Transition Metal Complexes Flashcards
Definition
the collective name for the dxy, dxz and dyz orbitals
T2g orbital
Show that the following complexes obey the 18-electron rule:
(a) [Ni(CO)4]
(b) [Mn(CO)5]-
Definition
The energy of the electron configuration in the ligand field minus the energy of the electronic configuration in the isotropic field
Crystal field stabilisation energy
In the elements, which orbital is filled first: 3d or 4s?
4s
Definition
Crystal field designation for an outer orbital complex; all t2g and eg orbitals are singly occupied before any pairing occurs
High spin
Definition
similar to the chelate effect however the effect is further enhanced by the cyclic conformation of the ligand. These ligands are not only multidentate, but because of their pre-organised structure there is little entropy penalty for wrapping it around the metal ion
Macrocycle effect
Predict which of the following complexes would be most stable:
(a) [Fe(OH2)6]2+
(b) [Fe(OH2)6]3+
Definition
a technique used to determine analyte concentration by measuring light absorption across the ultraviolet and visible light wavelengths
UV-vis spectroscopy
What causes low-spin arrangments?
When Δo is large and too much energy is required to promote electrons into the eg orbital
How are values of Δo measured experiementally?
UV-vis spectroscopy
Define
Coordination number
the number of atoms or ions immediately surrounding a central atom in a complex or crystal
Calculate CFSE for the low-spin and high-spin configurations of d7 . Which one looks as if it should be the most stable
Definition
A rule based on the fact that the valence shells of transition metals consist of nine valence orbitals (one s orbital, three p orbitals and five d orbitals), which collectively can accommodate 18 electrons as either bonding or nonbonding electron pairs
Eighteen electron rule
Using the formula Δ0 = hcNo/λ, convert the value of Δ0 = 12600 cm-1 for [Cu(OH2)6]2+ into kJ/mol
Definition
the difference in energy between eg and t2g
ΔO
Define
Lewis acid
a compound or ionic species which can accept an electron pair from a donor compound
Define
Metal-ligand charge transfer
when the electronic charge shifts from the MO with metal-like character to the ligand-like one resulting in the oxidation of the metal centre
Definition
a spectroscopic selection rule that states the a change in spin multiplicity is forbidden
Spin selection rule
Define
High spin
Crystal field designation for an outer orbital complex; all t2g and eg orbitals are singly occupied before any pairing occurs
Assuming that the following complexes obey the 18-electron rule, determine n in each complex:
(a) [Fe(CO)<em>n</em>]
(b) [Fe(CO)<em>n</em>(PPh3)2]
Define
Crystal field stabilisation energy
The energy of the electron configuration in the ligand field minus the energy of the electronic configuration in the isotropic field
Is [Fe(CN)6]4- more stable in high spin or low spin?
Low spin; CN is a carbon donor and therefore the crystal field splitting is large making it more stable to have low spin
Definition
the number of atoms or ions immediately surrounding a central atom in a complex or crystal
Coordination number
Define
Low spin
Crystal field designation for an inner orbital complex; contains electrons paired t2g orbitals before eg orbitals are occupied in octahedral complexes
Define
Lewis base
a compound or ionic species which can donate an electron pair to an acceptor compound
Definition
the collective name for the dz2 and the dx2-y2 orbitals
Eg orbital
What is the electron configuration of chromium metal?
Cr: [Ar] 4s13d5
How does oxidation state effect crystal field splitting?
The higher the oxidation state of the metal, the greater the crystal field splitting
Definition
a compound or ionic species which can accept an electron pair from a donor compound
Lewis acid
Define
UV-vis spectroscopy
a technique used to determine analyte concentration by measuring light absorption across the ultraviolet and visible light wavelengths
Definition
a spectroscopic selection rule that only applies to centrosymmetric molecules (those with an inversion centre) and atoms. It states that transitions can only occur if they result in a change in the value of the subsidiary quantum number, l. Thus, allowed transition are s -> p, p -> d and d ->f.
Laporte’s selection rule
Define
Macrocycle effect
similar to the chelate effect however the effect is further enhanced by the cyclic conformation of the ligand. These ligands are not only multidentate, but because of their pre-organised structure there is little entropy penalty for wrapping it around the metal ion
Show both high-spin and low-spin filling for d4
Ligands up to an including _______ in the spectrochemical series usually give rise to high spin complexes
H2O
How does the crystal field splitting change from the second transition series compared to the first?
