organo-PreÜx used before the
name of an element to indicate com-
pounds of the elements containing
organic groups (with the element
bound to carbon atoms). For exam-
ple, lead(IV) tetraethyl is an organo-
lead compound.
organometallic compoundA
compound in which a metal atom or
ion is bound to an organic group.
Organometallic compounds may
have single metal–carbon bonds,
as in the aluminium alkyls (e.g.
Al(CH 3 ) 3 ). In some cases, the bonding
is to the pi electrons of a double
bond, as in complexes formed be-
tween platinum and ethene, or to
the pi electrons of a ring, as in *fer-
rocene.
A
- Information about IUPAC nomenclature
- Further information about
nomenclature
organophosphorus compound
A compound that has at least one
carbon–phosphorus bond. Organo-
phosphorus compounds are used as
pesticides (as in malathion and
parathion) and as catalysts and sol-
vents. Phosphate esters, sometimes
included in this category, are em-
ployed forÜre-prooÜng textiles.
Orgel diagramA diagram showing
how the energy levels of a transition-
metal atom split when it is placed in
a ligandÜeld. The vertical axis shows
the energy and the horizontal axis
shows the strength of the ligand
Üeld, with zero ligandÜeld strength
at the centre of the horizontal axis.
That the splitting for dnis the same
as dn+5and the opposite of d10 –nis
readily seen on an Orgel diagram,
both for octahedral and tetrahedral
Üelds. The spectroscopic, optical, and
magnetic properties of complexes of
transition metals are made clear in
such diagrams. Orgel diagrams are
named after Leslie Orgel, who devel-
oped them in 1955.origin of elementsThe nuclear
processes that give rise to chemical
elements. There is not one single
process that can account for all the
elements. The abundance of the
chemical elements is determined not
just by the stability of the nuclei of
the atoms but also how readily the
nuclear processes leading to the exis-
tence of these atoms occur.
Most of the helium in the universe
was produced by fusion in the early
universe when the temperature and
the pressure were very high. Most of
the elements between helium and
iron were made in nuclear fusion re-
actions inside stars. Since iron is at
the bottom of the energy valley of
stability, energy needs to be put into
a nucleus heavier than iron for a fu-
sion reaction to occur. Inside stars
some heavy elements are built up by
the s-process, where s stands for
slow, in which high-energy neutrons
are absorbed by a nucleus, with the
resulting nucleus undergoing beta
decay to produce a nucleus with a
higher atomic number. Some heavy
elements are produced by the r-
process, where r stands for rapid,
which occurs in supernova explo-
sions when a great deal of gravita-
tional energy is released when a
large star that has exhausted its nu-
clear fuel collapses to a neutron star,
with the release of a very large num-
ber of neutrons.
ornithine (Orn) An *amino acid,
H 2 N(CH 2 ) 3 CH(NH 2 )COOH, that is not
a constituent of proteins but is im-
portant in living organisms as an in-
termediate in the reactions of the
*urea cycle and in arginine synthesis.ornithine cycle See urea cycle.
orpimentA natural yellow mineral
form of arsenic(III) sulphide, As 2 S 3.389 orpiment
o