Chemistry - A Molecular Science

Chapter 14 Inorganic Chemistry

Example 14.1

CoBr

is a green solid while CoCl 2

is a blue solid. Which is a stronger field ligand, 2

chloride ion or bromide ion? The stronger field ligand is the one with the larger value of

Δ. CoBr

appears green, which 2

means that it absorbs red light. CoCl

appears blue, so it absorbs orange photons. 2

Orange photons have more energy than red photons, so

Δ is larger for CoCl

, and 2

chloride is the stronger fiel

d ligand. [Note that the Co

2+ ion is octahedral and surrounded

by six anions in the solid, even though the formula indicates only two per cobalt. This is analogous to NaCl where each Na

1+ is surrounded by six Cl

1- ions.]

The magnitude of

can also influence the number of unpaired electrons on the metal, Δ

which is usually designated by the spin of the atom, ion, or molecule. The

spin

of a

species is the sum of the individual electron spins. If there are no unpaired electrons, then there is no spin because the sum of the m

quantum numbers of pairs

ed electrons is zero (

1 /^2

(^1) -
/); that is, the two spins cancel because thei^2
r magnetic fields are opposed. However, if
the two electrons have the same spin, then they sum as:
1 /^2

• (^1)
  / = 1. Similarly, three^2
  electrons with the same spin resu
  lt in a species with a spin of (3)(
  1 /^2
  ) =
  3 /^2

. As we shall see

in Section 14.6, it is the spin associated with an atom, molecule, or ion that dictates its magnetic properties.

Consider the case of Mn

2+

, which has a 3d

5 valence electron configuration. In the

absence of any ligands, the five d orbitals all have the same energy; and the five electrons would be unpaired, as shown in

Figure 14.5a. However, in

the presence of ligands the

(^) d
orbitals no longer have the same energy. Elect
rons seek the lowest energy situation, but
the orbital energy is not the only consideration. Recall that Hund's rule states that electrons remain unpaired in a sublevel as long as empty orbitals are available. They do not pair because paired electrons are closer than unpaired electrons and the energy of two negatively charged electrons increases when th
ey are close (like charges repel). The
amount by which their energy increases is called the
pairing energy
(PE)

. Thus, three

electrons enter the three low-energy d orbitals

without pairing, but the other two electrons

can either pair with the first set at the cost of the pairing energy (PE) or they can remain unpaired and enter the set of orbitals at highe

r energy at the cost of the orbital energy

difference (

). If Δ

< PE, it costs less energy for the elΔ

ectrons to occupy the higher energy

orbitals (Figure 14.5b). However, when

PE, it costs less energy for the electrons to Δ

pair (Figure 14.5c). Consequently, when Mn

2+ is surrounded by weak-field ligands, such

Δ > PE

Δ < PE

no ligand field weak ligand field strong ligand field high spin Mn

2+ low spin Mn

2+

Energy

ligand field strength

(a)

(b)

(c)

Figure 14.5 High and low spin Mn(II) Δ and the number of unpaired electrons in a Mn

2+ ion as a function

of ligand field strength. PE is the pairing energy.

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