24. Transition Metals and Coordination Compounds

What is the correct order when the following complexes are arranged in ascending order of the Δ values?

i) [Mn(NO₂)₆]^4–               ii) [Ni(CN)₄]^2–                 iii) [Zn(OH)₄]^2–

Which of these crystal-field splitting diagrams represents:

a. a weak-field octahedral complex of  Fe³⁺ ,

b. a strong-field octahedral complex of Fe³⁺ 

c. a tetrahedral complex of  Fe³⁺

d. a tetrahedral complex of Ni²⁺ (The diagrams do not indicate the relative magnitudes of ∆. ) [Find more in Section 23.6.]

<IMAGE> <IMAGE> <IMAGE> <IMAGE>

In acidic aqueous solution, the complex trans-[Co(en)₂Cl₁]²⁺(aq) undergoes the following substitution reaction:

trans-[Co(en)₂Cl₁]⁺(aq) + H₂O(l) → trans-[Co(en)₂(H₂O)Cl]²⁺(aq) + Cl^–(aq)

The reaction is first order in trans-[Co(en)₂Cl₂]⁺(aq), and the rate constant at 25°C is 3.2×10^–5 s^–1.

e. Draw a crystal field energy-level diagram for ​trans-[Co(en)₂Cl₂]⁺ that takes account of the fact that Cl^– is a weaker-field ligand than ethylenediamine.

Formation constants for the ammonia and ethylenediamine complexes of nickel(II) indicate that Ni(en)₃²⁺ is much more

stable than Ni(NH₃)₆²⁺:

(1) <REACTION>

(2) <REACTION>

The enthalpy changes for the two reactions, ΔH°₁ and ΔH°₂, should be about the same because both complexes have six Ni﹣N bonds. 

(a) Which of the two reactions should have the larger entropy change, ΔS°? Explain.

Predict the crystal field energy-level diagram for a linear ML2 complex that has two ligands along the <IMAGE>: <IMAGE> 

Nickel(II) complexes with the formula NiX₂L₂, where X is Cl^- or N-bonded NCS^- and L is the monodentate triphenylphosphine ligand P(C₆H₅)₃, can be square planar or tetrahedral.

(c) Draw possible structures for each of the NiX₂L₂ complexes, and tell which ones have a dipole moment.

Predict the crystal field energy-level diagram for a square pyramidal ML₅ complex that has two ligands along the <IMAGE> axes but only one ligand along the z axis. Your diagram should be intermediate between those for an octahedral ML₆ complex and a square planar ML₄ complex. <IMAGE>

Nickel(II) complexes with the formula NiX₂L₂, where X⁻ is Cl⁻ or N-bonded NCS⁻ and L is the monodentate triphenylphosphine ligand P(C₆H₅)₃, can be square planar or tetrahedral.

(a) Draw crystal field energy-level diagrams for a square planar and a tetrahedral nickel(II) complex, and show the population of the orbitals.

Explain why [CoCl₄]^2- (blue) and [Co(H₂O)₆]²⁺ (pink) have different colors. Which complex has its absorption bands at longer wavelengths?

<IMAGE>