3.4 chemistry of the d-block transition metals Flashcards
elements of the d-block have their outer electrons in the __ orbitals?
d orbitals
are all d-block elements transition metals?
no
what is a transition metal?
- a d-block element that forms one or more stable ions with partially filled d-orbitals
although zinc is part of the d-block, why is it not a transition element?
because it doesnt have a partially filled d sub-shell as an atom or in its stable ion Zn2+
what are some transition metal characteristics?
- variable oxidation states
- form complex ions
- form coloured ions in solution
- catalyse reactions either as elements or in compounds
what are the two exceptions to the electronic configuration rule that the 4s orbitals are filled before the 3d orbitals?
- chromium
- copper
Cr : [Ar] 3d^5 4s^1
Cu : [Ar] 3d^10 4s^1
when transition metal atoms form ions, what electrons are lost first?
the 4s electrons are lost first
when you write or draw the electronic configuration of any transition metal ion, they always have an empty 4s orbital
write the electronic configuration for Ni^2+
Ni : [Ar] 3d^8 4s^2
Ni^2+ : [Ar] 3d^8
write the electronic configuration for Fe^3+
Fe : [Ar] 3d^6 4s^2
Fe^3+ : [Ar] 3d^5
can transition metals have different oxidation states?
yes
why can transition metas have different oxidation states?
- because the electrons in the 4s and 3d orbitals have very similar energies
- so the energy required to remove/gain any of these electrons is similar
(a relatively similar amount of energy is required to gain or lose different number of electrons)
- the lower oxidation states are found as simple ions e.g Co^2+ or Fe^3+
- the higher oxidation states only exist when the metals are covalently bonded to very electronegative elements such as oxygen e.g CrO4 2- or MnO4 -
what is the most common oxidation states of chromium?
+2, +3 and +6
what is the most common oxidation states of manganese?
+2, +4 and +7
what is the most common oxidation states of iron?
+2 and +3
what is the most common oxidation states of cobalt?
+2 and +3
what is the most common oxidation states of copper?
+1 and +2
what does a transition metal complex consist of?
- a central metal ion surrounded by a number of molecules or ions called ligands
ligand definition
- a small molecule or ion with a lone pair of electrons that coordinate bonds to a central metal atom to form a complex ion
(lone pair donor)
the ligands are attached to the central ion by what?
coordinate bonds
the ligand supplies the two electrons which is what makes it a coordinate bond
when do ligands form coordinate bonds?
(transition metal ions have many orbitals available for bonding, many of which are empty)
- when an orbital, from the ligand, containing a lone pair of electrons overlaps with empty orbitals on the transition metal ion
how are ligands classified?
by the number of coordinate bonds that they can form in complexes
what are monodentate ligands?
- ligands that have one atom which can bond to the metal ion
what are examples of monodentate ligands?
- water
- ammonia
- chloride ion
- cyanide ion
what are bidentate ligands?
ligands that have two atoms which can bond to the metal ion
typically, complexes are either octahedral with six ligands arranged around the central metal ion (more common) or tetrahedral with four ligands (less common)
what is the bond angle of an octahedral shape?
90°
what is the bond angle of a tetrahedral shape?
109.5°
what is the colour of the octahedral complex [Fe(H2O)6] 2+?
pale green
what is the colour of the octahedral complex [Fe(H2O)6] 3+?
yellow
what is the colour of the octahedral complex [Cr(H2O)6] 3+?
dark green
what is the colour of the octahedral complex [Co(H2O)6] 2+?
pink
what is the colour of the octahedral complex [Cu(H2O)6] 2+?
(pale) blue
what is the colour of the octahedral complex [Cu(NH3)4(H2O)2] 2+?
royal blue
- the familiar colour of many Cu2+ aqueous solutions is due to the [Cu(H2O)6] 2+ ion.
