After adding the pieces of tin, dehydration and crystallization are carried out until the inorganic salt is obtained. In this compound, tin has lost two electrons from its valence shell to form bonds with chlorine atoms.
This can be better understood if we consider the valence configuration of tin (5s 2 5p x 2 p y 0 p z 0 ), of which the pair of electrons occupying the p x orbital is transferred to the H + protons , thus forming a diatomic hydrogen molecule. That is, this is a redox type reaction.
Properties of stannous chloride
Are the SnCl 2 bonds ionic or covalent? The physical properties of tin (II) chloride rule out the first option. The melting and boiling points for this compound are 247ºC and 623ºC, indicative of weak intermolecular interactions, a common fact for covalent compounds.
An isolated SnCl 2 molecule is illustrated in the image above .
The molecular geometry should be flat because the hybridization of the central atom is sp 2 (3 sp 2 orbitals and a pure p orbital to form covalent bonds), but the free pair of electrons occupies volume and pushes the chlorine atoms down, giving the molecule an angular geometry.
In the gas phase, this compound is isolated, so it does not interact with other molecules.
As the loss of the pair of electrons in the p x orbital , tin is transformed into the Sn 2+ ion and its resulting electron configuration is 5s 2 5p x 0 p y 0 p z 0 , with all its p orbitals available to accept bonds of other species.
The Cl – ions coordinate with the Sn 2+ ion to give rise to tin chloride. The electron configuration of tin in this salt is 5s 2 5p x 2 p y 2 p z 0 , being able to accept another pair of electrons in its free p z orbital .
For example, it can accept another Cl – ion , forming the complex of trigonal plane geometry (a pyramid with a triangular base) and negatively charged [SnCl 3 ] – .
SnCl 2 has high reactivity and a tendency to behave like Lewis acid (electron receptor) to complete its valence octet.
Just as it accepts a Cl – ion , the same occurs with water, which “hydrates” the tin atom by binding a water molecule directly to the tin, and a second water molecule forms hydrogen bond interactions with the first.
The result of this is that SnCl 2 is not pure, but coordinated with water in its dihydrated salt: SnCl 2 · 2H 2 O.
SnCl 2 is very soluble in water and in polar solvents, because it is a polar compound. However, its solubility in water, less than its weight by mass, activates a hydrolysis reaction (breakdown of a water molecule) to generate a basic and insoluble salt:
SnCl 2 (aq) + H 2 O (l) <=> Sn (OH) Cl (s) + HCl (aq)
The double arrow indicates that an equilibrium is established, favored to the left (towards the reactants) if the HCl concentrations increase. For this reason, the SnCl 2 solutions used have an acidic pH, to avoid the precipitation of the unwanted salt product of the hydrolysis.
Reacts with oxygen in the air to form tin (IV) chloride or stannic chloride:
6 SnCl 2 (aq) + O 2 (g) + 2H 2 O (l) => 2SnCl 4 (aq) + 4Sn (OH) Cl (s)
In this reaction, tin is oxidized, forming a bond with the electronegative oxygen atom and its number of bonds with chlorine atoms increases.
In general, the electronegative atoms of halogens (F, Cl, Br and I) stabilize the bonds of Sn (IV) compounds and this fact explains why SnCl 2 is a reducing agent.
When it oxidizes and loses all its valence electrons, the Sn 4+ ion is left with a 5s 0 5p x 0 p y 0 p z 0 configuration , the pair of electrons in the 5s orbital being the most difficult to be “snatched”.
Chemical structure of tin chloride
SnCl 2 has an orthorhombic-type crystalline structure, similar to rows of saws, in which the tips of the teeth are chlorides.
Each row is a chain of SnCl 3 forming a Cl bridge with another Sn atom (Cl-Sn (Cl) 2 -Cl- ···). Two chains, joined by weak interactions of the Sn — Cl type, constitute a layer of the arrangement, which is superimposed on top of another layer, and so on until defining the crystalline solid.
The free electron pair 5s 2 causes distortion in the structure because it occupies volume (the volume of the electron cloud).
The Sn can have a coordination number equal to nine, which is the same as having nine neighbors, drawing a trigonal prism with it located in the center of the geometric figure and the Cls at the vertices, in addition to other Cls located in each one. of the square faces of the prism.
This is easier to observe if you consider a chain where the Sn (dark gray spheres) point upwards, and the three Cls linked to it form the triangular floor, while the top three Cls form the triangular ceiling.
Uses / applications
In organic synthesis, it is used as a reducing agent for nitro aromatic compounds (Ar-NO 2 à Ar-NH 2 ). As its chemical structure is laminar, it finds use in the world of catalysis of organic reactions, as well as being a potential candidate for catalytic support.
Its reducing property is used to determine the presence of gold compounds, to coat glass with silver mirrors and to act as an antioxidant.
Also, in its trigonal pyramid molecular geometry (: SnX3 – M + ) it is used as a Lewis base for the synthesis of a vast amount of compounds (such as, for example, the Pt 3 Sn 8 Cl 20 cluster complex , where the free pair of electrons coordinates with a Lewis acid).
SnCl 2 can damage white blood cells. It is corrosive, irritant, carcinogenic, and has high negative impacts on the species that inhabit marine ecosystems .
It can decompose at high temperatures, releasing harmful chlorine gas. In contact with highly oxidizing agents, it triggers explosive reactions.