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Which one of the two structures of sodium thiosulfate is correct? One book says the second one (the one with co-ordinate bonds) while the other one says the first one?

possible Lewis structures of thiosulfate

According to me, the structure 2 is correct, as in that structure the octet of the central S atom is complete whereas in the first one there's an excess of it.

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    $\begingroup$ Take a look at the Wikipedia page. Na2S2O3 is the sodium salt of the thiosulfate dianion. The negative charge on the anion is highly delocalised... the sodium isn't directly bonded to any oxygens or silicons, rather associated with the anion as a whole $\endgroup$
    – NotEvans.
    Jul 17, 2016 at 17:54
  • $\begingroup$ chemistry.stackexchange.com/questions/24928/… $\endgroup$
    – Mithoron
    Jul 17, 2016 at 19:55
  • $\begingroup$ @user456 I would still like to remind you about our Be nice policy. First and foremost, if anything changes the meaning of your post, please roll it back. Second, check the edit history thoroughly before making any accusations. Thirdly, what's wrong with ... is hardly ever an appropriate response. $\endgroup$ Jul 19, 2017 at 11:01

2 Answers 2

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Both of your Lewis structures are wrong.

In both of the shown structures, the solid line from sodium to oxygen/ sulfur implies that there is a discrete bond between these atoms. This is not the case.

The sodium ions and the thiosulfate ions form ion pairs (or ion triples) and are attracted to each other via diffuse electrostatic interactions. Usually that is what we consider ionic bonding.

In aqueous solution the ions will be very mobile. Sodium ions may coordinate to one or more thiosulfate ions, and therein to more than one site of the thiosulfate ion. At the same time thiosulfate ions may coordinate to more than just two sodium ion, or fewer. In such solutions, the solutes also coordinate to the solvent.
Ignoring any external field, the oxygen within the thiosulfate ion are equal.

The Lewis structure best describing the thiosulfate molecular ion is having a +2 formal charge at the central sulfur, and a -1 charge on each of the ligand atoms oxygen/ sulfur. (See below for details.)
One possible (and more accurate) Lewis structure is therefore:

Lewis(-like) structure of disodium thiosulfate
(Note that the dashed lines do not represent bonds, but weakly coordinating interactions, but not necessarily all of them.)

More common are probably the following Lewis structure representations:

alternative Lewis structures for disodium thiosulfate

In solid state coordination of the ions depend on various factors, especially how much water is incorporated into the crystal structure. In any of these cases, it would be wrong to draw any bonds between the (molecular) ions. I'll include a few links to publications at the end of the post.


Additional thoughts of your input structures

Which structure of sodium thiosulfate is correct? [...]
cited structures from question

You noted correctly that in the first structure the central sulfur needs to "expand the octet". This would only be possible if d orbitals of said sulfur are involved. It has been proven many times that the contribution of d-orbitals in these hyper-coordinated species is minimal (usually below 1%).

The second structure is technically, or strictly speaking, not a Lewis structure, because dative bonds are not part of this description. However, if we allowed them as an extension, which has been proposed multiple times, then the arrow would need to point from the oxygen towards the central sulfur, as in those occurrences the more electronegative element would assume the higher electron density.


Disclaimer: The following section is a bit more advanced, and might not be appropriate to the casual reader, as another user pointed out that it might obfuscate the most important points about the structure of the anion.

Gas phase approach to determining the most likely Lewis structure

If we assume this molecule to be an ion-pair and calculate it in the gas phase, then we can draw a couple of conclusions, how a more accurate representation in the Lewis framework would look like.
I have performed a quick calculation on the DF-BP86/def2-SVP level of theory with Gaussian 09 and performed a natural bond order analysis (NBO 6.0) on it. The following picture shows the optimised geometry, alongside some inter-atomic distances.

molecular structure of disodium thiosulfate

As you can see, the sodium ions are coordinated to two oxygen, or one oxygen and a sulfur, respectively. The natural atomic charges reproduce well what I have initially stated, and reproduce well the Lewis structure I have suggested.

\begin{array}{rrr} \text{Atom}& \text{No}& \text{Natural Charge}\\\hline \ce{S} & 1 & 1.98 \\ \ce{S} & 2 & -0.73 \\ \ce{O} & 3 & -1.01 \\ \ce{O} & 4 & -1.01 \\ \ce{O} & 5 & -1.01 \\ \ce{Na} & 6 & 0.91 \\ \ce{Na} & 7 & 0.86 \\\hline \end{array}

In terms of the NBO analysis, we can back this up, as it indicates no lone pair at the central sulfur (S1), three lone pairs at the oxygen (O3, O4, O5) and the ligand sulfur (S2), and four single bonds. There are vacant acceptor orbitals at the sodium ions (Na7, Na8). Here is an abridged version of the analysis:

