Lecture 2 Flashcards
(15 cards)
Define metal complex
A metal complex is a combination of a Lewis acid (the central metal) with a number of Lewis bases (the ligands).
e.g. [Co(NH3)6]3+
What is a donor atom?
The atom in the Lewis base ligand that forms the bond to the central atom is called the donor atom
e.g. N is the donor atom when NH3 acts as a ligand
What is coordination number?
The coordination number (CN): number of ligand donor atoms to which the metal is directly bonded
e.g. CN=6
What factors determine coordination number?
- size of the central atom
- steric interactions between the ligands
- electronic interactions
What is a coordination compound?
The term coordination compound refers to
a neutral complex or an ionic compound in which at least one of the ions is a complex
e.g. [Co(NH3)6]Cl3
What were some of Werner’s early observations?
- two compounds, one violet and one green, had the same empirical formula CoCl3.4NH3
- just one chloride in each of the compounds
reacts with Ag+ to form AgCl - Werner suggested that the reactive chloride is not bound to the cobalt while the other two chlorides are: [CoCl2(NH3)4]+ Cl-
What did Werner discover about geometry?
He also proposed that the six bonded groups were arranged in a symmetric fashion around the cobalt atom.
Possible arrangements:
- planar hexagon (isomers=3)
- trigonal prismatic (isomers= 3)
- octahedral (isomers= 2)
What were Werners conclusions?
Since Werner could only isolate two compounds of this formula, the violet and the green one, he concluded that the most likely arrangement for the six ligands was octahedral. This conclusion is now known to be correct.
However, he had a deabte with Sophus Mads Jørgensen who favoured a chain based structural model
What are the 2 geometric isomers of an octahedral?
- cis and trans isomers
- Cis; The two ligands Y are arranged
adjacent to each other. (this was the violet Co complex) - Trans; The two ligands Y are arranged
opposite each other (this was the green Co complex) - mer and fac isomers
- Meridional; The three ligands X or Y all lie in the same plane
- Facial; The three ligands X or Y are adjacent
forming a triangular face of the octahedron (each ligand has a different type of line- dotted, dash and solid lines)
What is optical isomerism?
- A complex that is not superimposable on its own mirror image is chiral
- Forms enantiomers
- The two enantiomers rotate the plane of
polarised light by an equal amount, but in
opposite directions
What can you confirm optical activity?
- experiment to confirm octahedral geometry
- view along a 3-fold rotation axis of the
octahedron - note the handedness of the helix formed by the ligands:
- right rotation = ∆
- left rotation =
What are the similarities/ differences in enantiomers?
- most of the properties of two optical isomer are the same: colour, melting point, polarity, solubility, chemical reactivity (with non-chiral reagents)
- two optical isomers differ in: their reactivity with other chiral reagents and their interactions with polarised light
Co ordination number 6
- Six coordination is most common for d-block metal complexes.
- The majority of six coordinate complexes are
octahedral. - 3-fold rotation symmetry axis and 4-fold rotation symmetry axis
- cis/trans and mer/fac isomerism
- Important types of distortions resulting from stretching or compressing the octahedron;
1. along a 4-fold rotation symmetry axis- tetragonal distortion
2. along a 3-fold rotation symmetry axis- trigonal distortion
Co-ordination number > 6
- less common for 3d complexes
- (more important with 4d and 5d complexes: the larger central atom can accommodate more than 6 ligands)
Co-ordination number < 6
- CN 2; Linear
- CN 3; rare among d-metal complexes
- CN 4; very common, most important geometries; tetrahedral and square planar
Tetrahedral: favoured when the central atom is small and the ligands are large
Square planar; observed for metal ions with d8- configuration. They can exhibit geometric isomerism e.g. cisplatin - CN 5; less common
important geometries; trigonal bipyramidal, square pyramidal. The energy difference between the two structures is marginal and many complexes exist with intermediate
geometries.
Ligands often exchange between the sites: fluxional
