Download 5 - ChemWeb (UCC)

Exercise
Complete
and
balance
the
following
equations. Name the reaction type in each
case:
(a) N2 + H2
(b) P4 + 3O2
(c) SO2 + H2O
5.
Transition Elements
Section 23.7, p. 1010 - 1013
Elements in Groups 3 to 11 are the transition
elements or transition metals.
The valence shell of 1st row transition
elements consists of the 4s and the 3d subshells.
The valence electrons are the associated
electrons.
e.g. Cobalt, Co, [Ar]4s2 3d7 has 9 valence
electrons.
5.1 Compounds of transition elements.
Most transition metal compounds contain
covalent bonds between ligands and central
metal atom.
Some are simple polyatomic ions, e.g.
permanganate [MnO4]-, a polar covalent
compound. Electrons - shared between
central Mn and O ligands.
However most are "complex compounds" or
“complexes”.
These contain more
complicated ligands like H2O, NH3.
E.g. Fe forms complexes with six H2O
molecules, [Fe(OH2)6]Cl2, [Fe(OH2)6]SO4.
Bonding involves donation of an electron
pair from an atom of the ligand to an empty
orbital on the metal.
e.g. in [Fe(OH2)6]Cl2, each bond formed by
donation of one of the lone pairs on the O of
the H2O to an empty orbital on the Fe.
5.2. Shape
compounds
of
transition
element
Incompletely filled d orbitals, cannot use
VSEPR.
Compounds where the metal has a
coordination number of 6, i.e. 6 atoms or
groups bonded to it, are octahedral, e.g.
[Fe(OH2)6]Cl2, [Fe(NH3)6]Cl2,
[Fe(OH2)6]SO4. See handout.
Compounds where the metal has a
coordination number of 4, i.e. 4 atoms or
groups bonded to it are tetrahedral, e.g.
K2[FeCl4], K2[FeBr4].
2+
OH2
H2O
OH2
Fe
H2O
2Cl-
OH2
OH2
2+
NH3
H3N
NH3
Fe
H3N
NH3
2Cl-
NH3
2-
Cl
2K+
Fe
Cl
Cl
Cl
5.3 Hybridisation
Cannot use VSEPR for compounds with
incompletely filled d shells so determine
hybridisation by ‘inspection’.
Octahedral complexes
Six electron pairs around the transition
element centre, so need 6 hybrid orbitalsformed from one s, three p, and two d a.o.’s
so the T.E. is sp3d2 hybridised.
e.g. [Fe(NH3)6]Cl2
Fe is sp3d2 hybridised.
Tetrahedral complexes
Four electron pairs around the transition
element centre, so need 4 hybrid orbitals –
formed from one s, and three p a.o.’s so the
T.E. is sp3 hybridised.
e.g. K2[FeCl4]
Fe is sp3 hybridised
5.4 Oxidation states (O.S.s) of transition
elements
Same rules as before apply. For example,
MnF2
O.S. F = -1; Thus O.S. Mn = +2
KMnO4
O.S. O = -2; O.S. K = +1. Thus O.S. Mn =+7
K2[FeCl4]
O.S. Cl = -1; O.S. K = +1. Thus O.S. Fe = +2
[Fe(NH3)6]Cl2
O.S. Cl = -1;
Note: NH3 ligand has no overall charge.
O.S. Fe = +2
Question:
Calculate the O.S. of Fe in [Fe(OH2)6]Cl2.
Reactions of Transition Elements and Their
Compounds
E.g. Iron, Fe
1. Redox reactions
Examples:
(a) 2Fe + 3Cl2  2FeCl3
O.S. Fe goes from 0 to +3, i.e. Fe is oxidised
O.S. Cl goes from 0 to –1, i.e. Cl is reduced
(b) Fe + 2FeCl3  3FeCl2
(c) 2Fe + O2  2FeO
Question:
Calculate the O.S. of all the elements in
reactions (b) and (c) above. Hence, identify
the species being oxidised and the species
being reduced in each reaction.
2. Reactions with H2O – hydrolysis.
Examples:
FeCl2 + 6H2O  [Fe(OH2)6]Cl2
FeCl3 + 6H2O  [Fe(OH2)6]Cl3In both reactions, the Fe-containing products
are octahedral, i.e contain the [Fe(OH2)6]n+
cation.
3. Precipitation reactions
In the practical you do:
[Fe(OH2)6]2+(aq) + 3[C2O4]2[Fe(C2O4)3]4-(s) + 6H2O (l)
(aq)

[Fe(C2O4)3]4- is insoluble in water forming a
precipitate. The ligand [C2O4]2- has
exchanged with the H2O ligands around Fe.
Questions
1. Give the O.S. of the transition
element in the following complexes:
(a) [Fe(NH3)6]Cl3
(b) [Ni(H2O)6]F2
(c) Na2[CoCl4]
2. Draw the structure of the transition
element part of the following complexes
and name the type of geometry.
(a) [Fe(NH3)6]Cl3
(b) Na2[CoCl4]
3. Give the hybridisation of the transition
element in:
(a) [Fe(NH3)6]Cl3
(b) Na2[CoCl4]