Transition Metals Flashcards
Is 3d or 4s subshell filled first and what are the exceptions?
- 4s before 3d because empty 3d subshell is at a higher energy than empty 4s
- Exceptions: Cr and Cu
- Repulsion between paired 4s electrons in higher energy and lower stability than half-filled or fully-filled d orbital
How do d-block elements form ions?
d-block elements form ions by losing 4s electrons first, followed by 3d electrons
Why do you fill 3d last but remove from 4s subshell for transition metals forming ions?
When the 3d orbitals are occupied by electrons, these electrons from inner shell repel the 4s electrons further away from the nucleus and to higher energy level
What is a transition element?
A transition element is a d-block element that forms at least one stable ion with a partially filled d subshell
What are elements are in d-block but are not transition elements?
-Sc³⁺: no electrons in 3d
-Zn²⁺: fully filled 3d
What elements do not show typical properties of transition elements in some oxidation states?
-Cu⁺ do not show typical properties of transition elements because it has a fully filled d subshell
What are the physical properties of transition elements?
-Hard
-high densities
-high melting and boiling points
Across the period, what is the trend of transition elements atomic radii?
-Across the period, electrons are being added to the inner 3d subshell
-these inner 3d electrons shield valence 4s electrons from the attraction of the nucleus
-thus the influence of each additional proton in the nucleus as the proton number increases across the group is reduced considerably
-as a result, increase in effective nuclear charge is negligible, atomic radii remain almost constant
What boiling points do transition elements have and why?
-very high melting and boiling points
-4s and 3d electrons are very close in energy, hence both can be contributed to form the sea of delocalised electrons, thus the metals form cations of higher charge
- ionic radii of transition elements cations are smaller than typical s-block elements
-more energy is required to overcome the stronger electrostatic forces of attraction between transition metal cations and sea of delocalised electrons, thus has high m.p. and b.p.
What are the densities of transition elements like and why?
d-block elements are generally denser than s-block elements because atoms are smaller and more tightly packed together
What is the trend of densities of transition elements across the period and why?
Gradual increase in density across the period as atomic radii generally decrease while relative atomic mass increase
What is the trend of ionisation energies like for transition elements across the period?
-small variation in first and second ionisation energies
- 1st and 2nd ionisation energies typically involve the removal of 4s electrons
- across the period, the additional 3d electrons which are inner shell electrons shield the 4s electrons from increasing nuclear charge
- thus the increase in effective nuclear charge is insignificant
What are the characteristic chemical properties of transition elements?
- variable oxidation states
- catalytic properties
- formation of complexes
- formation of coloured compounds and ions
Why do s-block element not have variable oxidation states?
s-block elements are limited to oxidation states of +1 and +2 because once the s electrons are removed, further removal of inner-shell p electrons requires too much energy
Why are transition elements able to exhibit variable oxidation states?
-the closeness in energy of 3d and 4s electrons meant that both 3d and 4s electrons are available for bond formation
What is formula for the maximum number of oxidation states for transition elements?
Max. no. of oxidation states = no. of 4s e⁻ + no. of unpaired 3d e⁻
What are lower oxidation states of transition elements usually found in?
ionic compounds
What are higher oxidation states of transition elements usually found in?
compounds or ions that contain covalent bonds
What is a catalyst?
A substance that increases the rate of a chemical reaction but remains chemically unchanged at the end of the reaction by providing an alternative pathway of lower activation energy
What are the two types of catalysts?
- homogenous catalyst
- heterogenous catalyst
What are homogenous catalyst?
Catalyst that operates in the same physical phase as the reactants
What are heterogeneous catalyst?
Catalyst that operates in different physical phase to the reactants
What are the characteristics of transition elements that allow them to function effectively as homogeneous catalysts?
- ability of transition element to exist in different oxidation states
- relative ease at which the oxidation state can be converted from one to another
What is usually the phase for catalyst and reactants for heterogeneous catalyst?
- catalyst is usually in solid phase and provides sites at which the reaction can occur
- reactants are usually liquids or gases
What are the characteristics of transition elements that enable them to function effectively as heterogeneous catalyst?
-availability of energetically accessible vacant 3d subshells which allow the ready exchange of electrons to and from reactant particles, thus facilitating the formation of weak bonds with reactant particles
- availability of 3d and 4s electrons for bond formation with reactant particles
What is a complex?
A complex contains a central metal atom or ion bonded to one or more surrounding ligands
What is a ligand?
A ligand is an ion or molecule which contains at least one atom bearing one lone pair of electrons which can be donated into the energetically accessible vacant orbital of the central metal or ion, forming a dative bond
What is complex ion?