- one lone pair of electrons on the oxygen atom from each water molecule is used to form a coordinate bond to the metal ion
what is the colour of the tetrahedral complex [CoCl4] 2-?
blue
what is the colour of the tetrahedral complex [CuCl4] 2-?
yellow-green
when is the [CuCl4] 2- complex formed?
when the Cu2+ ions react with concentrated hydrochloric acid which displaces the water molecules
what does the overall charge on the complex depend on?
the charge on the central ion and the total charge due to the ligands
what is a ligand exchange reaction?
a reaction in which one ligand in a complex ion is replaced by a different one
is the ligand exchange reaction an equilibrium process?
yes
because a ligand exchange reaction complex contains two different ligands, there could be two different arrangements, but usually the two water molecules are opposite each other
ligand exchanges can also lead to a change in geometry of the complex ion
if ammonia solution is added to a solution containing [Cu(H2O)6] 2+, ammonia molecules replace four of the water molecules to form a new complex [Cu(NH3)4(H2O)2] 2+
- this is an equilibrium process
[Cu(H2O)6] 2+ + 4NH3 ⇌ [Cu(NH3)4(H2O)2] 2+ + 4H2O
blue ⇌ royal blue
if concentrated hydrochloric acid is added to a solution containing [Cu(H2O)6] 2+, six water molecules are replaced by four chloride ions
- this is a reversible reaction
- [Cu(H2O)6] 2+ + 4Cl- ⇌ [CuCl4] 2- + 6H2O
- blue ⇌ yellow-green
- concentrated HCl is used because it provides a very high concentration of chloride ions. The high chloride ion concentration shifts the equilibrium to the right giving a yellow-green colour. Adding water shifts the equilibrium to the left and the solution returns to blue
a similar reaction occurs with [Co(H2O)6 2+] and chloride ions, but the colours are different.
- if a large amount of chloride is used, for example by adding concentrated hydrochloric acid, the equilibrium shifts from the pink octahedral complex to the blue tetrahedral complex
- chloride ions are bigger than water molecules so there isnt room to fit six of them around the central metal ion
- [Co(H2O)6 2+ + 4Cl- ⇌ [CoCl4] 2- + 6H2O
- pink ⇌ blue
transition metal ions are only coloured when _________?
they form complexes
without ligands, all d orbitals in a transition metal ion have the same energy (they are degenerate)
how do transition metal ions form colour?
- when ligands approach the metal ion, they cause the energy of three of the d orbitals to become different to the other two. this splits the d orbitals into two sets: three of lower energy and two of higher energy
- an electron in a 3d orbital can move from a lower energy set to a higher energy set if it can gain sufficient energy
- when visible light is passed through a solution of this ion, some of the energy is absorbed which promotes an electron to a higher 3d orbital
- only one frequency (colour) of light is absorbed, which corresponds to the energy gap between the orbitals (∆E = hf)
- the colour you see is made up of the light frequencies that are not absorbed i.e those that are reflected
- when light of a particular colour is absorbed, its complimentary colour is reflected
why are compounds containing the complex [Cu(H2O)6] 2+ typically pale blue?
as they absorb light in the red region of the spectrum
the energy difference, ∆E, between the split 3d orbitals, and as such the colour, depends on what?
- the ligand
- the coordination number
- the transition metal ion
- the oxidation state of the metal
(different ligands cause different splitting of orbitals, so different frequencies are absorbed (giving a different ∆E) and different colours are produced)
are all complexes coloured?
no
- e.g Cu+, Sc3+
- copper (I) complexes have an electronic configuration with a full d sub-shell, while Sc3+ ions have an empty d sub-shell, meaning that electrons cannot move from lower to higher orbitals
- therefore copper (I) and scandium (III) complexes appear colourless
what is the colour of aqueous solutions of compounds containing the ion Cr3+?
(dark) green
what is the colour of aqueous solutions of compounds containing the ion Cr2O7 2-?
orange
what is the colour of aqueous solutions of compounds containing the ion Fe 2+?
pale green
what is the colour of aqueous solutions of compounds containing the ion Co 2+?
pink
what is the colour of aqueous solutions of compounds containing the ion CrO4 2-?
yellow
what is the colour of aqueous solutions of compounds containing the ion MnO4-?
purple
what is the colour of aqueous solutions of compounds containing the ion Fe 3+?
red-brown
what is the colour of aqueous solutions of compounds containing the ion Cu 2+?
pale blue
what are some examples of how transition metals are used as catalysts?
- iron in the haber process to create ammonia (heterogeneous)
- nickel to make margarine from the hydrogenation of vegetable oils (heterogeneous)
- vanadium (V) oxide in the contact process to create sulfuric acid
- manganese (IV) oxide in the catalytic decomposition of hydrogen peroxide (homogeneous)
what are the 2 types of catalysts?
