transition metals Flashcards
(25 cards)
What is an inner sphere complex?
This is the molecular unit comprised of the central metal and all directly bonded ligands.
What is an outer-sphere complex?
These are formed through interactions of the inner-sphere complex with ions or molecules that are not directly bonded to the metal.
E.g. the formation of ion pairs in solution and solvation
How are coordination complexes named?
If the coordination compound is a salt the cation is named first then the anion.
Irrespective of the nature of the complex (cation, anion, neutral) the ligands are named first in alphabetical order (ignoring any numerical prefixes) followed by the metal with its oxidation state in parentheses. For anionic Ligands that have names that end in –ide, -ite or –ate change the e to an o and get –ido, -ito and –ato. If the metal complex is anionic then the term –ate is added to the metal name.
What are the geometries for each coordination number?
3 coordinate - Typically trigonal planar: L-M-L angles at ~120
4 coordinate - Tetrahedral (Td): L-M-L angles at 109.5. Common for first row TMs but less so for 2nd and 3rd row. Square planar (D4h). L-M-L bond angles 90⁰. Common for 2nd and 3rd row TMs with d8 electronic configuration.
5 coordinate - Square pyramidal: all angles 90⁰. Trigonal bipyramidal: L-M-L axial-equatorial 90⁰, equatorial-equatorial 120⁰.
6 coordinate - Octahedral (Oh) – many complexes have this geometry, Trigonal prismatic - rare
What factors influence coordination number?
- Size of the metal ion – larger ions can accommodate more ligands.
- Charge on the metal ion – higher charge can mean greater attraction but smaller.
- Steric interaction between the ligands – larger ligands limit coordination number.
- Electronic structure of the metal ion – certain dn configurations can dictate coordination number and geometry.
What are the main types of isomerism in coordination complexes?
7
Structural isomerism – ionisation isomers and hydration isomers, Linkage isomerism, Coordination isomerism, Geometric isomerism, Stereoisomerism (– enantiomers and diastereomers)
What are ionisation isomers?
Same empirical formula but different inner sphere / outer sphere combinations. Ionisation isomers involve the exchange of anionic ligands.
What are hydration isomers?
Same empirical formula but different inner sphere / outer sphere combinations involving water exchange.
What are linkage isomers?
Occurs when a coordination compound contains a ligand that can bond to the metal atom through different atoms, resulting in different isomeric forms with the same molecular formula.
What are coordination isomers?
In coordination isomers, the anion and cation complexes of a coordination compound exchange one or more ligands. Coordination isomers are molecules with the same overall formula, but with different arrangements of ligands around a central metal ion, resulting in different physical and chemical properties.
What are geometric isomers?
Geometric isomers are two or more coordination compounds which contain the same number and types of atoms, and bonds, but different spatial arrangments.
Cis/trans
What are the steps of electron-counting in transition metal complexes (ionic model)?
1) know the O.S. (and hence the d-electron count) of the metal centre to do this we must:
>a) know the charges of the ligands bound to the metal;
>b) note the overall charge on the complex then O.S. = (overall charge) - (total charge from the ligands) and number of d electrons = element group No. – O.S.
2) add up the electron counts from the ligands and the metal centre to give total VE count.
Total VE = No. of d electrons from metal + No. of electrons donated by ligands
What is crystal field theory?
Crystal field theory (CFT) describes the breaking of orbital degeneracy in transition metal complexes due to the presence of ligands.
What is crystal field splitting ∆₀?
It requires more energy to have an electron in these orbitals than it would to put an electron in one of the other orbitals. This causes a splitting in the energy levels of the d-orbitals to create t2g (-2/5Δo) and eg (3/5Δo) orbital subsets. The separation between the t2g and eg orbital subsets is referred to as Δo (crystal field splitting energy).
What factors influence the magnitude of Δo?
Nature of the metal. Charge on the metal - Δo increases with increasing oxidation state of the metal ion. Higher the oxidation state, smaller size of the metal ion – resulting in shorter metal – ligand distance and stronger interaction energy. Position in a group (20-50% increase in Δo for each successive TM row). Identity of the ligand (Spectrochemical Series)
What happens in a tetrahedral (Td) crystal field?
Although none of the orbital lobes are directed exactly towards the ligand approach, the t2 set are more aligned than the e set and hence electrons in these orbitals experience a greater interelectronic repulsion.
Two things to note: the arrangement is the inverse of that seen for Oh.
The energy gap is defined by Δt which is 4/9Δo.
What is the formula for CFSE?
(H.S) CFSE = x(-0.4) + y(0.6) ∆o
(L.S.) CFSE = x(-0.4) + y(0.6) ∆o + P
x = No. of electrons in t2g; y = No. of electrons in eg; p is the pairing energy
Describe any magnetic properties.
Diamagnetism – very weak effect associated with paired electrons (all compounds have some diamagnetic properties).
Paramagnetism – weak effect due to the presence of unpaired electrons.
Ferromagnetism – strong effect – an example of a bulk magnetic property that results from long-range electron-electron interactions.
What is the impact of dn configuration on the size of metal ion?
sde … lie in __between__
The metal ions that are smaller than expected on the basis of a spherical distribution of electrons contain electrons in the t2g subset – these are not as effective at ‘repelling’ the ligands as they lie in orbitals between the axes.
What is the chelate effect?
-dentate ligands
The ligand(s)…. complexes of bi- and multidentate ligands are more stable than comparable complexes with solely monodentate ligands.
What is Jahn-Teller distortion?
When orbitals in the same level (degenerate) are occupied by different numbers of electrons, the molecule will distort to a lower energy form (J-T distortion). Lengthening of the axial bonds (along the z axis) results in electrons in orbitals containing a z component being lowered in energy (less repulsion – CF theory). The overall result is a lower overall energy for the distorted form compared to regular Oh.
What are the factors affecting ligand exchange?
The charge and size of the metal ion. The nature of the leaving group (M-X bond strength). Crystal field effects – changes in CFSE on going from ground state to excited state.
How are colours derived?
The colours derive from electronic transitions from the t2g subset to the eg subset and the position of the peak maxima can be used to determine Δo. Light is absorbed ONLY when it has the appropriate energy (i.e. equivalent to the separation of the t2g and eg states) to cause an electron to move from the lower state to the higher energy state.
Why is the solution of Mn(II) so pale and that of Zn(II) colourless?
ZnSO4 is colourless due to having full eg and t2g subsets, meaning that electrons cannot excite to go anywhere.
MnSO4 has d5 (high spin) so it cannot promote an electron to eg as the spin of the electron would have to change. -> spin selection rule.
