Why does concentration of solutions of alkali metals in liquid ammonia change the magnetic properties?

Question

In "NCERT Chemistry for Class XI - Volume II", in chapter "s-block elements", under the topic "Chemical Properties of alkali metals", it is given:

(vi) Solutions in liquid ammonia: The alkali metals dissolve in liquid ammonia giving deep blue solutions which are conducting in nature.

$$\ce{M + $(x + y)$ NH3 -> [M(NH3)_x]+ + [e(NH3)_y]- }$$

The blue colour of the solution is due to the ammoniated electron which absorbs energy in the visible region of light and thus imparts blue colour to the solution. The solutions are paramagnetic and on standing slowly liberate hydrogen resulting in the formation of amide.

$$\ce{M+(am) + e- + NH3(l) -> MNH2(am) + 1/2 H2(g)}$$

(where ‘am’ denotes solution in ammonia.) In concentrated solution, the blue colour changes to bronze colour and becomes diamagnetic.

In the last set of lines, it is given that in concentrated solution, the blue colour changes to bronze colour and the paramagnetic solution becomes diamagnetic.

How does the change in concentration affect the magnetic properties of the solution?

Wikipedia has nothing to offer to clarify my doubt.

Answer 1

I always suggest students to try Google Scholar (scholar.google.com) when a simple Google search fails. I just searched three keywords : alkali metals ammonia solutions and the third result is highly relevant.

When your book talks about "in concentrated solution", it means more alkali metal in liquid ammonia. This paper, which you should search in Google Scholar, is titled A Revised Model for Ammonia Solutions of Alkali Metals, Journal of the American Society, Year 1962, volume 84, pg 2264). It talks about the formation of dimers of metals of the type $\ce{M2}$, because as you increase the metal conc. in ammonia solution conductivity decreases. The authors write, raising objections to older models as completely wrong show the following evidence:

"We picture the » species as a quadrupolar ionic assembly of $\ce{e- + 2M+}$ in which there is little distortion of either the ammoniated electrons or ammoniated metal ions. Presumably the electrons and ions are held in a square or rhombic configuration."

In short the picture is not that simple an no wonder the authors of a grade 12 book never discuss more details. You can start from here and search more.