Would tin (II) sulfide be considered a covalent network solid?

Question

Considering that tin has a Pauling electronegativity of 1.96 and sulphur 2.58, and that a bond is considered to be ionic with a Pauling EN difference of approx. 1.7 at the least, would tin (II) sulfide be considered covalent?

This would be astonishing, since you would expect a metal—non-metal bond to be ionic. Indeed, many (non-academic) websites (such as this one) state that SnS is ionic. Evidence for the ionic character of tin (II) sulfide is the fact that it precipitates from solutions containing Sn(II) ions when adding hydrogen sulphide (which forms S(-II) ions) – thus, the two kinds of ions form a ionic compound (right?). Furthermore, SnS dissolves in hydrochloric acid, while typical covalently bonded networks such as silicon dioxide do not.

Finally, this paper states on the first page (which is freely accessible) that evidence for tin(II) salts with typical symmetrical ionic symmetries is completely lacking.

Thanks in advance, and happy holidays!

Answer 1

There is no sharp border between ionic and covalent compounds, much like there is no sharp border between a boy and a man. Some compounds between a metal and a non-metal, like $\ce{LiF}$ or $\ce{NaCl}$, are predominantly ionic, some (think of $\ce{Mn2O7}$ or $\ce{TiCl4}$) have markedly covalent character, and the rest are somewhere in between. As you see yourself, $\ce{SnS}$ is ionic enough to have certain kinds of reactions typical of ionic compounds, but not so ionic in terms of electronegativity or crystal structure.

Now, people in some areas of chemistry would put forth some definitions to tell covalent from ionic. Your "difference of electronegativities of at least 1.7" is one of those. Another is based on $\ce{MgO}$ and would use it as a border. There are a lot of definitions, and it's not like they are all wrong or useless; in fact, they are necessary to keep our stuff in order. One has to remember, though, that they are all slightly different, and all inherently arbitrary (much like the legal age limitations for driving, voting, drinking, and other activities, if we'd talk once more in terms of the "boy or man" analogy).

This must have been said here like a dozen times before, but whatever. Repetition is the mother of learning, they say.

Answer 2

This would be astonishing, since you would expect a metal—non-metal bond to be ionic. Indeed, many (non-academic) websites (such as this one) state that SnS is ionic.

Considering that tin has a Pauling electronegativity of 1.96 and sulphur 2.58, and that a bond is considered to be ionic with a Pauling EN difference of approx. 1.7 at the least, would tin (II) sulfide be considered covalent?

Yes tin (ii) sulfide is covalent and that website is rittled with non-ionic bonds. Percent ionic character can be estimated using equation: $$\mathrm{\% I.C. = 1-exp[ -0.25(X_A-X_B)^2]}$$ $X_A-X_B > 1.7$ suggests (there are deviations) that percent ionic character is greater than $50\%$ and substance can be called ionic. Otherwise bonds are probably more metallic or covalent. For the electronegativity values of tin (ii) sulfide you get an percent ionic character of $9.16\%$. $$\mathrm{\% I.C. = 1-exp[ -0.25(1.96-2.58)^2] = 9.16\% \ \ I.C.}$$ This means that tin (ii) sulfide bonds are about 90% covalent - compound is predominately covalently bonded.

Evidence for the ionic character of tin (II) sulfide is the fact that it precipitates from solutions containing Sn(II) ions when adding hydrogen sulphide (which forms S(-II) ions) – thus, the two kinds of ions form a ionic compound (right?).

Precipitating from soluble salts has nothing to do with the ionic nature and if anything would suggest the opposite as ionic substances typically dissolve well in water.

Furthermore, SnS dissolves in hydrochloric acid, while typical covalently bonded networks such as silicon dioxide do not.

Tin (ii) sulfide is not actually soluble in hydrochloric acid, but dissolves in it much like many metals, forming tin (ii) chloride and hydrogen sulfide which are both soluble. Silicon dioxide is oxophilic and much prefers bonding to the oxygen over the chlorine, and thus will not dissolve.

One thing to note is that tin is a metal by a narrow margin and in fact the $\beta$-tin is non-metallic and stable below $13.6^\circ C$.