Increases by 50%
What causes high-spin arrangements?
When Δo is small
Define
Barycentre
the zero-energy reference in octahedral complexes
Definition
a compound or ionic species which can donate an electron pair to an acceptor compound
Lewis base
Define
Laporte’s selection rule
a spectroscopic selection rule that only applies to centrosymmetric molecules (those with an inversion centre) and atoms. It states that transitions can only occur if they result in a change in the value of the subsidiary quantum number, l. Thus, allowed transition are s -> p, p -> d and d ->f.
Define
Eg orbital
the collective name for the dz2 and the dx2-y2 orbitals
Definition
a model that describes the breaking of degeneracies of electron orbital states, usually d or f orbitals, due to a static electric field produced by a surrounding charge distribution
Crystal field theory
What is the general rule used to determine the value of the crystal field theory?
X- donor < O donor < N donor < C donor
What happens when 5 degenerate d orbitals are split into two sets?
3 t2g orbitals of lower energy and 2 eg orbitals of higher energy are produced
Define
Spin selection rule
a spectroscopic selection rule that states the a change in spin multiplicity is forbidden
Definition
Crystal field designation for an inner orbital complex; contains electrons paired t2g orbitals before eg orbitals are occupied in octahedral complexes
Low spin
Define
Chelate effect
the enhanced affinity of chelating ligands for a metal ion compared to the affinity of a collection of similar nonchelating (monodentate) ligands for the same metal
What is the electron configuration of Fe3+ ?
[Ar] 3d5
What factors effect crystal field splitting?
- The identity of the metal
- The oxidation state of the metal
- The number of the ligands
- The nature of the ligands
What does the splitting of d orbitals lead to?
Overall lowering of energy and consequently the complex is more stable than before the splitting
Define
ΔO
the difference in energy between eg and t2g
In compounds, what is an easy way to determine the number of d electrons?
The group number of the metal
Define
Crystal field theory
a model that describes the breaking of degeneracies of electron orbital states, usually d or f orbitals, due to a static electric field produced by a surrounding charge distribution
What is the energy separation between the t2g and the eg orbitals called?
Δoct
Definition
a geometric distortion of a non-linear molecular system that reduces its symmetry and energy. This distortion is typically observed among octahedral complexes where the two axial bonds can be shorter or longer than those of the equatorial bonds
Jahn-Teller distortion
Define
Eighteen electron rule
A rule based on the fact that the valence shells of transition metals consist of nine valence orbitals (one s orbital, three p orbitals and five d orbitals), which collectively can accommodate 18 electrons as either bonding or nonbonding electron pairs
What does the 18-electron rule state?
a complex is stable if there are 18 electrons around the metal
What is the electron configuration of V3+ ?
[Ar] 3d2
Definition
a substance whose molecules can form several bonds to a single metal ion. i.e. multidentate
Chelating agent
Definition
the zero-energy reference in octahedral complexes
Barycentre
Define
Chelating agent
a substance whose molecules can form several bonds to a single metal ion. i.e. multidentate
Definition
the enhanced affinity of chelating ligands for a metal ion compared to the affinity of a collection of similar nonchelating (monodentate) ligands for the same metal
Chelate effect
Define
T2g orbital
the collective name for the dxy, dxz and dyz orbitals
Definition
when the electronic charge shifts from the MO with metal-like character to the ligand-like one resulting in the oxidation of the metal centre
Metal-ligand charge transfer
Define
Jahn-Teller distortion
a geometric distortion of a non-linear molecular system that reduces its symmetry and energy. This distortion is typically observed among octahedral complexes where the two axial bonds can be shorter or longer than those of the equatorial bonds
How does the number of ligands effect crystal field splitting?
The crystal field splitting is greater for a larger number of ligands
Convert the value of ΔO= 12600cm-1 for[Cu(OH2)6]2+ into kJ mol-1
ΔO = E = hvNO = hcNO/λ
Where h = 6.63 x 10-34 (J s; Plank’s constant)
c = 3 x 108 (m s-1; Speec of light)
NO = 6.022 x 1023 (mol-1; Avogadro’s constant)
Which two selection rules determine whether a particular transition is allowed or forbidden?
Laporte’s selection rule
Spin selection rule
What type of molecules does Laporte’s selection rule apply to?
Molecules that have a centre of inversion
According to Laporte’s selection rule, why are tetrahedral complexes more strongly coloured than octahedral ones?
