Metal ions in Biochemistry & Coordination Chemistry Flashcards
What are the roles of metal ions ?
- Charge carrier
- Osmotic balance
- Triggers (nerve signalling and muscle contraction)
- Structural (bones and proteins)
- Catalysts
- Transport and storage (i.e. O2)
Bulk metals used by nature ?
- Make up 1% of the human body weight
- Na+, Mg2+, K+ and Ca2+
Trace metals used by nature?
V, Cr, Mn, Fe, Co, Ni, Cu, ZN, Mo an W
Specific role of Na and K ?
- Charge carrier
- Osmotic balance
Specific role of Mg ?
- Structural role
- Hydrolytic cleavage
Specific role of Ca ?
- Structural role
- Trigger
- Charge carrier
Specific role of Zn ?
- Structural role
- Hydrolytic cleavage
Specific role of Fe and Cu ?
- O2 transport, storage and/or activation
- Electron transport
- Superoxide (O3-) breakdown
Specific role of Mo ?
- Oxo (O) transfer
- N2 activation
Specific role of Ni ?
- Methane (CH4) production
- H2 activation
- Hydrolysis
Specific role of Mn ?
- O2 evolution
- Peroxide breakdown
- Superoxide breakdown
Specific role of Co ?
- Free radical reactions
- Nucleophilic metal ion
What is the job of P and S ?
PO3R2- and SO3R- make organic (CHNO) compounds soluble in water as anions
What is the job of Na+ and K+?
- Counterions and electrical carriers – can’t use Ca2+ or Al3+ as these would precipitate organic anions
- Also serve as messengers, activating Na+ or K+ specific ATPases
What is the job of Mg2+ ?
Used as a weak Lewis acid inside cell and binds to anionic surfaces such as RNA and ribosomes
What is the job of Ca2+ ?
Kept out of cell and is used as a temporary reflux trigger (i.e. nerve impulses)
What does the number of available oxidation states help predict ?
Some of the roles of the metal
Irving-Williams Series ?
- Relative stability of metal complexes
- Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II)
- The ionic radius is expected to decrease regularly for Mn2+ to Zn2+
- Stability increases in this order
Metal ions do not freely exist in nature, they are always surrounded by ?
Ligands, from simple like H20 to more complex haems
For instance, whenever metals dissolve in water, they are hydrated (coordinated) by ? to form ?
For instance, whenever metals dissolve in water, they are hydrated (coordinated) by water molecules to form a charged complex
What does the water ‘complex’ contain ?
The water ‘complex’ has a positive charge (n+) and requires anions for charge
balance
What is a Lewis acid and a Lewis base ?
- Lewis acid = Electron pair acceptor
i. e. metal ions - Lewis base = electron pair donor
i. e. ligands
What are Soft acids and bases?
Large species that are easily polarisable (think of a bean bag)
What are Hard acids and bases?
Small species that are not easily polarised (think of a marble)
L (Lewis base) must have electrons to ‘donate’ to ?
M (Lewis acid)
What can a Lewis base either be ?
Neutral (uncharged) or charged (usually negative, e.g. CN-)
In water, what is the most common ligand ?
Water
What is a complex ion ?
When the coordination complex carries a charge
In regards to the periodic table, where is the electronegativity the highest?
- Electronegativity is higher on the right side of the periodic table
- Electronegativity is higher on the top of the periodic table
Explain electronegativity in ionic bonds ?
Large electronegativity difference
Explain electronegativity in covalent bonds ?
Small electronegativity difference
What is meant by the coordination number and oxidation ?
- Coordination number: the number of ligands bound to the metal
- Oxidation state: the charge of the ‘free’ metal ion
What is the most common coordination environment ?
Octahedral (Coordination number: 6)
What is important to note in regards to the octahedral geometry and it’s ligands ?
In octahedral geometry, the ligands lie on the axis. Thus, the orbitals that are on the axis (dz2 and dx2-y2) are higher in energy than the ones that lie in-between the axis (dzx, dyz, and dxy). In a sense, the orbitals on the axis are essentially ‘repelled’ by the electrons of the ligand
What is meant by Δo ?
- The energy difference between the two levels (ligand-field splitting)
- The magnitude of Δo is ligand dependent
What does the size of Δ depend on ?
Depends on L (Lewis base)
What are ligands causing a small Δ called ?
Low field ligands (H20)
What are ligands causing a large Δ called ?
High field ligands (CN-), forces electrons into lower levels
What gives metal complexes magnetic properties ?
Unpaired electrons
Addition of a cyanide solution to a colourless [Fe(H2O)6]2+ aquo solution results in ?
The appearance of a blue colour (technically this may be spin forbidden)
The excitation of an electron in [Fe(CN)6]4- is responsible for ?
The blue colour of this complex
What are colours in solution sometimes caused by ?
Excitation of lower energy to higher energy d orbitals
The ability of ligands to spit the d orbitals varies according to ?
The spectrochemical series (ligands based on the strength of their interaction with metal ions)
What is meant by weak field ligands ?
Ligands that cause a transition metal to have a small crystal field splitting, which leads to high spin
What is meant by strong field ligands ?
Ligands that produce a large crystal field splitting, which leads to low spin
What is the Chelate effect ?
Entropy favours multi-dentate ligands over monodentate ligands
