Dictionary of Chemistry [6th Ed.]

the structure is a mer-isomer (merid-
ional). If the three X ligands are all
on one face of the octahedron and
the three Y ligands are on an oppo-
site face, then it is a fac-isomer(fa-
cial). See also ambidentate; e–z
convention.

isomers See isomerism.

isometric1.(in crystallography)
Denoting a system in which the axes
are perpendicular to each other, as in
cubic crystals. 2.Denoting a line on
a graph illustrating the way in which
temperature and pressure are inter-
related at constant volume.

isomorphismThe existence of two
or more substances (isomorphs) that
have the same crystal structure, so
that they are able to form *solid solu-
tions.

isonitrile(isocyanide; carbylamine)
An organic compound containing the
group –NC, in which the bonding is
to the nitrogen atom.

297 isopolymorphism

i

trans-isomer cis-isomer

X

Y

M Y

Y

Y

X

fac-isomer mer-isomer

Y

X

Y

M X

Y

Y

X

X

Y

M Y

Y

X

X

X

Y

M Y

X

Y

Isomerism

A

• Information about IUPAC nomenclature

iso-octaneSee octane; octane

number.

isoplethA vertical line in a liq-

uid–vapour phase diagram consisting

of a line of constant composition of

the whole system as the pressure is

changed. The word isopleth comes

from the Greek for ‘equal abun-

dance’. See also tie line.

isopoly compoundSee cluster

compound.

isopolymorphismThe property of

a substance with more than one crys-

talline structure that is isomorphous

with the crystalline structures of an-

other substance (see polymorphism).

For example, antimony(III) oxide,

Sb 2 O 3 , has both rhombic and octahe-

dral structures and these are isomor-

phous with the similar structures of

arsenic(III) oxide, As 2 O 3 ; both oxides

therefore exhibit isopolymorphism.