If the complex carries an overall charge, it is a complex ion
What is coordination number of central metal atom or ion?
The number of co-ordinate bonds from the ligands to central atom or ion
Why is Fe²⁺ able to form dative bond with 4 or 6 ligands when its orbitals are all half-filled?
- For 4 ligands: 4s electron get move to 3d and get paired, resulting in 4 vacant orbitals (4s, 4p) to form 4 coordinate bonds
- For 6 ligands, 2 of the 3d e⁻ move and become paired, resulting in 6 empty orbitals to form 6 coordinate bonds (2 from 3d, 4s, 4p)
(just rmb it as electrons will basically squash itself and pair themselves to have vacant orbitals to form dative bonds)
How to name complex?
- Name cation then anion
- Name ligand before metal, if more than one ligand of the same kind present, indicate number of ligands by prefixes
- Name central atom and indicate oxidation state using roman numerals
What is the prefix name for H2O and what’s the co-ordinating atom?
aqua
H2O
What is the prefix name for NH3 and what’s the co-ordinating atom?
ammine
NH3
What is the prefix name for CO and what’s the co-ordinating atom?
carbonyl
CO
What is the prefix name for Cl⁻ and what’s the co-ordinating atom?
chloro
Cl⁻
What is the prefix name for CN⁻ and what’s the co-ordinating atom?
cyano
CN⁻
What is the prefix name for SCN⁻ and what’s the co-ordinating atom?
S-thiocyanato
SCN⁻
What is the prefix name for OH⁻ and what’s the co-ordinating atom?
hydroxo
OH-
What is the prefix name for H₂NCH₂CH₂NH₂ and what’s the co-ordinating atom?
ethane-1,2-diammine
H₂NCH₂CH₂NH₂
What prefix to use for the ligand?
di, tri, tetra, penta, hexa
or bis, tris, tetrakis
How to name the central atom in cationic and anionic complex?
Cationic complex: name of metal
Anionic complex: modify the name of the metal to end with “-ate”
What is the name of V in anionic complex?
vanadate
What is the name of Cr in anionic complex?
chromate
What is the name of Mn in anionic complex?
manganate
What is the name of Fe in anionic complex?
ferrate
What is the name of Cu in anionic complex?
cuprate
What is the name of Zn in anionic complex?
zincate
What is the name of Al in anionic complex?
aluminate
What is the name of Pb in anionic complex?
plumbate
What are ligands classified according to?
Number of co-ordinate bonds that the ligands form with central atom of ion
What are the types of ligands?
- monodentate
- polydentate
* bidentate- hexadentate
What are monodentate ligands?
A monodentate ligand is one which can form only one co-ordinate bond with a central atom or ion
What are some examples of monodentate ligands?
- :NH3
- H₂O:
- :Cl⁻
- :CN⁻
What are polydentate ligands?
A polydentate ligand is one which can form more than one co-ordinate bond with a central atom or ion
What are polydentate ligands also called and why?
Polydentate ligands are also called chelating agents because of their ability to form a claw-like grip on metal atom or ion
Do chelating ligands or monodentate ligands form more stable complexes and why?
- Chelating ligands form more stable complexes with central metal atom or ion compared to monodentate ligands
- because the claw-like grip of polydentate ligands can hold the central atom or ion more securely
-chelating process is both enthalpy and entropy driven
What are bidentate ligands?
A bidentate ligands is one that can form two co-ordinate bonds with a central atom or ion
What are some examples of bidentate ligands?
- ethane-1,2-diamine (en)
- ethanedioate ion (oxalate ion)
- 2-hydroxybenzoate ion
How are the co-ordinate bonds of bidentate ligands placed around the central atom?
The coordinate bonds are adjacent to each other because the ligand cannot stretch so far
What is a hexadentate ligand?
A hexadentate ligand is one which can form six co-ordinate bonds with a central atom or ion
What is an example of a hexadentate ligand?
EDTA⁴⁻ ion
What are the uses of EDTA⁴⁻ ion?
- It is used in trace amounts in food such as salad dressing where by forming complexes EDTA⁴⁻ effectively prevents metal ions from catalysing reactions which lead to food spoilage
- used as antidote for poisoning by heavy metal ions such as lead because it binds itself to lead(II) ions and permits them to be expelled by the body
- prevent blood from clotting during operations by chelating with calcium ions
- added to shampoo to help soften the water
What is the shape of complexes with coordinate number of 2?
Linear
What is the shape of complexes with coordinate number of 4?
-tetrahedral
-square planar
What is the shape of complexes with coordinate number of 6?
octahedral
What is the wavelength of visible light?
