Sodium metal on dissolution in liquid ammonia gives a deep blue solution due to the formation of:

We start by recalling a well-known property of alkali metals: when an alkali metal such as sodium ($$Na$$) is added to liquid ammonia ($$NH_3$$) at low temperature, the metal actually dissolves rather than merely dispersing. This dissolution is accompanied by an intense deep blue colour.

To understand the origin of this colour, we first write the qualitative representation of the species formed when sodium enters liquid ammonia. Metallic sodium contains free electrons. Upon dissolution, the metal atoms ionise according to the idea

$$Na \;\;\longrightarrow\;\; Na^{+} \;+\; e^{-}$$

Inside the liquid ammonia medium, both the sodium cations and the electrons become surrounded (solvated) by ammonia molecules. The solvated species can be expressed symbolically as

$$Na^{+}(NH_3)_x \quad\text{and}\quad e^{-}(NH_3)_y$$

where $$x$$ and $$y$$ denote the coordination numbers (often 4-6 for the cation and about 4 for the electron). These solvated electrons are popularly called “ammoniated electrons.”

The deep blue colour of the solution is due to these ammoniated electrons. Their presence introduces broad absorption bands in the visible region, leading to the characteristic blue appearance. Neither sodamide ($$NaNH_2$$) nor a simple sodium-ammonia complex alone can account for this colour; it is specifically the solvated (ammoniated) electrons that are responsible.

Hence, when the question asks for the species responsible for the deep blue solution, the correct choice must be the one that explicitly mentions these solvated electrons.

Hence, the correct answer is Option D.