Chemistry Reference
In-Depth Information
preference for O-donors, though they can accommodate other donors, even including form-
ing some metal-carbon bonded species. All form [M(OH
2
)
n
]
3
+
(where
n
> 6), such as
[Nd(OH
2
)
9
]
3
+
, although these readily hydrolyse; this tendency increases from La to Lu (as
the ionic radius decreases). Chelating ligands give the most stable complexes, in line with
expectations developed for d-block metals in Chapter 5; basically, they follow the normal
rules of complexation developed in detail for d-block elements.
The row below the lanthanoids is the
actinoids
(also called the actinides), which are
mainly synthetic elements. Those that are found on Earth naturally are isotopically long-
lived thorium and uranium, but all are radioactive. After their d-block parent actinium, in
principle the f orbitals are then filled for the following elements. However, energies of 5f
and 6d are so close that elements immediately following Ac (and their ions) may have
electrons in both 5f and 6d orbitals, at least until 4 or 5 electrons have been entered, when
5f alone seems to be more stable. This means that the early actinoid elements tend to show
more d-block character (variable oxidation states and associated chemistry). Consequently,
resemblance of the series to the parent is less marked than with the lanthanoids, at least
until americium. Only after americium (about half-way along the series) are the elements
similar and lanthanoid-like in chemistry, with only the M(III) oxidation state stable. Earlier
elements, such as uranium, display oxidation states of up to M(VI); in fact, U(VI) is the
most common oxidation state for that element. High coordination numbers (up to fourteen)
are characteristic; for example, [Th(NO
3
)
6
]
2
−
is twelve-coordinate, as each nitrate ion acts
as an O,O-chelate ligand. Their solution chemistry is often complicated; hydrolysis in water
(even to oxo species) is common. Elements above fermium are short-lived, and isolable
only in trace amounts; others can be prepared in gram or even kilogram amounts. Being
radioactive and in most cases rare synthetic elements, they are not met by most scientists
let alone in everyday life, although they sometimes find an application.
6.2.5
Beyond Natural Elements
The Periodic Table has been extended from U (Z
112 since 1940 by
synthetic methods. Some synthetic elements have extremely long half-lives (e.g.
248
96
Cm,
t
1/2
3.5
=
92) to currently Z
=
10
5
yr), others moderate half-lives (e.g.
249
97
Bk,
t
1/2
300 d) or short half-lives
(e.g.
261
104
Rf,
t
1/2
65 s). Their syntheses have involved fusion and bombardment reactions,
for example:
×
249
97
Bk
18
8
O
260
103
Lr
4
2
He
3
1
0
n
+
→
+
+
Separation of a new element is a key problem. Separations involve methods such
as volatilization, electrodeposition, ion-exchange, solvent extraction and precipitation/
adsorption. Separation relies on the unique chemistry of each element; although not heavy
elements, but useful as an illustration,
64
30
Zn/
64
29
Cu are separated by dissolution in dilute
HNO
3
followed by selective electrodeposition of Cu (a very simple task, as the Cu
II/0
and
Zn
II/0
1V).
The totally synthetic fourth row of the d block has now been created fully, with all
member elements prepared, albeit in tiny amounts, and characterized. Lifetimes of these
new radioactive elements are not long, so their coordination chemistry has not been explored
in any detail. However, it is very likely they will behave like their third row analogues.
They are more chemical curiosities than applicable species at this time; however, they do
stand as monuments to the human inventive spirit and technological capacity.
redox potentials differ by
∼
