Metal and Ligand Coordination Preference Pt2 Flashcards
There are few examples of supramolecular complexes formed from d⁰ to d⁴ metal ions compared to many examples from d⁵ to d¹⁰ ions
What are the reasons for this?
- The Irving Williams series: metal-ligand coordination strength is weaker in the early d-block due to the larger size of metal ions and so complexes are less stable
- Early in the d-block, the d-orbitals are higher in energy and so match less well with the energy of the ligand orbitals for covalent bonds (note some early d metal ions have no d-electrons, e.g. Sc³⁺, Y³⁺)
- Some metals form oxo ions e.g. VO₂⁺ for Vandium(V) and VO²⁺ for vanadium (IV) and this makes it more difficult to form larger coordination complexes
What factors affect whether a metal complex is high spin or low spin
d⁵ metals can be high spin or low spin meaning?
Can have pair or unpaired electrons
Mn(II) and Fe(II) are the most common example of d⁵ complexes
What are these metals like?
- These ions are both relatively hard therefore like hard ligands, like anionic oxygen ligands
Why is a high spin configuration partictuarly favoured with d⁵
As it has the maximum amount of exchange energy energy due to having five paired electrons
Hence losts of complexes are
Fe(III) form a ….. complex with strong field ligands?
low spin
d⁶ can be high spin or low spin meaning…
can have 3 or 1 paired electrons
why is low spin more common in d⁶
Which d⁶ elements are always low spin?
Because the low-spin orientation has the maximum amount of crystal field stabilisation energy possible
4d and 5d elements
Fe(II) can be high spin or low spin depending on…
…whether the ligand is a weak or strong field ligand
What types of ligands do d⁶ metals bind to?
These metal ions are softer and prefer nitrogen donors (such as pyridines) and phosphines - tend to be neutral donors
Why are d⁷ a little more complex?
- Ions with a d⁷ configuration can for 4, 5 and 6-coordination complexes in a range of geometries and so are difficult to predict. Octahedral is a good first guess
- Also all paramagnetic as has an odd number of electrons
- [Co(OH₂)₆²⁺ + 4Cl⁻ → [CoCl₄]²⁻ + 6H₂O
- e.g. Co(II) changes geometry during this reaction
d⁸ metals on the other hand are very predictable because…
- With the exception of Ni(II), these are almost exclusively square planar
- This is because in the square planar geometry all the low energy orbitals are filled and the high energy dx²-y² is vacent
- e.g. Ni(II), Pd(II), Pt(II), Rh(II), Ir(I)
Why is Ni(II) the exception in d⁸ complex?
- Ni(II) is a 3d metal and therefore had a low Δo
- It can for 4-coordinate high spin tetrahedral complexes with weak field ligands
- and 4-coordinate square planar complexes with strong field ligands
- BUT also forms 6-coordinate octehedral complexes and some 5-coordinate complexes too
Cu(Ii) is a d⁹ metal which forms octahedral complexes that have…
- a Jahn-Teller Distortion (usually an axial elongation
- This lowers the overall energy of the electrons (2 in the dz² fall in energy and only 1 in the dx²-y² rises in energy)
What are the properties of a d¹⁰ metal?
- There is now a filled d shell, the d-electron count no longer affects the geometrical preferences
- It makes these species diagmagnetic
- e.g. Cu(I), Ag(I), Au(I), Zn(II)
Cu(I) prefers…
… tetrahedral coordination
Zn(II) form…
… a mixture of 4-coordinate tetrahedral, 5-coordinate, and 6-coordinate octahedral
Cd(II) often forms…
… octehedral complexes
Au(I) prefers…
… linear complexes with only two ligands (these are 14 electron complexes)
Ag(I) forms…
… tetrahedral complexes most commonly but can form linear complexes with negatively charged ligands
What are two most common geometries for 5-coordinate complexes…
What is the difference in characteristics between a hard acid and a soft acid?
- Hard: higher charge (positive or negative) and smaller size (same charge over smaller area)
- Atoms nearer the top of the periodic table tend to be hard
- Soft: the opposite - lower charge, larger and lower down periodic table
What do we need to consider if metal-ligand bonds are made reversibly
- Need to thing about rate which metal-ligand bonds are made and broken (kinetics), as well as bond strength (thermodynamics)
- This can vary how hard/soft ligand is, but exchange rate of various metal ions with water can be a good guide
What is the goldilocks place within this diagram?
- If exchange is too slow then we do not get the error checking we need for the initally formed undesired structures to break apart before reforming the desired structure
- If exchange is too quick, then this often correlated with metal ligand bonds not being strong enough to hold more complex structures together
What factors affect the location of these metals on this diagram?
- Higher charge - slower exchange
- Small ionic radii - slower exchange (rates decrease across 3d metal)
- Lower period - slower exchange (metal ligand bonding strong with higher Δ)
- Electronic configuration also has an affect
What does denticity mean?
denticity of a ligand describes how many bonds that ligands makes to one metal centre
What is a Chelating ligand?
- Ligands that make two or more bonds to the same metal centre are known as chelating ligands
- Chelating ligands bond far more strongly to metal centres than monodentate ligands
What are the geometric requirement of say a chelating bidentate ligand?
- Chelating ligands have geometric requirements which govern which positions around a metal complex they can bond to
- With bidentate ligands the second coordination site must be in a cis relationship to the first coordination site
What are 4 common bidentate ligands?
- bipy
- phen
- dppe
- en
What are 2 commoon tridentate ligand examples?
- Terpy/tpy
- pdca/pydca
What are the first geometric requirements for a chelating tridentate ligand?
- Tridentate ligands that are relatively relatively rigid and planar can only coordinate to three positions in a meridional arrangment (3 positions that lie in the same plane)
What is the other geometric requirement for chelating tridentate ligands?
- Coordinate in a facial manner
- Ligands where the three donor atoms are contained in a macrocycle (large ring) are restricted to coordinating in this manner
What does Topicity mean?
The number of different metal centres that a ligand links between
(We need to use multitopic ligands (link two or more metals) to build structures containing multiple metal centres)
Most multitopic ligands are often designed to be…
…rigid and planar
(they would not be able to connect to two different metal centres if this wasn’t the case)
How does the chelate affect link to intramolecular processes?
- Once the first metal is bound, the ligand can…
- bind to the same metal again to form a chelate via an intermolecular process
- OR bond to a different metal via an intramolecular process
- The intramolecular process is almost always more favourable than the intermolecular (entropy gain by reducing no of particles within solution)
How can we favour an intermolecular process over the usually favoured intramolecular process?
We must disfavour the formation of a chelate by giving the ligand a rigid structure which prevents more than one donor atom coordinating to the same metal ion
The following ligand is…
both ditopic and bidentate (chelating)
In some ligands not all the possible donor atoms will always bond to metals in any structure we make
What affects this?
- Steric hindrance may prevent some atoms from coordinating to metals
- Ligands that can chelate will generally outcompete monodentate ligands
- More electron rich ligands will typically coordinate over less electron rich ligands
How does the following ligand bind?
It is tritopic and monodentate
How does the following ligand bind?
- bitopic and bidentate
