Chemistry Reference
In-Depth Information
The molecular formula:
like the molecular drawing, we can eventually establish that we are dealing with a
[Co(NH
3
)
6
]
3
+
cation and three NO
3
−
anions, with the metal being cobalt(III);
importantly, the set of square brackets here also defines the presence of a complex unit,
and everything within the square brackets 'belongs' to the one complex unit, linked by
covalent bonds - the central atom (metal) which is the focus of bonding invariably comes
first in the formula.
The molecular name:
in this case, we notice there are two 'words', one for the cation and the other for the
anions;
the metal and its oxidation state are defined clearly;
the ligand name is defined clearly, along with a prefix that relates to the number of
ligands;
the anion name is defined clearly, although the number must be worked out from the
charge on the cation, demanding a knowledge of the charge (or lack of charge) of the
ammonia ligands.
What we hope is clear from all of the above is that all three approaches require some
basic knowledge of aspects of coordination chemistry to extract the full information from
the names; this is, of course, an inevitability of any language - some concepts of context
and 'unspoken' rules are necessary.
However, let's move on to see how we can interpret molecular formula and names, and
then step forward to some simple rules of nomenclature and examples. How we can 'read'
inorganic nomenclature is exemplified below. Note how the metal is placed first in the
formula representation of the complex unit and last in the written name; some other aspects
are obviously common to both representations (Figure A.2).
Some basic rules for naming compounds are given below. These may not make you an
expert, but will allow some understanding of how names are generated; only extensive use
(as with any language) brings expertise. Fortunately, structural formulae are now frequently
met and allow an escape from the more demanding naming methodology.
ligands
ligands
ligand
arrangement
metal
ligand
arrangement
metal
counter ion
counter ion
trans
-[CrBr
2
(NH
3
)
4
](OOCCH
3
)
trans-tetra
amminedibromo
chromium
(III) acetate
oxidation state
n
umber of
each ligand
square brackets
defining complex unit
number of
each ligand
Figure A.2
