three or more binding sites held together by COVALENT
BONDs, and which defines a molecular cavity in such a
way as to bind (and thus “hide” in the cavity) another
molecular entity, the GUEST(a cation, an anion, or a
neutral species), more strongly than do the separate
parts of the assembly (at the same total concentration
of binding sites). The ADDUCTthus formed is called a
“cryptate.” The term is usually restricted to bicyclic or
oligocyclic molecular entities, as seen here.
Corresponding monocyclic ligand assemblies
(CROWNs) are sometimes included in this group, if they
can be considered to define a cavity in which a guest
can hide. The terms podandand spherandare used for
certain specific LIGAND assemblies. Coplanar cyclic
polydentate ligands, such as PORPHYRINs, are not nor-
mally regarded as cryptands.
See alsoHOST.
crystal A three-dimensional solid formed by regular
repetition of the packing of atoms, ions, or
molecules.
crystal classes Three-dimensional point groups. The
32 possible unique combinations of rotation, reflec-
tion, inversion, and rotoinversion symmetry elements
in three dimensions. The morphology of every crystal
must be based on one of these collections of symmetry.crystal field Crystal field theory is the theory that
interprets the properties of COORDINATIONentities on
the basis that the interaction of the LIGANDs and the
CENTRAL ATOMis a strictly ionic or ion-dipole interac-
tion resulting from electrostatic attractions between the
central atom and the ligands. The ligands are regarded
as point negative (or partially negative) charges sur-
rounding a central atom; covalent bonding is com-
pletely neglected. The splitting or separation of energy
levels of the five degenerate dorbitals in a transition
metal, when the metal is surrounded by ligands
arranged in a particular geometry with respect to the
metal center, is called the crystal field splitting.crystal lattice A pattern of particle arrangements in
a crystal. There are 14 possible lattice patterns.crystal lattice energy The amount of energy that
holds a crystal together.crystalline solid A solid characterized by a regular,
ordered arrangement of particles; atoms, ions, or
molecules assume ordered positions.64 crystal
Examples of structures of cubic crystal