Coordination Chemistry Fundamentals

Question 1

Define the term complex / coordination compound

Answer 1

A positively charged central metal ion, called an acceptor, surrounded by ions or molecules called ligands

Question 2

Define the term ligand

Answer 2

Ligands are electron donators, often nucleophiles or Lewis Bases that surround the acceptor ion/atom.

Question 3

Define the term denticity and explain the different types of denticity

Answer 3

Denticity refers to the number of atoms which donate a pair of electrons per ligand.

Monodentate/Unidentate ligands have one donor atom e.g. OH2, NH3, NR3 (neutral) and F-, Cl-, OH-, CN- (anionic)

Bidentate/Didentate Ligands have two donor atoms per ligand e.g. Bipyridine (Two nitrogens/Phosphorouses on the same molecule which each have a lone pair)

A tridentate ligand has three donor atoms e.g. diethylenetriamine (Three Ns or Ps)

A tetradentate has four. (Ns and Ps)

Hexadentate ligands include [EDTA]4- which bind strongly to acceptor with two Ns and four Os in a tetrahedral geometry

Question 4

What is an ambidentate ligand

Answer 4

1. Ligand that has more than one possible ligand as part of that ligand e.g. SCN- vs NCS- (electron pair can coordinate via the sulphur or the Nitrogen)

E.g. NO2- coordinates via the N, whereas ONO- coordinates via the O

E.g. SCN- (thiocyanate) coordinates through the S whereas NCS- coordinates via the N (isothiocyanate)

Question 5

What is a bridging ligand

Answer 5

A ligand simultaneously attached to two or more central acceptors (usually metallic)

Some cases e.g. CN- can be an ambidentate bridging ligand if the C and N both act as bidentate ligands

Question 6

Explain the difference between Coordination number and Denticity

Answer 6

Coordination number (CN) is the number of ligand atoms directly bonded to the central acceptor in the complex.

Denticity is the number of atoms per ligand which donate electrons

Monodentate will have a CN of 1
Co(H20)6 has a CN of 6
Multidentate will have a CN of the denticity.
CN can range between 2-12

Question 7

Give the geometries of different coordination numbers

Question 8

How are ligands bound to the acceptor

Answer 8

Ligands held to acceptor via dative bonds

Question 9

What is a transition metal

Answer 9

Transition metals are elements with a partially filled d- orbital in at least one common oxidation state

E.g. Cu+ : [Ar]3d10 which is not a valid
Cu2+ : [Ar]3d9 so Cu is a transition metal

Question 10

State which d-block elements are not transition metals

Answer 10

1. Zinc (3d10)
2. Scandium (3d0)

Question 11

Give the different shapes that a bidentate ligand can form with the acceptor ion

Answer 11

1. Four membered
2. Five membered
3. Six membered

Question 12

What is chelation

Answer 12

Chelation is the binding of chelate ligands which bind and form multiple rings around the central atom e.g. [EDTA]4-

All dentate ligands form chelated complexes as they all lead to the formation of rings around the central atom.

Question 13

Which transition metal shapes correspond to specific Coordination numbers?

Answer 13

Coordination number of 4 implies tetrahedral or square planar geometry

Coordination number of 6 implies octahedral geometry

Coordination number of 5 (rarer) implies trigonal bipyramid or square based pyramid

Question 14

Explain what is meant by the term nuclearity

Answer 14

Nuclearity is the subcript of the metal in the equation, denoting the number of central metal atoms in the complex.