8/13/2019 The Beautiful Pure Color of Gems

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8/13/2019 The Beautiful Pure Color of Gems

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It is this splitting of the 3dorbital energies (symbolized by ) that explains the color and

magnetism of complex ions of the first-row transition metal ions. For example, in an octahedral

complex of Co3+

(a metal ion with six 3delectrons), there are two possible ways to place the electrons

in the split 3dorbitals (Fig.21.22).If the splitting produced by the ligands is very large, a situation

called the strong-field case,the electrons will pair in the lower-energy t2g orbitals.This gives a

diamagnetic complex in which all the electrons are paired. On the other hand,if the splitting is small

(the weak-field case), the electrons will occupy all five orbitals before pairing occurs.In this case the

complex has four unpaired electrons and is paramagnetic.

The crystal field model allows us to account for the differences in the magnetic properties of

Co(NH3)63+

and CoF63-

. The Co(NH3)63+

ion is known to be diamagnetic and thus corresponds to the

strong-field case, also called the low-spin case,since it yields the minimumnumber of unpaired

electrons.In contrast, the CoF63-

ion, which is known to have four unpaired electrons, corresponds to

the weak-field case, also known as the high-spin case,since it gives the maximumnumber of unpaired

electrons.

The reason that the Ti(H2O)63+

ion absorbs a specific wavelength of visible light can be traced

to the transfer of the lone delectron between the split dorbitals, as shown in Fig.21.25.A given

photon of light can be absorbed by a molecule only if the wavelength of the light provides exactly

the energy needed by the molecule.In other words, the wavelength absorbed is determined by the

relationship