Transition Metal Chemistry Flashcards
Topic 5 - Paul Newman
Coordination chemistry
The study of how ligands bind to central metal ions to form coordination complexes.
Benefits of coordination chemistry
- Uses in healthcare e.g., heart imaging agents, diagnostics, anti cancer drugs
- Environmental uses, can be used to eliminate free radicals in the atmosphere.
Trends in the periodic table
- Atomic radius decreases from left to right
- Radii of the 2nd and 3rd rows are very similar.
- Electronegativity decreases down a group
- Bond strength decreases down a group
Ligands
Atoms that are able to donate a pair of lone electrons to form a dative/coordinate bond. Generally non-metallic p block elements, e.g., O, N, P, S.
Inner-sphere complex
Molecular unit comprised of a central metal ion and all directly bonded ligands.
Outer-sphere complex
Formed through electrostatic interactions between the central metal ion and ligands, rather than dative bonds. Examples include ion pairs formed in solution and solvation.
Coordination number
The total number of dative bonds in a complex.
Mono dentate ligands
Species with one lone pair of electrons e.g., ammonia, hydride ions etc.
Bidentate ligands
A species with two lone pairs that can form two dative bonds e.g., 1,2-diaminoethane, 2,2-bipyridine.
Bridging
If a ligand is not monodentate and has two potential electron donors, it can ‘bridge’ between multiple metals
Ambidentate ligands
A species that has two different atoms that can bind to a metal ion, however, only one can bind at a time.
Nomenclature: salts
If the compound is a salt, the cation is named before the anion.
Nomenclature: general
Irrespective of the charge, ligands are named in alphabetical order (ignoring numerical prefixes), followed by the metal with the oxidation state in brackets.
Nomenclature: ligands
Some do not use their usua names e.g., H2O - aqua, NH3, ammine, CO - carbonyl.
For anionic ligands, usual suffixes (ide, ite, ate) are changed (ido, ito, ato).
Nomenclature: metal ions
If it is an anion, the suffix -ate is added to the metal name.
Nomenclature: ambidentate ligands
The greek symbol κ (kappa) is used to denote which atom is donating the electrons in the complex ion.
Two- coordinate
Linear geometry, most common for d10 metal ions
Three-coordinate
Trigonal planar geometry
Four-coordinate
Tetrahedral (common for first row TMs) and square planar (2nd and 3rd row d8 TMs)
Five-coordinate
Square pyramidal and trigonal nipyramidal
How does Zn differ from other TMs?
Can exist in any number of geometries
Six-coordinate
Most common format
Octahedral and Trigonal prismatic (much more rare)
Higher coordination numbers
Mostly early TMs in groups 3-5, especially f block elements
Factors influencing coordination number
Size: larger ions can have more ligands
Charge: higher charge can allow for greater attraction but smaller size
Steric interaction: larger ligands limit coord number
Electronic structure: certain dn configurations can dictate coord number, and early TMs accept more electrons from ligands.
Ionisation Isomers
Species with the same empirical formula but different inner/outer sphere combinations. Invlove the exchange of anionic ligands.
Hydration isomers
Same empirical formula, but water ligands are attached to the central metal ion differently.
Linkage isomers
Where the molecular formula is the same but ambidentate ligands are attached at different sites to the central metal ion.
Coordination isomers
The anion and cation complexes of a coord compound exchange one or more ligands.
Geometric isomers
Same molecular formula, different arrangement of ligands in space.
Meridonial arrangement
Where three ligands of the same species are placed in a straight line along one of the diagonals of an octahedral complex.
Facial arrangement
Where three ligands of the same species are placed on the same face of an octahedral complex.
Stereoisomerism
Found in tetrahedral and in octahedral complexes.
Bis stereoisomerism
A complex is bis if it has two identical ligands. Cis isomerism occurs when the bis ligands are placed next to one another. Trans isomerism occurs when they are opposite one another.
Tris stereoisomerism
A complex is tris if it has three identical ligands. Stereoisomerism can occur depending on the orientation of the identical ligands.
Number of valence orbitals in TMs
9 valence orbitals and 9 valence bonding molecular orbitals, if all are filled, bonding energy is optimised.
What does it mean for a complex to follow the 18e rule
Referred to as coordinatively saturated or electron precise.
Usually stable, do not readily undergo substitution reactions.
Complexes tend to include 2nd and 3rd row TMs.
Ligands without lone pairs
To coordinate to a metal, a ligand can use electron density in its HOMO to donate to the LUMO of the metal. This leads to caveats in symmetry and energy.
Electron counting
Electron counting: covalent method
Disassemble the complex, giving the lone pairs (and therefore charge) back to the ligands.
Determine ligand charge by hypothetically adding hydrogen ions until the compound is neutral…
Electron counting: ionic method
Find the oxidation state of the central metal ion by knowing the charges of ligands, knowing the overall charge of the complex, and finding the OS from the difference.