- homogeneous
- heterogeneous
what are homogeneous catalysts?
- catalysts in the same physical state as the reactants
how do homogeneous catalysts work?
- by using their variable oxidation states to oxidise/reduce a reactant, making it more reactive
- the transition metal can then be converted back to its original oxidation state by reaction with another molecule
- the catalysed reaction takes place via intermediate species
what are heterogeneous catalysts?
- catalysts in the different physical state to the reactants
how do heterogeneous catalysts work?
- by a process called surface adsorption
- the process works by:
• the incoming reactant molecules to bond to the surface of the metal, usually by attracting the surface electrons. if they form covalent bonds with the metal surface, this is called chemisorption but if they are held by intermolecular forces it is known as physical adsorption
• the incoming molecules are brought into close proximity with each other and make bonds between each other
• the transition metal regains the electrons as the product(s) leave - reaction occurs on the surface of the catalyst
if you add sodium hydroxide solution to most transition metal ions, what will you get?
a coloured ppt
if you add sodium hydroxide to most transition metal ions, you will get a coloured ppt. if the ppt dissolves on addition of excess sodium hydroxide solution, then the transition metal ion is _____?
amphoteric
transition metal ion: Cr3+
- what is the colour of the solution?
- what is the observation when OH- (aq) is added?
- what is the observation when excess OH- (aq) is added?
- equations
- green
- grey-green ppt
- precipitate dissolves giving a deep green solution
- Cr3+ (aq) + 3OH- (aq) —> Cr(OH)3 (s)
- Cr(OH)3 (s) + 3OH- (aq) —> [Cr(OH)6] 3- (aq)
transition metal ion: Fe2+
- what is the colour of the solution?
- what is the observation when OH- (aq) is added?
- what is the observation when excess OH- (aq) is added?
- equation
- pale green
- dark green ppt
- no further change
- Fe2+ (aq) + 2OH- (aq) —> Fe(OH)2 (s)
transition metal ion: Fe3+
- what is the colour of the solution?
- what is the observation when OH- (aq) is added?
- what is the observation when excess OH- (aq) is added?
- equation
- yellow
- red-brown ppt
- no further change
- Fe3+ (aq) + 3OH- (aq) —> Fe(OH)3 (s)
transition metal ion: Cu2+
- what is the colour of the solution?
- what is the observation when OH- (aq) is added?
- what is the observation when excess OH- (aq) is added?
- equation
- pale blue
- pale blue ppt
- no further change
which two elements in the d-block of the periodic table are not technically transition metals?
- scandium
- zinc
- they dont form ions with an incomplete d-subshell
what are some examples of transition metals that have variable oxidation states?
- Fe2+ and Fe3+
- Cu+ and Cu2+
- Cr2+ and Cr3+
what is a complex ion?
- a central metal ion surrounded by ligands
(the coordinate bonding of ligands to a central metal ion results in thr formation of complexes)
what is meant by coordination number?
the number of coordinate bonds formed with a central metal ion
why do transition metals form coloured complexes?
- when visible light hits a transition metal ion, electrons are excited to higher energy levels
- some frequencies of the visible light are absorbed when electrons jump up to higher energy orbitals
- the rest of the frequencies of visible light are transmitted or reflected
- these frequencies combine to make the complement of the colour of the absorbed frequencies
- this creates the colour of the complex
(different ligands lead to different splittings sand therefore different colours)
what can lead to a colour change in a transition metal complex?
- change in oxidation number of the ion
- change in the ligand
- change in coordination number of the complex
what is the shape of the [CrCl4] - complex? bond angle?
- tetrahedral
- 109.5°
what is the shape of the [Cr(NH3)6] 3+ complex? bond angle?
- octahedral
- 90°
what is the shape of the [Cu(NH3)4(H2O)2] 2+ complex?
- octahedral
- 90°
what is the shape of the [Cu(NH3)4(H2O)2] 2+ complex?
- octahedral
- 90°
what is the equation and observations for the ligand exchange reaction between copper hexaaqua ions and chloride ions?
