Transition Metals Flashcards
Characteristics of transition metals?
Why do these occur?
-complex formation
-catalytic activity
-formation of coloured ions
-variable oxidation states
-occur due to incomplete d subshell in the atom/ion
Define transition metals
elements with an incomplete d-subshell that can form at least one stable ion with an incomplete d-subshell
Why is zinc not a transition metal
can only form 2+ ion, which has a complete d subshell
Define complex
a central metal ion surrounded by ligands
Define ligand
An atom, ion or molecule which can donate a lone electron pair.
Define coordination number
number of co-ordinate bonds formed to a central metal ion.
electron configuration of Cr and Cr3+
[Ar] 3d5 4s1
[Ar] 3d3
electron configuration of Cu, Cu+ and Cu2+
[Ar] 3d10 4s1
[Ar] 3d10
[Ar] 3d9
What are monodentate ligands?
Give examples
-ligands that can only form one dative bond to the central metal ion
e.g.
-water (H2O) molecules
-ammonia (NH3) molecules
-chloride (Cl–) ions
-cyanide (CN–) ions
What are bidentate ligands?
Give examples
-ligands that can each form two dative bonds to the central metal ion
-due to each ligand having two atoms with lone pairs of electrons
e.g.
-1,2-diaminoethane (H₂NCH₂CH₂NH₂)
-also written as ‘en’
-ethanedioate ion (C₂O₄²⁻)
-also written as ‘ox’
What are multidentate ligands?
Give examples
-ligands with more than two atoms with lone pairs of electrons
-so can form more than two dative bonds
e.g.
-EDTA4- (hexadentate ligand as it forms 6 dative covalent bonds to the central metal ion)
Complexes with water & ammonia molecules
-neutral ligands
-contain a lone pair of electrons
-can be used to form a dative covalent bond with the central metal ion
water: lone pair on oxygen atom
ammonia: lone pair on nitrogen atom
-water and ammonia are small ligands
-6 of them can usually fit around a central metal ion
-each donates a lone pair of electrons, = 6 dative bonds
-coordination number = 6
-overall charge of a complex = sum of charges on the central metal ion and each ligands
Describe what complexes with hydroxide & chloride ions are like
-hydroxide ligands are small, so 6 of them can fit around a central metal ion
-complex will have a coordination number of 6
-Cl- ligands are large ligands
-only 4 of them will fit around a central metal ion
-complexes with 4 chloride ligands have a coordination number of 4
-tetrahedral shape due to minimised repulsion
109.5
when do complexes have a linear shape
give the bond angle and examples with their uses
-central metal atoms or ions with two coordinate bonds
180°
usually Cu+ or Ag+ is the central metal ion with two coordinate bonds formed to two ammonia ligands
-diaminesilver(I) ion, [Ag(NH₃)₂]⁺
-present in Tollens’ reagent
-used to test for the aldehyde functional group in organic molecules
-silver(I) ion is reduced to silver atoms
-produce a silver mirror on the test tube walls
when do complexes have a tetrahedral shape
-if there are four coordinate bonds
e.g. complexes with four chloride ions
when do complexes have a square planar shape
-complexes with four coordinate bonds
-usually cyanide ions (CN-) ligands
-cisplatin
90o
when do complexes have a octahedral shape
-central metal atom or ion forms 6 coordinate bonds
90
geometric
Geometrical (cis-trans) isomerism
Even though transition element complexes do not have a double bond, they can still have geometrical isomers
Square planar and octahedral complexes with two pairs of different ligands exhibit cis-trans isomerism (this is a special case of E-Z isomerism)
An example of a square planar complex with two pairs of ligands is the anti-cancer drug cis-platin
Whereas cis-platin has beneficial medical effects by binding to DNA in cancer cells, trans-platin cannot be used in cancer treatment
As long as a complex ion has two ligands attached to it that are different to the rest, then the complex can display geometric isomerism
Examples of octahedral complexes that exhibit geometrical isomerism are the [Cu(NH3)4(H2O)2]2+ and [Ni(H2NCH2CH2NH2)2Cl2]2+ complexes
[Ni(H2NCH2CH2NH2)2Cl2]2+ can also be written as [Ni(en)2Cl2]2+
Like in the square planar complexes, if the two ‘different’ ligands are adjacent (next) to each other then that is the ‘cis’ isomer, and if the two ‘different’ ligands are opposite each other then this is the ‘trans’ isomer
In [Cu(NH3)4(H2O)2]2+, the two water ligands are adjacent to each other in the cis isomer and are opposite each other in the trans isomer
Transition metal complexes and optical isomers
-octahedral complexes with bidentate ligands also have optical isomers
-means that both forms are non-superimposable mirror images of each other
-have no plane of symmetry
-one image cannot be placed directly on top of the other
-optical isomers only differ in their ability to rotate the plane of polarised light in opposite directions
Examples:
-[Ni(H2NCH2CH2NH2)3]2+ and [Ni(H2NCH2CH2NH2)2(H2O)2]2+
-ligand 1,2-diaminoethane (H2NCH2CH2NH2) can be written as ‘en’
-ligand ethanedioate ion (C2O4 2-) can be written as ‘ox’
How to draw stereochemical formulae
-solid line = bond in the same plane as the paper
-dotted line = bond receding behind the plane of the paper (can also be hatched or shaded wedges)