Tetrahedral complexes do not have a centre of inversion so Laporte’s rule does not apply to them like it does octahedral complexes which means that more transitions are allowed
What orbital transitions occur in ligand-metal charge transfer?
p -> d
When can metal-ligand charge transfer occur?
When the ligand has an empty p orbital that can accept electrons from the metal
Why do d5 complexes only give a series of very weak absorption bands?
According to the spin selection rule, changes in spin multiplicity is forbidden. Since d5 complexes have 1 electron in each orbital no transitions are possible that wouldn’t break this rule
(a) TiO2 is the most common white pigment, why is it white?
(b) Explain why [Mn(OH2)6]2+ is a very pale pink colour
(c) Explain why metal chromates (CrO42-) have been used as yellow pigments
Why are tetrahedral complexes almost always high spin?
ΔT = 4/9 ΔO
How does the splitting diagram for a tetrahedral complex compare to an octahedral complex? Why?
It is inverted; t2 (xy, xz, yz) orbitals are higher in energy, e (x2-y2, z2) orbitals are lower
Since tetrahedral complexes have only four ligands instead of six, the ligands don’t quite point directly at the three d orbitals, causing the crystal field splitting to be much less than in the octahedral case making them less stable
What geometric formation do complexes with 8d electrons tend to form?
Square planar
True or False:
ΔSP > ΔO
True
Show arrangement of electron in an a) octahedral, b) tetrahedral and c) square planar d8 metal ion
Which octahedral complexes are effected by Jahn-Teller distortion? Why?
Octahedral complexes of d9, low-spin d7 and high spin d4 due to unsymmetrically filled eg orbitals
True or False:
Jahn-Teller distortion decreases stability of complexes
False; the distortion increases crystal field splitting and enhances stability
What is the chelate effect caused by?
Differences in reaction entropy between chelated and nonchelated complexes in dilute solution. The chelation reaction results in an increase in the number of independent molecule in solution. By contrast, the nonchelating reaction produces no net change. The former therefore has the more positive reaction entropy and hence is the more favoured process
Define
Antibonding orbital
a type of molecular orbital (MO) that weakens the chemical bond between two atoms and helps to raise the energy of the molecule relative to the separated atoms
Define
Destructive interference
the interference of two waves of equal frequency and opposite phase, resulting in their cancellation where the negative displacement of one always coincides with the positive displacement of the other
Define
Dissociation constant
a quantity expressing the extent to which a particular substance in solution is dissociated into ions, equal to the product of the concentrations of the respective ions divided by the concentration of the undissociated molecule
Define
Electrode potential
the electromotive force of a cell built of two electrodes
Define
Equilibrium constant
a number that expresses the relationship between the amounts of products and reactants present at equilibrium in a reversible chemical reaction at a given temperature
Define
Formation constant
the equilibrium constant for the formation of a complex ion from its components in solution
Define
Hess’s law
A law that states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes
Define
Hund’s rule
every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin
Define
Irving-Williams series
the relative stabilities of complexes formed by transition metals as represented in a diagram
Define
Latimer diagram
a summary of the standard electrode potential data of that element
Define
Ligand field theory
one of several theories that describe the electronic structure of coordination or complex compounds, notably transition metal complexes. It factors in the effects of π bonds as well as σ bonds
Define
Oxidation
the loss of electrons during a reaction by a molecule, atom or ion
Define
Pauli exclusion principle
in an atom or molecule, no two electrons can have the same four electronic quantum numbers
Define
π-acids
a ligand that accepts electron density into it’s p orbital from the metal center
Define
π-bases
a ligand with filled π-type orbitals that donates electron density to the metal center
Define
π bond
a covalent bond formed between two neighboring atom’s unbonded p-orbitals
Define
σ bond
the strongest type of covalent chemical bond that are formed by head-on overlapping between atomic orbitals
Define
σ-donor ligands
A ligand that when coordinated to metal, it forms one single sigma bond
Define
Reduction
reaction that involves the gaining of electrons by one of the atoms involved in the reaction
Define
Thermodynamic cycle
a series of thermodynamic processes which returns a system to its initial state
Definition
a type of molecular orbital (MO) that weakens the chemical bond between two atoms and helps to raise the energy of the molecule relative to the separated atoms
Antibonding orbital
Definition
the interference of two waves of equal frequency and opposite phase, resulting in their cancellation where the negative displacement of one always coincides with the positive displacement of the other
Destructive interference
Definition