[Cu(H2O)6] 2+ (aq) + 4Cl- (aq) —> [CuCl4] 2- (s) + 6H2O (l)
- the blue solution forms a yellow solution
what is the equation and observations for the ligand exchange reaction between cobalt hexaaqua ions and chloride ions?
[Co(H2O)6] 2+ (aq) + 4Cl- ⇌ [CoCl4] 2- (aq) + 6H2O (l)
- the pink solution forms a dark blue solution
what colour are the compounds [CoCl4] 2- and [CuCl4] 2-?
[CoCl4] 2- —> blue
[CuCl4] 2- —> (yellow-)green
what colour are the compounds [Co(H2O)6] 2+ and [Cu(H2O)6] 2+?
[Co(H2O)6] 2+ —> pink
[Cu(H2O)6] 2+ —> blue
what colour is the compound [Cu(NH3)4(H2O)2] 2+?
(royal) blue
why do transition metals make good homogenous catalysts?
- transition metals have variable oxidation states, making them good homogeneous catalysts
- the variable oxidation states mean they are able to oxidise and reduce reactants and intermediates to form the desired products
(the ability of transition metals to form more than one stable oxidation state means that they can accept and lose electrons easily. this enables them to catalyse certain redox reactions)
why do transition metals make good heterogeneous catalysts?
- using the 3d and 4s electrons of three atoms on the catalyst surface, transition metals can form weak bonds with reactants, making them more reactive
what is the contact process?
- an industrial process used to make sulfuric acid
- it is made by oxidising sulfur dioxide in the presence of a solid catalyst
- sulfur trioxide is then reacted with water to form sulfuric acid
what conditions are required for the contact process?
- temperature around 450°C
- vanadium oxide catalyst (V2O5)
- pressure of 2 atm
give the equation for the reaction of Cr(III) ions with excess NaOH
[Cr(H2O)6] 3+ + 6OH- —> [Cr(OH)6] 3- + 6H2O
give the equation for the reaction of Fe(II) ions with excess NaOH
[Fe(H2O)6] 2+ + 2OH- —> Fe(H2O)4(OH)2 + 2H2O
- the green solution forms a green ppt
give the equation for the reaction of Fe(III) ions with excess NaOH
[Fe(H2O)6] 3+ + 3OH- —> Fe(H2O)3(OH)3 + 3H2O
- the orange solution forms a brown ppt
give the equation for the reaction of Cu(II) ions with excess NaOH
[Cu(H2O)6] 2+ + 2OH- —> Cu(H2O)4(OH)2 + 2H2O
what are the possible ions of scandium? most common?
what colour?
+3 (colourless)
what are the possible ions of chromium? most common?
what colour?
+2
+3 (most common) (dark green)
+6 (pale straw)
what are the possible ions of manganese? most common?
what colour?
+2 (most common) (pale pink)
+3 (colourless)
+4 (colourless)
+6 (colourless)
+7 (purple)
what are the possible ions of iron?
most common?
what colour?
+2 (pale green)
+3 (most common) (red-brown)
what are the possible ions of cobalt? most common?
what colour?
+2 (most common) (pink)
+3
+4
what are the possible ions of nickel ?
most common?
what colour?
+2 (most common) (green)
+4
what are the possible ions of copper?
most common?
what colour?
+1 (colourless)
+2 (most common) (pale blue)
a solution containing [Cu(H2O)6] 2+ ions is blue. explain the origin of this colour [3]
- ligands cause d-orbitals to split into 3 lower and 2 higher energy levels
- electrons move from lower level to higher level by absorbing some frequencies (electrons absorb energy and are promoted to a higher energy level)
- light not absorbed gives colour seen
ligands are able to donate a lone pair of electrons into available orbitals of the central metal ion
‘the presence of a ligand causes the d-orbital to split’
why are chromium and copper exceptions to the rules of electronic configuration?
- the d sub-energy level is described as being degenerate
- it is especially stable if it is half full (d^5) or completely full (d^10)
- Cr is so close to having a half full d sub-energy level that it takes one of the 4s electrons to achieve it early
- Cu is so close to having a full d sub-energy level
what is the oxidation state formed by an element in its compounds determined by?
- the maximum number of electrons it can lose without requiring so much energy to remove the electrons that the energy cannot be recovered in bonding
what are the possible ions of zinc ?