430nm to 670nm
What happens if the wavelength is greater than 700nm?
infra-red radiation
What happens if wavelength is less than 400nm?
UV light
What is the colour observed of a substance due to?
If certain wavelengths are absorbed and other wavelengths are reflected or transmitted, the substance appears coloured, the colour is due to the reflected or transmitted wavelengths
What is white light?
visible light
What happens when white light falls on a substance?
- It can be totally reflected, substance appears white
- It can be totally transmitted, substance appears colourless
- It can be totally absorbed, substance appears black
What is the observed colour of the substance?
Complement of the colour absorbed
What is the wavelength range and colour?
violet: 400-450nm
blue: 450-490nm
green: 490-550nm
yellow: 550-580nm
orange: 580-650nm
red: 650-700nm
What are the pairs of complementary colour?
- red, green
- yellow, violet
- orange, blue
Why are transition metal complexes coloured?
- In an isolated gaseous metal ion, all the five 3d orbitals are degenerate
- In a complex ion, the presence of ligands cause the 3d orbitals to split into two sets of non-degenerate orbitals with different energies
- the difference in energies, ΔE between the two sets of 3d orbitals is relatively small and falls within the energy range of the visible spectrum of the electromagnetic spectrum
- visible light of the right energy is absorbed to promote the electrons from d orbitals of lower energy to partially filled/empty d orbitals of higher energy
- colour observed is the complement of the colours absorbed
What is the formula for the energy gap?
ΔE = hf = hc/λ
h = Planck constant
c = speed of light in vacuum
f = frequency of radiation
λ = wavelength of radiation
What happens to the wavelength absorbed when the energy gap is smaller?
If the energy gap is smaller, light with longer wavelength has to be absorbed for d-d transition and vice versa
(think of the formula)
What must the central metal ion have for transition metal complex to show colour due to d-d transition?
Central metal atom or ion must have:
- at least one d electron to be promoted
- an empty or partially filled 3d orbital at the upper level to accommodate the d electron to be promoted
What is the colour of transition metal complexes and compounds that do not satisfy conditions for d-d transition?
White in solid form and colourless in solution
Are Sc³⁺ and Ti⁴⁺ coloured and why?
- they have d⁰ configuration
- these metal ions do not possess any d electrons, hence d-d transition is not possible
- hence there is an absence of colour
Are Zn²⁺ and Cu⁺ coloured and why?
- they have d¹⁰ configuration
- these metal ions do not possess an empty or partially filled d orbital of higher energy level to accommodate the d electron to be promoted, hence d-d transition is not possible
- hence there is an absence of colour
Are non d-block metals coloured and why?
-these metal ions do not have d electrons
- energy gap is too large for the transition of electrons of a lower energy to a higher energy and requires absorption of radiation outside the visible region (eg. uv light)
- thus they are not coloured
What is the crystal field theory?
- In an octahedral complex, ligands are modelled as six-point negative charges that surround the positively-charged transition metal ion
- dz² and dₓ²₋y² orbitals have lobes that project towards the negative charges, greater repulsion between the orbitals on the ligands and the dz² and dₓ²₋y² orbitals
- thus 3dz² and 3dₓ²₋y² orbitals exist at higher energy level
- dxy, dxz, dyz orbitals have lobes that project between the charges
- weaker repulsion between orbitals on the ligands and the dxy, dxz, dyz orbitals
- 3dxy, 3dxz, 3dyz orbitals exist at lower energy level
-this causes splitting of the degenerate d orbitals into two different energy levels
How are the d orbitals splitted in tetrahedral complexes?
- Higher energy: 3dxy, 3dxz, 3dyz orbitals
- lower energy: 3dz² and 3dₓ²₋y²
What are the factors affecting the colour of complexes?
- colour of complex is dependent on the energy gap, ΔE between two sets of d orbitals which is dependent on:
* identity of metal: complexes of different metals have different energy gaps, ΔE- oxidation state: compounds of same element with different oxidation state show different colours
- nature of ligand: different ligands will split d orbitals by different extents
What is Ks?
complexation stability constant
What is a ligand exchange reaction?
- When transition element ions are in aqueous solutions, they will automatically become hydrated: water molecules will surround the ion and act as ligands by forming dative covalent bonds to the central metal ion
- when there are other potential ligands present in the solution, there is a competing equilibrium in ligand exchange and most stable complex ion will be formed
What does Ks indicate and what does a higher Ks imply?
- Ks indicates the inherent tendency of a particular ligand to replace water in the aqua-complex
- Higher Ks implies a stronger binding ligand
What is the Ks for this equation: [a]²⁺ + 5b ⇌ [c]²⁺ + 4b and what is not included in the expression and why?
