versity (1968) and Faraday Medallist and Lecturer of
the Chemical Society of London (1970).
He died on March 3, 1999, at age 94, at his home
in Ottawa, Canada, and left a second wife, Monika,
and son and daughter.
Hess’s law of heat summation If a reaction goes
through two or more steps, the enthalpy of reaction is
the sum of the enthalpies of all the steps; the enthalpy
change is the same regardless of the number of steps.
heterobimetallic complex A metal complex having
two different metal atoms.
heteroconjugation (1) Association between a base
and the conjugate acid of a different base through a
HYDROGEN BOND(B′...HB+or A′H...A–). The term has
its origin in the CONJUGATE ACID-BASE PAIRand is in no
way related to conjugation of ORBITALs. Heteroassocia-
tion is a more appropriate term.
(2) Some authors refer to CONJUGATED SYSTEMs
containing a heteroatom, e.g., pyridine, as “heterocon-
jugated systems.” This usage is discouraged, since it
inappropriately suggests an analogy to HOMOCONJUGA-
TION(2) and conflicts with the currently accepted defi-
nition of that term.
heterocyclic amine An amine in which nitrogen is
part of a ring.
heterocyclic compound A cyclic organic compound
containing one or more atoms other than carbon in its
ring.
heterogeneous catalyst A catalyst that exists in a
different phase (solid, liquid, or gas) from the reac-
tants; a contact catalyst that furnishes a surface at
which a reaction can occur.
heterogeneous equilibrium Equilibria involving
species in more than one phase.
heterogeneous mixture A mixture that does not
have uniform composition and properties throughout,
where parts differ in composition or state of matter.heteroleptic Transition-metal or main group com-
pounds having more than one type of LIGAND.
See alsoHOMOLEPTIC.heterolysis(heterolytic reaction) The cleavage of a
COVALENT BONDso that both bonding electrons remain
with one of the two fragments between which the bond
is broken, e.g., A—B →A++B–. Heterolytic bond fission
is a feature of many BIMOLECULAR REACTIONs in solu-
tion (e.g., ELECTROPHILIC substitution and NUCLE-
OPHILICsubstitution).
See alsoHETEROLYTIC BOND-DISSOCIATION ENERGY;
HOMOLYSIS.heterolytic bond-dissociation energy The energy
required to break a given BONDof some specific com-
pound by HETEROLYSIS. For the DISSOCIATIONof a neu-
tral molecule AB in the gas phase into A+and B–, the
heterolytic BOND-DISSOCIATION ENERGYD(A+B–) is the
sum of the bond-dissociation energy, D(A–B), and the
adiabatic ionization energy of the radical A.minus the
electron affinity of the radical B..heteronuclear Consisting of different elements.heteroreceptor A RECEPTORregulating the synthesis
or the release of mediators other than its own ligand.
See alsoAUTORECEPTOR.heterotrophic organisms Organisms that are not
able to synthesize cell components from carbon dioxide
as a sole carbon source. Heterotrophic organisms use
preformed oxidizable organic SUBSTRATEs, such as glu-
cose, as carbon and energy sources, while energy is
gained through chemical processes (chemoheterotro-
phy) or through light sources (photoheterotrophy).heterotrophic organisms 127