Chemistry - A Molecular Science

Chapter 14 Inorganic Chemistry

on the bond axes (where they point directly at the ligands) are higher than the energies of those that lie between the axes (where they

do not point directly at the ligands). The

symbol

is used to represent the energy differeΔ

nce between the two sets of d orbitals. The

magnitude of

depends on both the metal and the ligand, and it dictates many of the Δ

properties of the complex. Ligands that cause

to be large are called Δ

strong-field ligands

,

while ligands for which

is small are called Δ

weak-field ligands

. In our discussions, we

will use the CN

1- ion as an example of a strong-field ligand, and the Cl

1- ion and H

O 2

molecule as examples of weak-field ligands.

Many transition metal complexes are colored,

and this color can often be used as an

indicator of the magnitude of

. Recall from our discussion in Chapter 2 that color results Δ

from the absorption of light energy corresponding to electronic transitions that promote electrons from lower energy levels into higher energy levels. The frequency of the absorbed light,

, and the difference between the two energy levels, ν

E, are related by Δ

E Δ

= h

, where h is Planck’s constant. It is ofteν

n the case that the color of transition metal

complexes is due to electrons changing d

orbitals.

White light is the result of all visible co

lors. When white light shines on a colored

substance, however, some of the colors are ab

sorbed. Those colors that are not absorbed

can be either reflected or transmitted to the ey

e. We perceive this reflected or transmitted

portion as the color of the substance. Consequent

ly, the characteristic color of a material is

not the color of light that it absorbs; rather it is the mixture of the remaining, unabsorbed colors that are observed. In other words, it appears as its

complementary color

.* The

approximate relationship between

observed and absorbed colors is summarized in Table

14.1. Furthermore, the energy of a photon is proportional to its frequency, and so the frequency of the photon that is absorbed is a direct indicator of the magnitude of

. The Δ

relative energies of the visible photons are given in the last column of Table 14.1. Thus, a transition metal compound that appears yellow absorbs photons in the violet, while a transition metal compound that appears red absorbs green light. The energy of violet light is greater than that of green light, so we conclude that

is larger for the compound that Δ

appears yellow.

y

z

Energy

(^2) z
(^2) x - y
2
Δ
x
x
y
y
z
x
z
xy yz xz
Figure 14.4 Relative energies of the d orbitals

• A color and its complement contain all colors. Thus, a paint consisting
  of a mixture of a color and its comple
  mentary color will absorb all visible
  light and appear black. Table 14.1

. Complementary colors

A compound observed to be

Absorbs

photons in the

Increasing photon energy

green red lowest blue orange

violet yellow

red green

orange blue

yellow violet highest

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