Add up the e-counts to give the total VE count.
Homoleptic complex
A complex wherein all the ligands are identical.
Crystal field splitting energy dependent on
The nature of the ligands
The charge on the metal ion
Whether the metal is 3d, 4d or 5d.
Free ions in a vacuum
All d orbitals have equivalent energy (degenerate). If the free ions are placed in a spherical field of negative charge they remain degenerate but increase in energy.
Why are dz2 orbitals differently shaped?
Wave function characteristics: has cylindrical symmetry around the z-axis due to its wave function characteristics.
Nodal plane: causes the doughnut shape that is found on the xy plane, where other orbitals will have planar nodes that bisect their lobes.
Crystal field theory purpose
A model that explains how the energy levels of a TM’s d orbitals change when surrounded by ligands, thus affecting the properties of the coordination compound.
Crystal field theory key points
- Considers metals and ligands as point charges
- M-L interaction is limited to electrostatic repulsion and splitting of M d orbitals.
- can be used to explain spectroscopy, magnetism and reactivity trends of d-block compounds.
Point charge
A theoretical charge that exists in a single point in space.
Loss of degeneracy in d orbitals
Occurs when d orbitals become split into different energy levels when surrounded by ligands. Typically occurs because electron density from ligands interacts differently with some orbitals than others, leading to energy splitting.
Loss of degeneracy - stronger ligands
Stronger ligands can lead to larger energy gaps between d orbitals.
Crystal field splitting energy Δo
The difference in energy between the highest and lowest energy levels of d orbitals in a metal ion.
Factors influencing CFSE
- Nature of the metal
Mn2+ < Fe3+ < Pt4+ - Charge on a metal
Higher oxidation state = stronger interaction energy - Position in a group
The sizes of 4d and 5d orbitals are larger compared to 3d, leading to stronger overlap with ligands. - Identity of the ligand
Spectrochemical series.
Tetrahedral crystal field
Four ligands attached to the central metal ion. Orbitals split into top 3 and bottom 2 due to poor ligand overlap ( orbitals are on the axes whereas the ligands aren’t). Tetrahedral complexes are usually high spin by default as they have a tetrahedral splitting constant, which is typically smaller than the CFSE.
Octahedral crystal field
Six ligands attached to the central metal ion, d orbital splits into t2g (bottom 3 energy levels) and eg (top 2 energy levels). eg orbitals are on the axes and therefore move electrons to a higher orbital energy (PE higher than CFSE) whereas t2g are have lower PE energy and don’t have orbitals on the axes, so electrons pair instead.
Crystal field theory equation
E ∝ (q1)(q2)/r
E = bond energy
q = charges of the interacting ions
r = distance separating the ions.
Predictions based on CFT
Predicts that cations of lower charge e.g. K+ and Na+ would form few coordination compounds. For TMs, it is more complicated because they have varying numbers of d electrons in non-spherically-symmetric orbitals, meaning the shapes and occupations of the orbitals become factors that help determine bond energy and properties.
Crystal field splitting energy
When ligands approach a transition metal ion, some can experience more repulsions from d electrons based on the geometrical structure of the molecule. The electrostatic environment creates a splitting in energy.
Low spin
When a complex has few unpaired electrons and a strong field strength.
High spin
When a complex has many unpaired electrons and weaker field strength.
CFSE and pairing energy
CFSE and pairing energy. Electrons will take the path that requires the least amount of energy. If the pairing energy is greater than the CFSE, then the e- will move to a higher energy orbital, however if the CFSE is higher than the pairing energy, the e- will pair up rather than moving singly to a higher energy orbital.
How to determine high/low spin
- What is the geometry of the complex?
- Is the ligand strong or weak?
- What is the d electrons configuration for the central metal ion?
- Is the splitting energy large or small?
Spectrochemical series
I− < Br− < S2− < SCN− < Cl− < NO3− < N3− < F− < OH− < C2O42− < H2O < NCS− < CH3CN < py < NH3 < en < bipy < phen < NO2− < PPh3 < CN− < CO
En ligand
diaminoethane C2N2H8
Py ligand
pyridine C5H5N
Phen ligand
1,10-phenanthroline C12H8N2
Paramagnetic vs Diamagnetic Octahedral complexes
Determined by spin states, if there are unpaired electrons, paramagnetic(attracted to magnetic field) . If all electrons are paired, diamagnetic (unaffected by a magnetic field).
Square planar crystal field
Four ligands attached to central metal ion. However, the ligand electrons are only attracted to the xy plane. Four different energy levels. Tends to be low spin as it has the square planar splitting energy, which tends to be larger than CFSE.
Crystal field stabilisation energy
CFStE = (no e- in lower energy d orbitals)(energy diff between split orbitals) - (no e- in higher energy d orbitals)(energy diff)
