The table below gives stepwise (K_n) and overall (beta_n) formation complexes fo

Question

The table below gives stepwise (K_n) and overall (beta_n) formation complexes for the [M(bpy)_2]^2+ (bpy = 2,2'-bipyridine) complexes in aqueous solution, with values missing in one box for each complex. [M(OH_2)_8-2n]^2+ + bpy [M(bpy)_n(OH_2)_6-2n]^2+ + 2 H_2O Fill in the missing values in each of the empty boxes. Explain the trend in the stepwise stability constants: K_1 > K_2 > K_3 for the Mn(II), Co(II), and Ni(II). Explain why the K_1 values are increasing in the trend Mn(II) K_2 (the answers to Question 3 might help you)

Explanation / Answer

Use the equation, = K1K2K3

a. K2 (Mn) : / K1K3 = 5.9 * 103

(Fe) : K1K2K3 = 6.3 * 1015

K1 (Co) : / K2K3 = 4.3 * 105

K3 (Ni) : / K1K2 = 1.6 * 106

b. The successive K values show a steady decrease for simple substitution of coordinated water molecules when there is no change of stereochemistry and the metal ligand bond energies do not change appreciably with the no of ligands. Several factor contribute:

1. Coulombic factor: This involves increasing electrostatic repulsion between the ligands (their peripheral electrons).

2. Steric hindrance: Here the ligand is bulkier than water. So there will be steric hindrance in the formation of the complex.

3. Statistical factor: The initial species has 6 sites from where the it can lose two water ligands. After first substituting the complex has 4 sites from where it can lose two water ligands. Thus, probability factor decreases with each substitution. So, initial complex has higher probability for first substitution. Second species has lower probability for the second substitution and so on.

c. The stability of complexes of +2 ions follows a common order with various ligands ( Irving- Williams Order): Mn(II) < Fe(II) < Co(II) < Ni(II)

The explanations are...

1. The   ionic radius is expected to decrease regularly from Mn(II) to Ni(II). This is the normal periodic trend and would account for the general increase in stability. As metals decreases in size the metal ligand bond becomes shorter which is the cause of increasing stability.

2. The Crystal Field Stabilization Energy (CFSE) increases from zero for Mn(II) to a maximum at Ni(II). This makes the complexes increasingly stable.

d. The reason behind this is the change of spin state from second complex, [Fe(bpy)2(H2O)2] to third complex ,[Fe(bpy)3]. [Fe(bpy)(H2O)4] and [Fe(bpy)2(H2O)2] are high spin complexes. Spin state changes in [Fe(bpy)3]. It is a low spin complex which makes the metal ligand bonds much shorter here than the former two complexes. Increasing stability in [Fe(bpy)3] increases the value of K3 very sharply.

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