Multidentate ligands Flashcards
a monodentate ligand is
one that forms one dative bond with a metal ion
this means it uses one lone pair on one atom to form the dative bond
a bidendate ligand is
one that forms two dative bonds with a metal ion
a multidentate ligand is
one that forms several dative bonds with a metal ion
an example of a bidendate ligand is 1,2-diaminoethane, with formula:
it acts as a bidendate ligand because:
NH2CH2CH2NH2
abbreviated to ‘en’
it uses the lone pair of electrons on each nitrogen atom to attack to the metal ion
the equation showing the formation of the [Ni(en)3]2+ is from a haxaaqua metal ion is:
[Ni(H2O)6]2+ + 3en → [Ni(en)3]2+ + 6H2O
EDTA ‘s formula is :
(COOH-CH2)2-N-CH2-CH2-N-(CH2-COOH)2
EthyleneDiamineTetraAcetic (EDTA)
the EDTA molecule forms the ion……………
EDTA4- as each COOH group loses a H+ to form 2 COO- groups on each of the two Nitrogen atoms in the molecule
the EDTA4- ion has ………lone pairs of electrons
6 lone pairs, one lone pair on each O in the COO- groups and one lone pair on each N atom
this means the EDTA4- ion forms 6 dative bonds, so is a hexadentate ligand
most complexes are……………….. in the sense that they ……… decompose …………
most complexes are stable in the sense that they do not decompose readily
the stability of complexes refers to
a comparison of the stabilities of two complexes in which the number of ligands has changed
the equation for the reaction where the monodentate ligands in hexaaquacopper(II) are exchanged by bidentate ligands, en, is:
comment on the stability of the product and refer to the system’s entropy
[Cu(H2O)6]2+ + 3en → [Cu(en)3]2+ + 6H2O
here, the number of species has increased from 4 (total of reactants) to 7 (products), meaning the system is more disordered so there is an increase in ∆Ssystem. ligand exchange reactions of this sort lead to an increase in stability of the products compared to the reactants, so the formation of the product is favoured. this is because 6 H2O ligand have been replaced by only 3 en ligands.
the equation for the ligand exchange reaction where the H2O ligands in haxaaquacopper(II) are replaced by the EDTA4- ion is:
[Cu(H2O)]2+ + EDTA4- → [Cu(EDTA)]2- + 6 H2O
here six H2O ligands are replaced by one EDTA4- ligand, so the toal number of species has increased from two to seven, meaning ∆Ssystem increases more and the product is favoured more so
the simplified explanation of the role of haemoglobin in oxygen transport is :
- haemoglobin consists of 3 main parts, the largest of which is protein (globin)
- within the protein, there are 4 haem groups that are made up of mostly C and H atoms
- inside each haem group, there are 4 N atoms that hold an Fe2+ by forming dative bonds with a square planar structure
- there is a fifth dative bond to the Fe2+ ion form the protein, globin
- when blood passes through the lungs, haemoglobin collects oxygen molecules and transports tgem to cells
- when haemoglobin collects O2, the O2 molecules act as a ligand by using one its its lone pairs of e- to forma dative bond with a Fe2+ ion
the reason why CO, carbon monoxide, is dangerous is :
- the dative bond formed by O2 with haemoglobin is not particularly strong, meaning it can easily be released in the cell
- the dative bond formed by the lone pair on CO is much stronger, so a ligand exchange reaction occurs, and the CO less easily removed from the haemoglobin, preventing blood from transporting O2 to cells