-solid wedge = bond coming out of the paper
What is ligand substitution
-one ligand in a complex is replaced by another
-forms a new complex that is more stable than the original one
-can be partial or complete substitution
-complex ion can change its charge or remain the same depending on the ligand involved
-no changes in coordination number, or the geometry of the complex, if the ligands are of a similar size
-if the ligands are of a different size, for example water ligands and chloride ligands, then a change in coordination number and the geometry of the complex will occur
Complete substitution without change in coordination number in cobalt(II) complexes
The [Co(H2O)6]2+(aq) complex ion is pink in colour
If ammonia solution is added to [Co(H2O)6]2+, a pale yellow / straw coloured solution will be formed
Complete ligand substitution of the water ligands by ammonia ligands has occurred
[Co(H2O)6]2+ (aq)
+ 6NH3 (aq) → [Co(NH3)6 ]2+ (aq) + 6H2O (l)
pink solution yellow solution
If excess concentrated ammonia solution is added to [Co(H2O)6]2+, a brown solution will be formed
The ammonia ligands make the cobalt(II) ion so unstable that it readily gets oxidised in air to cobalt(III), [Co(NH3)6]3+ (aq)
Upon dropwise addition of sodium hydroxide (NaOH) solution to [Co(H2O)6]2+(aq), a blue precipitate is formed
Partial ligand substitution of two water ligands by two hydroxide (OH-) ligands has occurred
[Co(H2O)6]2+ (aq)
+ 2OH- (aq) → Co(OH)2(H2O)4 (s) + 2H2O (l)
pink solution blue precipitate
cause of incomplete ligand substitution
-unfavourable energetics of the reaction and instability of the product
-copper(II) ions illustrate this behaviour with ammonia
-different sized ligands can also lead to incomplete substitution
Incomplete substitution in copper(II) complexes:
-hexa-aqua complexes are most common when a transition element ion is in solution, (i.e. it has six water ligands attached to it)
-e.g., Cu2+(aq) is [Cu(H2O)6]2+(aq)
-[Cu(H2O)6]2+ (aq) complex ion is pale blue in colour
-upon dropwise addition of sodium hydroxide (NaOH) solution, a light blue precipitate is formed
-partial ligand substitution of two water ligands by two hydroxide ligands has occurred
[Cu(H2O)6]2+ (aq) + 2OH- (aq) → Cu(OH)2(H2O)4 (s) + 2H2O (l)
blue solution light blue precipitate
-addition of excess concentrated ammonia (NH3) solution = pale blue precipitate dissolves to form a deep blue solution
-partial ligand substitution has occurred
Cu(OH)2(H2O)4 (s) + 4NH3 (aq) → [Cu(NH3)4(H2O)2 ]2+ (aq) + 2H2O (l) + 2OH- (aq)
light blue precipitate deep blue solution
-adding concentrated NH3 solution dropwise to the [Cu(H2O)6]2+ (aq), instead of NaOH solution, the same light blue precipitate would form
-again, the pale blue precipitate will dissolve to form a deep blue solution, if excess ammonia solution is then added
Change in co-ordination number
-water ligands in [Cu(H2O)6]2+ can also be substituted by chloride ligands, when concentrated HCl is added
-reversible reaction
-complete substitution of the water ligands causes blue solution to turn yellow
[Cu(H2O)6]2+ (aq) + 4Cl- (aq) → [CuCl4 ]2- (aq) + 6H2O (l)
blue solution yellow solution
-coordination number has changed from 6 to 4, as the chloride ligands are larger than the water ligands
-so only 4 will fit around the central metal ion
-some of the [Cu(H2O)6]2+ complex ion will still be present in the solution
-mixture of blue and yellow solutions in the reaction mixture will give it a green colour
-adding water to the solution → chloride ligands are displaced by the water molecules → the [Cu(H2O)6]2+ (aq) ion and blue solution will return
Incomplete substitution in cobalt(II) complexes:
The water ligands in [Co[H2O)6]2+ can also be substituted by chloride ligands, upon addition of concentrated hydrochloric acid
The complete substitution of the water ligands causes the pink solution to turn blue
[Co(H2O)6]2+ (aq) + 4Cl- (aq) → [CoCl4 ]2- (aq) + 6H2O (l)
pink solution blue solution
Like with [Cu(H2O)6]2+ above, the coordination number has changed from 6 to 4, because the chloride ligands are larger than the water ligands, so only 4 will fit around the central metal ion
Adding water to the solution will cause the chloride ligands to be displaced by the water molecules, and the [Co(H2O)6]2+ (aq) ion and pink solution will return
Describe the Haem Complex
-complex with iron(II) at its centre
-O atoms form a dative covalent bond with the Fe(II)
-enables oxygen molecules to be transported around the body in the blood
-O2 molecules are not very good ligands and bond weakly to the iron(II)
-allows them to break off easily and be transported into cells
-CO is toxic AS it is a better ligand than oxygen
-binds strongly and irreversibly to the iron(II)
-prevents O2 from being carried to the cells
-if oxygen attached to the haemoglobin (oxyhaemoglobin) is replaced by carbon monoxide (carboxyhaemoglobin), a darker red colour is produced in the haem complex
Signs of carbon monoxide poisoning
-anaemia occurs when a person does not have enough haemoglobin in their blood
-due to a loss of blood or deficiency in iron
-can be restored by taking iron sulfate tables in the diet