+2 (only)
what does the term ‘permanganate’ mean?
it means any compounds containing the manganate ion
is there such a thing as a copper (II) ion or a chromium (III) ion on their own?
no - instead they exist bonded to water molecules or other electron pair donors
the chromate (CrO4 2-) and dichromate (Cr2O7 2-) ions exist in equilibrium when in aqueous solution:
Cr2O7 2- + H2O ⇌ 2CrO4 2- + 2H+
- in acidic conditions, the dichromate ion is the predominant species so solution appears orange
- in alkaline conditions, the chromate ion is the predominant species so will therefore appear yellow
what is a coordinate bond?
- a covalent bond where the electron pair is donated from the same atom
(the ligand in a coordinate bond donates both electrons that make up the bond)
coordination bonds have enthalpies (strenghts) of the same magnitude as those of other covalent bonds
how do you write the formula for ligands?
e.g [Cu(NH2CH2CH2NH2)2 (H2O) (OH)]
[metal ion centre, ligands]
- ethylenediaminetetraacetic acid (EDTA) is an example of a hexadentate ligand
- has 6 lone pairs which bond to a central metal ion
- EDTA has a 4- charge
(dont need to learn - just info)
what is the coordination number of octahedral ligands?
6
- as there are 6 sets of lone pairs being donated by the 6 surrounding ligands
what is the coordination number of tetrahedral ligands?
4
- as there are 4 sets of lone pairs being donated by the 4 surrounding ligands
what are the two criteria that must be satisfied if the ion is to be coloured?
- there must be a splitting of the d-orbitals (this only happens in the presence of ligands and so only complex ions are coloured)
- the d-orbitals must be partially filled (if the d orbitals are empty then there are no electrons which can be excited into the higher energy d-orbitals and the ions will be colourless. if the d orbitals are full then there are no empty orbitals into which the electrons can be excited and the ions will be colourless)
- it is also possible to determine the concentration of a solution containing a coloured ion using a technique called ultra-violet and visible spectrophotometry (UV-vis)
- as the absorbance of a solution is proportional to its concentration, the concentration of any solution can be determined by comparing its absorbance to the absorbance of a solution of known concentration
dont think need to know - just info
what are the 2 types of reactions that complexes broadly undergo?
- acid-base
- ligand substitution/exchange
what are the acid-base complex reactions u need to know?
- aqueous copper (II) ions with hydroxide ions
- aqueous cobalt (II) with hydroxide ions
- aqueous iron (II) with hydroxide ions
- aqueous iron (III) with hydroxide ions
- aqueous chromium (III) with hydroxide ions
acid-base complex reaction: aqueous copper (II) ions with hydroxide ions
- info
- formula
- observations
- aqueous solutions of copper (II) contain the blue, octahedral hexaaquacopper (II) ion
- as a 2+ ion, the solutions are weakly acidic but protons can be removed by bases
- the copper ions react accordingly to the following equation
[Cu(H2O)6] 2+ (aq) + 2OH- (aq) —> [Cu(OH)2(H2O)4] (s) + 2H2O (l)
blue —> gelatinous pale blue ppt. insoluble in excess NaOH
acid-base complex reaction: aqueous cobalt (II) with hydroxide ions
- info
- formula
- observations
- aqueous solutions contain the pink, octahedral hexaapuacobalt (II) ion
- hexaaqua ions can also be present in solid samples of the hydrated salts
- as a 2+ ion, the solutions are weakly acidic but protons can be removed by bases
- [Co(H2O)6] 2+ (aq) + 2OH- (aq) —> [Co(OH)2(H2O)4] (s) + 2H2O (l)
- pink —> blue ppt. insoluble in excess NaOH
acid-base complex reaction: aqueous iron (II) with hydroxide ions
- formula
- observations
- [Fe(H2O)6] 2+ (aq) + 2OH- (aq) —> [Fe(OH)2(H2O)4] (s) + 2H2O (l)
- pale green —> gelatinous green ppt. insoluble in excess NaOH
acid-base complex reaction: aqueous iron (III) ions with hydroxide ions
- formula
- observations
- [Fe(H2O)6] 3+ (aq) + 3OH- (aq) —> [Fe(OH)3(H2O)3] (s) + 3H2O (l)
- pale yellow/brown —> rusty/orange brown ppt. insoluble in excess NaOH
acid-base complex reaction: aqueous chromium (II) ions with hydroxide ions
- formula
- observations
- info
- [Cr(H2O)6] 3+ (aq) + 3OH- (aq) —> [Cr(OH)3(H2O)3] (s) + 3H2O (l)
- green solution —> grey-green gelatinous ppt. ppt redissolves in excess NaOH
- [Cr(OH)3(H2O)3] (s) + 3OH- (aq) —> [Cr(OH)6] 3- (aq)
- grey-green ppt —> dark green solution
in some cases in ligand exchange reactions, the coordination number is reduced from 6 to 4. why?