Ks = [c²⁺][[b]⁴]/ [a²⁺][[b]⁵]
- H2O is not included because water in aqueous solution is in excess and any water produced is negligible compared to the water already present
What affects ligand exchange reactions?
Ligand exchange reactions are reversible and the relative concentration of ligands will influence ligand exchange
Which one is more stable: complexes involving polydentate ligand or complexes involving monodentate ligands?
- complexes involving polydentate ligands usually greater than complexes involving monodentate ligands
- assuming all co-ordinate bonds are of similar strength, the greater the number of bonds, the more difficult it is to remove the ligands and therefore the more stable the complex ion
What do ligands behave as?
Ligands behave as nucleophiles as they bond to the central metal atom or ion using a lone pair of electrons
What is the relationship between the strength of ligand and nucleophiles?
Generally, stronger binding ligands are also stronger nucleophiles as their electron pair more readily available to form a co-ordinate bond
How is ligand exchange reaction different from substitution reaction?
Unlike substitution, the number of sites available might change due to factors like ligand size
What is the effect of ligand exchange on E values?
Replacement of water ligands in an aqua-complex by other ligands can cause significant changes in electrode potential values
What is haemoglobin and what is its function?
Haemoglobin is the red pigment in blood and it is a protein found in red blood cells that carries oxygen from the lungs to the body’s tissues
What is the structure of haemoglobin?
- Within haemoglobin, there are 4 haem groups
- Each haem group consists of a Fe²⁺ ion with a co-ordination number of 6
- Five of the co-ordinate sites are occupied by nitrogen: four of the N atoms form a porphyrin ring system to form haem complex and the fifth N atom is to form a complex protein known as globin
- the sixth site can accomodate different ligands
How does haemoglobin work to release oxygen?
The sixth site of haem group can accommodate an oxygen molecule as a ligand via a co-ordinate bond, Fe²⁺-O₂ bond is relatively weak, allowing the oxygen molecule to be released to cells, the site is then occupied by H2O ligand
What are the effects of CO on haemoglobin?
CO may replace the H2O ligand on the sixth site, however, they are strongly and often irreversibly bonded at this site and this destroys haemoglobin’s oxygen-carrying capacity
What happens when Cu²⁺ salts is added to water?
Cu²⁺ dissolve in water to give blue solution due to the ion [Cu(H₂O)₆]²⁺ formed
What happens when NH₃(aq) is added to Cu²⁺ (aq)?
- When NH₃ (aq) is gradually added, a blue precipitate is first formed
* NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq)+ OH⁻ (aq)
* [Cu(H₂O)₆]²⁺ (aq) + 2OH⁻ (aq) → Cu(OH)₂ (s) + 6H₂O (l) - When excess NH₃ (aq) is added, blue ppt. dissolves to produce a dark blue solution
* Cu(OH)₂ (s) + 4NH₃ (aq) + 2H₂O (l) ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ (aq) + 2OH⁻ (aq)
What happens when excess Cl⁻ (aq) is added to Cu²⁺ (aq)?
-formation of yellow complex
[Cu(H₂O)₆]²⁺ (aq) + 4Cl⁻ (aq) → [CuCl₄]²⁻ (aq) + 6H₂O (l)
What happens when I⁻ (aq) is added to Cu²⁺ (aq)?
- redox reaction
- off-white ppt. is formed in brown solution
2Cu²⁺ (aq) + 4I⁻ (aq) → 2CuI (s) + I₂ (aq)
What colour is Cr³⁺ (aq) and what can it react with?
- Green [Cr(H₂O)₆]³⁺ (aq)
- It is acidic and can react with carbonates
[Cr(H₂O)₆]³⁺ (aq) + H₂O (l) ⇌ [Cr(H₂O)₅OH)]²⁺ (aq) + H₃O⁺ (aq)
What happens when NaOH (aq) is added to Cr³⁺ (aq)?
- a grey-green ppt. is formed which dissolves in excess NaOH (aq) to produce a dark green solution
- [Cr(H₂O)₆]³⁺ (aq) + 3OH⁻ (aq) → Cr(OH)₃ (s)+ 6H₂O (l)
- Cr(OH)₃ (s) + 3OH⁻ (aq) ⇌[Cr(OH)₆]³⁻ (aq)
What happens to CrO₄²⁻ and Cr₂O₇²⁻?
