An Introduction to Environmental Chemistry

The Oceans 199

O

C

H 2 C O–

H 2 C

C

O

O–

N:

O

H 2 C

CH 2

:N

CH 2

CH 2

C

O–

C

O–

O + M+

EDTA

O

C

H 2 C O

H 2 C

C

O

O

N

O

H 2 C

CH 2

N

CH 2

CH 2

C

O

C

O

O

EDTA–metal complex

M

Fig. 2Chelation between a metal ion (M+) and ethylenediaminetetra-acetic acid (EDTA), a well-known
laboratory ligand. Note that the six coordination bonds (white) are made between dissociated -O-groups
and lone pairs of electrons (denoted by double dots) on the N atoms of the EDTA. Similar coordination
bonds are made in fulvic acid chelates.

Table 1Percentage of each major ion in seawater
present in various ion pairs.

Na+ Mg^2 + Ca^2 + K+

Free ion 98 89 89 99
MSO 4 210 10 1
MHCO 3 — 11 —
MCO 3 —— — —

Cl- SO 42 - HCO 3 - CO 32 -

Free ion 100 39 81 8
NaX — 37 11 16
MgX — 20 7 44
CaX — 44 21
KX — 1 ——
Mg 2 CO 3 —— — 7
MgCaCO 3 —— — 4

M, cation; X, anion; —, species present at less than
1%.

Ligands and chelation

Beside water, other polar molecules and some

anions coordinate to metal ions; such firmly

bonded molecules are known as ligands. Each

ligand contains at least one atom that bears

a lone pair of electrons. Lone pairs of

electrons can be envisaged as non-bonded

electrons that are grouped at the extremities

of an atom’s electron orbital, and these

‘plug-in’ to gaps in the electron orbital of the

metal ion forming a coordinate bond. Most

ligands form only one coordinate bond with

a metal ion, for example the Cl-anion

which helps keep metal ions in solution

in mid-ocean ridge hydrothermal systems.

In some cases, however, a ligand may form

more than one link with the metal ion.

The complex ions that form between these

ligands and cations are known as chelates

or chelated complexes from the Greek chelos

meaning ‘a crabs claw’. The ligand is

envisioned to have a claw-like grip on the

metal ion, binding it firmly and forming a

stable complex ion (Fig. 2), which is much

more soluble than the metal ion itself.

Natural chelates include fulvic and humic

acids formed by the degradation of soil

organic matter.