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Chapter 23: Problem 79

Identify the complexes which are expected to be coloured. Explain [1994 - 2 Marks] (i) \(\left[\mathrm{Ti}\left(\mathrm{NO}_{3}\right)_{4}\right]\) (ii) \(\left[\mathrm{Cu}\left(\mathrm{NCCH}_{3}\right)_{4}\right]^{+} \mathrm{BF}_{4}^{-}\) (iii) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+} 3 \mathrm{Cl}^{-}\) (iv) \(\mathrm{K}_{3}\left[\mathrm{VF}_{6}\right]\)

Short Answer

Expert verified
Complexes (iii) [Cr(NH3)6]3+ and (iv) K3[VF6] are expected to be colored.

Step by step solution

01

Understanding complex formation

We need to identify which complexes are expected to be colored. This depends on the presence of unpaired electrons in the d orbitals of the metal ions forming the complexes.
02

Analyze [Ti(NO3)4]

Titanium in this complex appears as Ti(IV), which has a 3d^0 electron configuration. Since there are no d electrons, this complex is not expected to be colored.
03

Analyze [Cu(NCCH3)4]+BF4-

Copper in this complex is typically Cu(I) with a 3d^10 configuration, resulting in no unpaired d electrons. Therefore, this complex is not expected to be colored.
04

Analyze [Cr(NH3)6]3+ 3Cl-

Chromium in this complex is Cr(III) with a 3d^3 configuration, which means there are unpaired d electrons. The presence of unpaired electrons causes d-d transitions, making this complex colored.
05

Analyze K3[VF6]

Vanadium in this complex is V(III) with a 3d^2 configuration. This means there are unpaired d electrons, leading to possible d-d transitions, indicating that this complex is colored.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Crystal Field Theory
Crystal Field Theory (CFT) is essential in understanding the behavior of coordination complexes. At its core, CFT explains how metal ions in coordination complexes bond with surrounding ligands. These ligands act as an electric field, causing the metal鈥檚 d orbitals to split into different energy levels. Here's a breakdown of the fundamental aspects of CFT:
  • d Orbital Splitting: Normally, the d orbitals of metal ions are of equal energy or "degenerate." However, when surrounded by ligands, these orbitals split into higher and lower energy levels due to the repulsion between the d electrons of the metal and the electrons in the ligands. The specific pattern of this splitting depends on the geometry of the complex and the nature of the ligands.

  • Octahedral Complexes: In an octahedral complex, like \([ ext{Cr}( ext{NH}_3)_6 ext{]}^{3+}\), the d orbitals split into two sets of energy levels: \(e_g\) (higher energy) and \(t_{2g}\) (lower energy). Such splitting influences whether complexes are colored.

  • Importance for Color: The energy difference between these split d orbitals, known as the "crystal field splitting energy," can involve electronic transitions when they absorb visible light, thus causing color in the complex.
Crystal Field Theory is vital for explaining why certain complexes appear colored, influenced by the d-d transitions within the split d orbitals.
d-d Transitions
d-d transitions are a fascinating aspect of coordination chemistry that is integral to why complexes of transition metals exhibit color. When light hits a coordination complex, these transitions can occur due to the movement of electrons between split d orbitals as described by Crystal Field Theory. Key points about d-d transitions include:
  • Energy Absorption: In complexes like \([ ext{Cr}( ext{NH}_3)_6 ext{]}^{3+}\), light provides energy that is absorbed as electrons jump from lower energy d orbitals to higher energy ones within the split pattern. The specific wavelength absorbed corresponds to the observed color, since the color observed is typically complementary to the absorbed one.

  • Presence of Unpaired Electrons: d-d transitions require unpaired d electrons. As we saw with \([ ext{Cr}( ext{NH}_3)_6 ext{]}^{3+}\) and \( ext{K}_3[ ext{VF}_6]\), these complexes contain unpaired electrons, allowing these transitions to contribute to their color.

  • Influence of Ligands: The ligands surrounding the metal ion influence the extent of d orbital splitting, thereby affecting the energy required for d-d transitions, and ultimately determining the color observed.
Understanding d-d transitions helps clarify why certain coordination complexes are vibrant in color, while others remain colorless.
Unpaired Electrons
Unpaired electrons in coordination complexes can be a game-changer when determining the magnetic and optical properties of the complex. The presence of unpaired electrons is directly tied to whether a complex will show color through d-d transitions. Let鈥檚 explore the role of unpaired electrons:
  • Determining Color: As identified in \([ ext{Cr}( ext{NH}_3)_6 ext{]}^{3+}\) and \( ext{K}_3[ ext{VF}_6]\), unpaired electrons in these complexes permit the d-d transitions needed for color manifestation. Without unpaired electrons, like in \([ ext{Cu}( ext{NCCH}_3)_4 ext{]}^+\) which has a 3d10 configuration, no d-d transitions, and thus no color, occur.

  • Electron Configuration Influence: The number of unpaired electrons depends on the complex's electron configuration. For example, Cr(III) tends to have a 3d3 configuration, leading to unpaired electrons, whereas Cu(I) usually has a completely filled 3d10 configuration, meaning no unpaired electrons are present.

  • Magnetic Properties: Unpaired electrons not only influence color through d-d transitions but also affect the magnetic properties, rendering such complexes paramagnetic, as contrasted with complexes lacking unpaired electrons which are diamagnetic.
The presence of unpaired electrons in the metal ions of a coordination complex plays a crucial role in dictating its color and magnetic behavior.

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