Chemistry - A Molecular Science

Chapter 14 Inorganic Chemistry

spin makes the synthesis of molecular magnets challenging. However, one method that has met with some success is to synthesize mate

rials involving different ions with different

spins. If the spins align as represented in Figur

e 14.19c, there is a net ‘up spin’, and the

material is magnetic even though the neighboring spins are opposed. This type of magnetic material is called a

ferrimagnet

. Ferrimagnetic materials have been prepared from metal

cyanide coordination complexes similar to th

e blue dye known as Prussian Blue. These

materials are constructed starting with octahedral metal cyanide complexes M(CN)

. 6

While the carbon atoms of the cyanide ligand are coordinated to one metal, M, the nitrogen atoms of the cyanide ligands can also

act as a Lewis base that can coordinate to

another Lewis acidic transition metal, M’. The cyanide ligand

bridges

the two metals

(M:C

N:M’). The two metals are arranged into≡

a crystalline solid with a face-centered

cubic packing analogous to the sodium chloride structure. One such compound is Cs

Mn[V(CN) 2

]. The Mn(II) has five unpaired electrons, while the V(II) has only three. 6

The structure of the material is represente

d in Figure 14.20. The cyanide bridge assures

that there is an alternation of vanadium and manganese ions in all three directions (-Mn:N

C:V:C≡

N:Mn-) and imposes the required alte≡

rnation of spins of different

magnitudes.

14.7

CHAPTER SUMMARY AND OBJECTIVES The bonding between a transition metal and its

ligands can be described as a Lewis acid

(metal)-Lewis base (ligand) interaction. The ligand

coordinates

(bonds) to the metal, and

the geometry of the ligands about the

metal is referred to as the metal’s

coordination

geometry

; the number of ligands bound to

the metal is the metal’s

coordination number

.

Ligands forming more than one me

tal-ligand bond are said to be

bridging ligands

if the

bonds are to two different metals or

chelating ligands

if they bond to only one metal.

The lone pairs on the ligands cause the energi

es of metal d orbitals directed along the

bonding axes (z

2 and x

2 -y

2 orbitals) to be higher than those directed between the axes (xy,

xz, yz orbitals). The energy difference between the two sets of d orbitals is given the symbol

. The magnitude of Δ

depends on the metal and Δ

the ligands. Ligands causing

large

’s are Δ

strong-field ligands,

while those causing small

’s are Δ

weak-field ligands

. If

is larger than the pairing energy, the electrΔ

ons pair before occupying the higher energy

set of orbitals, and the metal is said to have a

low spin

configuration. If

is small, the Δ

electrons occupy the higher set of orbita

ls before pairing, and the metal has a

high spin

(a) (c)

ferr

omagnetic

antiferr

omagnetic

ferr

imagnetic

(b)

Figure 14.19 Electron spin and magnet type a) Neighboring units have a net spin oriented in the same direction; the substance is magnetic. b) Adjacent units interact to produce opposing spins and a nonmagnetic material. c) Adjacent units have opposed spins, but the spins are of different magnitudes and the material is magnetic. The fact that the

↑ is longer than the

↓ is

used to indicate the different ma

gnitudes of the spin.

nitrogencarbonmanganesevanadium

Figure 14.20 Structure of Cs

Mn[V(CN) 2

] 6

The cesium ion in the center of each cube is omitted for simplicity.

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