9.5 Bonding in Coordination Compounds 21
The crystal field splitting in this case is denoted by ∆t (the subscript “t” indicating tetrahedral complex). In the tetrahedral field, since the d orbitals are not interacting directly with the ligand field, the splitting of d orbitals is less than that in the octahedral complexes. t < o t = 49 o The lower value of crystal field splitting (∆t) in tetrahedral complexes may also be attributed to the lesser number of ligands (4).
Color in Coordination Compounds Most of the transition metal complexes are colored in their solution or solid state. The transition metals have the ability to absorb some of the radiations from the visible spectrum of light and transmit other radiations. When a transition complex is placed in white light, it absorbs certain portion of this white light and the portion which is not absorbed is reflected back from the complex. It is this reflected portion that imparts color to the complex. Thus, the actual color of the complex is depicted by the reflected light and not the light it absorbs. Crystal field theory explains the color in coordination compounds and attributes it to the d−d transition of the electron between the split t2g and eg levels. The energy gap between t2g and eg levels is very low in case of transition metal complexes and when light falls on them, the electrons in the lower energy level jump to the higher energy level. The absorbed light is actually that portion of light which is sufficient to excite the electrons from lower energy level to the higher energy level. The portion of light reflected back is responsible for the color of the complex. Table 9.4 shows the color of the light absorbed by a coordination entity and the color of the coordination entity. Consider the example of the complex [Ti(H2O)6]3+, in which Ti (III) has a single d electron and hence d1 configuration. The d electron will occupy t2g orbital. On irradiation with visible light, it should be possible for the ion to capture a quantum of radiation with frequency ∆o/h, (where h is Planck’s constant) and get excited from t2g orbital to eg orbital. Solutions containing the hydrated Ti3+ ion are reddish−violet in color because yellow and green light are absorbed to excite the electron and the transmitted light is the complimentary color.
Key Point
In absence of a ligand, the crystal field splitting does not occur, so d−d transitions are not possible and the substance is colorless. For example, hydrated copper sulphate (CuSO4∆ 5H2O) is blue in color whereas anhydrous CuSO4 is white.
Limitations of Crystal Field theory Crystal field theory successfully explains the formation, structures, color and magnetic properties of coordination compounds. However, the theory has certain limitations: 1. According to crystal field theory, anionic ligands are considered as point charges and hence should exert the greatest splitting effect. However, they occur at the low end of spectrochemical series.
Table 9.4 Relationship between the colors of the coordination entities and the colors absorbed by them.
Chap_09.indd 21
Coordination Entities
Color of Light Absorbed
Color of Coordination Entity
[CoCl(NH3 )5 ]2+
Yellow
Violet
[Co(NH3 )5 (H2O )]3+
Blue Green
Red
[Co(CN)6 ]3−
Ultraviolet
Pale Yellow
[Cu(H2O )4 ]2+
Red
Blue
[ Ti(H2O )6 ]3+
Blue Green
Violet
[Co(NH3 )6 ]3+
Blue
Yellow Orange
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