Metal and Ligand Coordination Preferences Pt1 Flashcards
(34 cards)
What does Supreamolecular mean?
When we have more than one molecules or more than one component coming together to form a larger structure
Organic molecules are typically held together by strong covalent bonds, one at a time during irreversible reactions
Why is metal-ligand coordination different?
How does this affect the synthesis process?
- Although M-L bonds have covalent character, they are made in reversible processes and can break apart after they have formed
- Bonds are hence referred to as dynamic
- Means components must be designed to the pieces all come together at once
The synthesis process in inorganic chemistry is called (M-L bonds)?
Self-assemby or self-organisation
Despite the large size of particular supramolecular compounds, they are much easier to made than organic compounds because…
- It is only made from two different pieces that are mixed…
- An organic ligands
- A metal ion (from a metal salt)
- Where the construction rules are encoded within the pieces
What is a key energetic feature of the self-assembly process?
are ususally the most thermodynamically stable arrangement we can have in the system
i.e., bonds form and break reversibly until we reach equilibrium where all the pieces are in the structure at the lowest energy
Why do they structures require sufficiently strong metal-ligand bonds?
The metal-ligand bonds are sufficiently strong to hold the structure together well and for equilibrium to be entirely on the side of the product we want to make
Why does both kinetic and thermodynamics have to be considered in metal-ligand bond formation?
- Bond formation must be reversible so that there is an opportunity for error correction if an incorrect structure forms first
- Therefore a metal-ligand combination is needed where:
- Bonds for sufficiently strong to hold the structures together
- But not too strongly so that incorrect connections cannot come apart
When choosing a metal ion for a suparmolecular compound, it needs…
To have the correct coordination geometry (e.g., ocetahedral, tetrahedral, square planar etc)
When choosing a ligand for a supramolecular compounds, it needs…
- …the correct arrangement of donor atoms, considering two main ligand properties
- How many bonds a ligand makes to each metal (denticity)
- How many different metals a ligand connects (topicity)
What are the main properties of Supramolecular structures?
- Supramolecular structures contain two or more molecules held together by non-covalent bonds
- The metal-ligand coordination bonds often have covalent character but are dyanmic
- There is also non-covalent interactions that can also help to stabilise the structure
What types of non-covalent interactions occur within supramolecular compounds?
- Hydrogen bonds
- Interactions between charges
- Van der Waals (dispersion)
- Aromatic interactions/π-π stacking
What is this structure?
Triple Helicates
M₂L₃ structure
What is this structure?
3 x 3 grid
M₉L₆
What is this structure?
Metal-organic cage
M₂L₄
What is this structure
Two interwoven rings
[2] catenate (with metal)
[2] catenane (without metal)
We can also form extended polymeric metal-coordination complexes
Ones with well-defined void spaces are called…
Metal organic frameworks (MOFs)
Why are alkai metal ions not very good at making these coordination structures?
- Low charge (+1) and so complexes are often weak + ligands in rapid exchange - are hard and oxophillic
- Closed shell electron configuration with ions, hence no orbitals for covalent bonding and ligand interactions are purely ionic
What is a Crown-ether ligand
How are they named?
Its an oxo-carbon ring
The best ligand for the alkali metal ions are the…
Crown-ether ligands
The larger the alkali metal ions bing more strongly in the larger cavities due to better size matching
The strongest binding combination is 18-crown-6 with K⁺
(however, not the greatest for the structures we want to make)
What are the properties of the group 2 alkaline earth metal ions?
- Have closed shell configurations
- metal-ligand interaction bonding is purely ionic in character
- Have a higher charge of +2
- Hard and perfect oxygen donors (can interact with nitrogen)
What are the best ligands for group two alkaline earth metal ions?
- Best ligands are typically oxygen atoms carrying a formal negative charge
- Common ligands are carboxylates and phosphates
- (used in ATP, chlorophyll in biology as well as MOFs in synthetic chemistry)
What is the most common way a group 2 metal bonds to a carboxylate?
A bidentate ligand is rare due to forming a strained four-membered ring
What are the Lanthanides?
And what oxidation state do they tend to form?
Group 14 and the first row of the f-block metals
+3 oxidation state
What is the electron configuration of La(III)
La (III) = [Xe]
Same configuration of a noble gas
Leaves them with only f electrons as valance electrons (for all other Lanthanides apart from La)
