction and two hydroxide ions removed hydrogen ions from two of water ligands and converted them into water molecules. Two water molecules have lost hydrogen ions are now hydroxide ligands - colour of ppte changes to pink
If aq ammonia used as alkali - added to excess there is obs not made with aq sodium hydroxie - ppte dissolves to form brown ppte
[Co(H2O)6]2+ + 2NH3 -> [Co(H2O)4(OH)2] + 2NH4+
Forming brown ppte
[Co(H2O)4(OH)2] + 6NH3 –> [Co(NH3)6]2+ + 4H2O + 2OH-
Brown solution changes colour due to oxidation by oxygen in atmosphere - oxidation number of cobalt from +2 to +3 and yellow [Co(NH3)6}3+ ion forms - resulting solution looks darker than yellow
2) When conc acid slowly added to solution with hexaaquacobalt(II) ion the pink solution changes to blue
[Co(H2O)6]2+ + 4Cl- —-> [CoCl4]2- + 6H2O
All six water ligands replaced by 4 chloride ions - change in coordination number from 6 to 4 and charge on complex 2+ to 2- so not change in oxidation number