a quantity expressing the extent to which a particular substance in solution is dissociated into ions, equal to the product of the concentrations of the respective ions divided by the concentration of the undissociated molecule
Dissociation constant
Definition
the electromotive force of a cell built of two electrodes
Electrode potential
Definition
a number that expresses the relationship between the amounts of products and reactants present at equilibrium in a reversible chemical reaction at a given temperature
Equilibrium constant
Definition
the equilibrium constant for the formation of a complex ion from its components in solution
Formation constant
Definition
A law that states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes
Hess’s law
Definition
every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin
Hund’s rule
Definition
the relative stabilities of complexes formed by transition metals as represented in a diagram
Irving-Williams series
Definition
a summary of the standard electrode potential data of that element
Latimer diagram
Definition
one of several theories that describe the electronic structure of coordination or complex compounds, notably transition metal complexes. It factors in the effects of π bonds as well as σ bonds
Ligand field theory
Definition
the loss of electrons during a reaction by a molecule, atom or ion
Oxidation
Definition
in an atom or molecule, no two electrons can have the same four electronic quantum numbers
Pauli exclusion principle
Definition
a ligand that accepts electron density into it’s p orbital from the metal center
π-acids
Definition
a ligand with filled π-type orbitals that donates electron density to the metal center
π-bases
Definition
a covalent bond formed between two neighboring atom’s unbonded p-orbitals
π bond
Definition
the strongest type of covalent chemical bond that are formed by head-on overlapping between atomic orbitals
σ bond
Definition
A ligand that when coordinated to metal, it forms one single sigma bond
σ-donor ligands
Definition
reaction that involves the gaining of electrons by one of the atoms involved in the reaction
Reduction
Definition
a series of thermodynamic processes which returns a system to its initial state
Thermodynamic cycle
What does ligand field theory factor in that crystal field theory doesn’t?
Pi bonds
What type of bond is this?
Electron density appears along the internuclear axis and cylindrical symmetry around the internuclear axis. That means if we rotate the orbitals around the z-axis there is no change of phase
Sigma bond
What type of bond is this?
A nodal plane exists along the internuclear axis, lack of cylindrical symmetry. That means if we rotate the orbitals around the z-axis there is a change of phase
Pi bond
In sigma donor ligands, if no pi bonding is present the energy difference between the t2g and eg orbitals corresponds to what?
Δoct
Sigma donors _________ electrons from lone pairs to the metal centre from either the s or p orbitals
Sigma donors donate electrons from lone pairs to the metal centre from either the s or p orbitals
What are the valence shell atomic orbitals for a first row transition metal?
3d, 4s and 4p
Combinations of the metals and ligand orbitals generate _____ bonding and ____ anti-bonding molecular orbitals
Combinations of the metals and ligand orbitals generate 6 bonding and 6 anti-bonding molecular orbitals
When there is no pi bonding present, which is the only molecular orbital that does not interact with the ligand group?
t2g orbital
True or False:
π-acids donate electron density to the metal centre
False
π-bases donate electron density to the metal centre
How does π-bases affect the molecular orbital splitting diagram from the sigma only version?
How does π-acids affect the molecular orbital splitting diagram from the sigma only version?
What does the formation constant represent?
It indicates the strength of binding of the ligand relative to water: if Kf is large, the incoming ligand binds more strongly than the solvent; if Kf is small, the ligand binds more weakly than water
What is the inverse of Kf? Why is it useful?
The inverse of each Kf is the dissociation constant, Kd , which is often preferred when we are interested in the concentration of ligand that is required to give a certain concentration of complex. Kd is the ligand concentration required to obtain 50% complexations
Why is there a consistent decrease in the stepwise formation constant as you replace ligands in a complex?
The decrease in the stepwise formation constant reflects the diminishing statistical factor as successive ligands are replaced, coupled with the fact that an increase in the number of bound ligands increases the likelihood of the reverse reaction
If Kf<em>n </em>> Kf<em>n+1,</em> what has most likely occurred?
A major change in the electronic structure of the complex as more ligands are added (i.e. change in spin)
Explain the shape of the plot observed in the Irving Williams series. Explain the general increase left to right, the deviation from linear and the maximum at Cu
True or False:
The smaller the value of EO , the greater the tendency for reaction to occur left to right
False
The greater the value of EO , the greater the tendency for reaction to occur left to right