-yellow CrO₄²⁻ (aq) and orange Cr₂O₇²⁻(aq) can be converted to each other depending on the pH of the solution
* 2CrO₄²⁻ (aq) + 2H⁺ (aq) ⇌ Cr₂O₇²⁻(aq) + H₂O (l)
* Cr₂O₇²⁻ (aq) + 2OH⁻ (aq) ⇌ 2CrO₄²⁻ (aq) + H₂O (l)
What happens to MnO₄⁻ (aq) under acidic conditions?
- Purple MnO₄⁻ can be reduced under acidic conditions to produce pale pink Mn²⁺
MnO₄⁻ (aq) + 8H⁺ (aq) + 5e⁻ ⇌ Mn²⁺ (aq) + 4H₂O (l)
What happens to MnO₄⁻ (aq) under alkaline conditions?
- Purple MnO₄⁻ can be reduced under neutral/slightly alkaline conditions to produce dark brown MnO₂ (s)
MnO₄⁻ (aq) + 4H⁺ (aq) + 3e⁻ ⇌ MnO₂ (s) + 2H₂O (l)
What colour is Fe³⁺ (aq) and what can it react with?
- Yellow [Fe(H₂O)₆]³⁺ (aq)
- It is acidic and can react with carbonates
[Fe(H₂O)₆]³⁺ (aq) + H₂O (l) ⇌ [Fe(H₂O)₅(OH)]²⁺ (aq) + H₃O⁺ (aq)
What happens when SCN⁻ (aq) is added to Fe³⁺ (aq)?
-ligand exchange reaction occurs and the formation a blood red complex
[Fe(H₂O)₆]³⁺ (aq) + SCN⁻ (aq) ⇌ [Fe(SCN)(H₂O)₅]²⁺ (aq) + H₂O (l)
What happens when NaOH (aq) is added to Fe²⁺ (aq)?
- A green ppt. is formed
- Upon leaving in air, green ppt. turns red-brown
* [Fe(H₂O)₆]²⁺ (aq) + 2OH⁻ (aq) → Fe(OH)₂ (s) + 6H₂O (l)
* 4Fe(OH)₂ (s) + O₂ (g) +2H₂O (l) → 4Fe(OH)₃ (s)
What happens when CN⁻ (aq) is added to Fe²⁺ (aq)?
-Ligand exchange reaction occurs
[Fe(H₂O)₆]²⁺ (aq) + 6CN⁻ (aq) ⇌ [Fe(CN)₆]⁴⁻ (aq) + 6H₂O (l)
What happens when Cl⁻ (aq) is added to Ag⁺ (aq)?
- white ppt. formed
* Ag⁺ (aq) + Cl⁻ (aq) → AgCl (s) - ppt. dissolved in NH₃ (aq) to produce a colourless solution
*AgCl (s) + 2NH₃ (aq) ⇌ [Ag(NH₃)₂]⁺ (aq) + Cl⁻ (aq)
What species do Cr exist at in +3 oxidation state and its respective colours?
- [Cr(H₂O)₆]³⁺: green
- Cr(OH)₃: grey-green ppt.
- [Cr(OH)₆]³⁻: dark green
What species do Cr exist at in +6 oxidation state and its respective colours?
- CrO₄²⁻: yellow
- Cr₂O₇²⁻: orange
What species do Mn exist at in +2 oxidation state and its respective colours?
Mn²⁺: pale pink
What species do Mn exist at in +4 oxidation state and its respective colours?
MnO₂: dark brown ppt.
What species do Mn exist at in +6 oxidation state and its respective colours?
MnO₄²⁻: dark green
What species do Mn exist at in +7 oxidation state and its respective colours?
MnO₄⁻: purple
What species do Fe exist at in +2 oxidation state and its respective colours?
- [Fe(H₂O)₆]²⁺: pale green
- Fe(OH)₂: green ppt.
What species do Fe exist at in +3 oxidation state and its respective colours?
- [Fe(H₂O)₆]³⁺: pale yellow
- Fe(OH)₃: red-brown ppt.
- [Fe(SCN)(H₂O)₅]²⁺: blood-red
What species do Cu exist at in +2 oxidation state and its respective colours?
- [Cu(H₂O)₆]²⁺: pale blue
- [Cu(NH₃)₄(H₂O)₂]²⁺: dark blue
- [CuCl₄]²⁻: yellow
- Cu(OH)₂: pale blue ppt.
What species do Cu exist at in +1 oxidation state and its respective colours?
CuI: off-white ppt.
What species do Ag exist at in +1 oxidation state and its respective colours?
- AgCl: white ppt.
- AgBr: cream ppt.
- AgI: yellow ppt.
- [Ag(NH₃)₂]+: colourless
What is absorption of a solution (A) related to?
Absorption of a solution at a particular wavelength is directly proportional to concentration of species (c) responsible for its absorption