- because the incoming ligands are larger than H2O
- the incoming ligands are negatively charged - H2O ligands are neutral
- therefore, the complex is more stable if tetrahedral - less repulsion between ligands
what are the ligand exchange reactions u need to know?
- aqueous copper (II) ions with conc HCl
- aqueous cobalt (II) ions with conc HCl
- aqueous copper (II) ions with ammonia, NH3
- aqueous cobalt (II) ions with ammonia, NH3
ligand exchange reactions : aqueous copper (II) ions with conc HCl
- formula
- observations
- [Cu(H2O)6] 2+ (aq) + 4Cl- (aq) ⇌ [CuCl4] 2- (aq) + 6H2O (l)
- pale blue solution (octahedral) —> yellow-green solution (tetrahedral)
(adding excess water reverses the reaction)
ligand exchange reactions : aqueous cobalt (II) ions with conc HCl
- formula
- observations
- [Co(H2O)6] 2+ (aq) + 4Cl- (aq) ⇌ [CoCl4]2- (aq) + 6H2O (l)
- pale pink solution (octahedral) —> blue solution (tetrahedral)
ligand exchange reactions : aqueous copper (II) ions with ammonia, NH3
- formula
- observations
- [Cu(H2O)6] 2+ (aq) + 2NH3 (aq) —> [Cu(OH)2(H2O)4] (s) + 2NH4 + (aq)
- pale blue solution —> blue ppt. soluble in excess NH3
- the ammonia molecules act as bases to abstract two protons from the copper complex
- excess ammonia results in the formation of a ligand sub reaction
- [Cu(OH)2(H2O)4] (s) + 4NH3 (aq) —> [Cu(NH3)4(H2O)2] 2+ (aq) + 2H2O + 2OH- (aq)
- blue ppt —> royal/deep blue solution
ACID BASE REACTION FOLLOWED BY LIGAND SUBSTITUTION
what is a catalyst?
- a substance which alters the rate of a reaction with itself being chemically unchanged at the end of
how does a catalyst work?
- it lowers the activation energy for the reaction by providing an alternative route
ligand exchange reactions : aqueous cobalt (II) ions with ammonia, NH3
- formula
- observations
- [Co(H2O)6] 2+ (aq) + 6NH3 (aq) —> [Co(NH3)6]2+ + 6H2O
- solution changes from pink to yellow straw colour
in ligand exchange reactions, why can one ligand be swapped for another ligand?
EXAMPLES
- the ligands e.g NH3 and H2O are similar in size and are both uncharged
- this means they can be exchanged without a change in coordination number via a ligand exchange reaction
- e.g the Cl- ligand is much larger than the NH3 and H2O ligands meaning substitution with this ligand results in a change in coordination number for that complex. these substitutions take place when concentrated HCL is reacted with the metal complex
with copper complexes and NH3 in excess, this substitution in ligand exchange reactions is incomplete and the complex has formed a combination of water and ammonia ligands
why are concentrated solutions of NH3 and HCl used in reactions?
- to push the equilibrium to the RHS
the maximum absorbance for the [Co(H2O)6]2+ ion occurs at a wavelength of 518nm. calculate the energy of this absorbance in kJmol^-1 [3]
f = 5.79 x10^14
E=hf = 3.84 x10^-19 (x L /1000)
E = 231 kJmol^-1
give two reasons why transition metal compounds can act as catalysts [2]
- variable oxidation states
- partially filled 3d energy levels
- ability to absorb ‘molecules’
- form complexes (or temporary bonds) with reacting molecules
why can transition elements form compounds with a variety of of oxidation states [1]
the energies of the d orbitals are similar to