     (Occupancy)   Bond orbital / Coefficients / Hybrids
 ------------------ Lewis ------------------------------------------------------
(skipping core orbitals)
  24. (1.98855) LP ( 1) S  2            s( 90.65%)p 0.10(  9.34%)d 0.00(  0.01%)
  25. (1.88728) LP ( 2) S  2            s(  0.00%)p 1.00( 99.89%)d 0.00(  0.11%)
  26. (1.84883) LP ( 3) S  2            s(  1.80%)p54.53( 98.09%)d 0.06(  0.11%)
  27. (1.97170) LP ( 1) O  3            s( 77.50%)p 0.29( 22.49%)d 0.00(  0.01%)
  28. (1.83558) LP ( 2) O  3            s(  0.67%)p99.99( 99.25%)d 0.11(  0.07%)
  29. (1.82232) LP ( 3) O  3            s(  0.03%)p99.99( 99.89%)d 2.42(  0.08%)
  30. (1.97170) LP ( 1) O  4            s( 77.50%)p 0.29( 22.49%)d 0.00(  0.01%)
  31. (1.83558) LP ( 2) O  4            s(  0.67%)p99.99( 99.25%)d 0.11(  0.07%)
  32. (1.82232) LP ( 3) O  4            s(  0.03%)p99.99( 99.89%)d 2.42(  0.08%)
  33. (1.96648) LP ( 1) O  5            s( 77.67%)p 0.29( 22.33%)d 0.00(  0.01%)
  34. (1.83753) LP ( 2) O  5            s(  0.02%)p99.99( 99.91%)d 4.78(  0.07%)
  35. (1.82147) LP ( 3) O  5            s(  0.00%)p 1.00( 99.93%)d 0.00(  0.07%)
  36. (1.95903) BD ( 1) S  1- S  2
               ( 56.47%)   0.7515* S  1 s( 25.58%)p 2.86( 73.06%)d 0.05(  1.36%)
               ( 43.53%)   0.6598* S  2 s(  7.70%)p11.86( 91.35%)d 0.12(  0.95%)
  37. (1.97694) BD ( 1) S  1- O  3
               ( 34.82%)   0.5901* S  1 s( 24.89%)p 2.97( 74.04%)d 0.04(  1.07%)
               ( 65.18%)   0.8073* O  3 s( 21.84%)p 3.57( 77.98%)d 0.01(  0.19%)
  38. (1.97694) BD ( 1) S  1- O  4
               ( 34.82%)   0.5901* S  1 s( 24.89%)p 2.97( 74.04%)d 0.04(  1.07%)
               ( 65.18%)   0.8073* O  4 s( 21.84%)p 3.57( 77.98%)d 0.01(  0.19%)
  39. (1.97758) BD ( 1) S  1- O  5
               ( 34.64%)   0.5886* S  1 s( 25.28%)p 2.91( 73.66%)d 0.04(  1.06%)
               ( 65.36%)   0.8085* O  5 s( 22.34%)p 3.47( 77.48%)d 0.01(  0.18%)
 ---------------- non-Lewis ----------------------------------------------------
  40. (0.05958) LV ( 1)Na  6            s( 95.91%)p 0.04(  3.91%)d 0.00(  0.18%)
  41. (0.10019) LV ( 1)Na  7            s( 96.97%)p 0.03(  2.93%)d 0.00(  0.10%)
  42. (0.25853) BD*( 1) S  1- S  2
               ( 43.53%)   0.6598* S  1 s( 25.58%)p 2.86( 73.06%)d 0.05(  1.36%)
               ( 56.47%)  -0.7515* S  2 s(  7.70%)p11.86( 91.35%)d 0.12(  0.95%)
  43. (0.24584) BD*( 1) S  1- O  3
               ( 65.18%)   0.8073* S  1 s( 24.89%)p 2.97( 74.04%)d 0.04(  1.07%)
               ( 34.82%)  -0.5901* O  3 s( 21.84%)p 3.57( 77.98%)d 0.01(  0.19%)
  44. (0.24584) BD*( 1) S  1- O  4
               ( 65.18%)   0.8073* S  1 s( 24.89%)p 2.97( 74.04%)d 0.04(  1.07%)
               ( 34.82%)  -0.5901* O  4 s( 21.84%)p 3.57( 77.98%)d 0.01(  0.19%)
  45. (0.24610) BD*( 1) S  1- O  5
               ( 65.36%)   0.8085* S  1 s( 25.28%)p 2.91( 73.66%)d 0.04(  1.06%)
               ( 34.64%)  -0.5886* O  5 s( 22.34%)p 3.47( 77.48%)d 0.01(  0.18%)

Key: LP = lone pair, BD = bond, LV = lone valency, BD* = "antibond"

Alternative coordinations

There are at least two other structures of the stoichiometry $\ce{Na2S2O3}$, essentially with the same bonding motif, but different coordinations of the sodium atoms. The first is about $\pu{2.4 kJ mol-1}$ more stable than the previously shown.

alternative molecular structure of disodium thiosulfate

The second is about $\pu{3.2 kJ mol-1}$ higher in energy.

alternative molecular structure of disodium thiosulfate

These are electronic energies only, but they show that the sodium atoms are not discretely bound and may interchange positions quite freely.
Given that disodium thiosulfate probably only exists in a crystal lattice which cannot be described by Lewis structures, or in solution where we have to worry about other effects and equilibria, it is obvious, that there cannot be a single Lewis structure for this compound. It simply does not exist as a discrete molecule.


Literature (selection) of crystal structures

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Three cases:
In an aqueous environment, the anion $\ce{(S2O3)^2-}$ is surrounded by water as well as $\ce{Na+}$. In this case, none of the above.

In crystalline form, the $\ce{S-S}$ bond length is close to single $\ce{S-S}$ bond which implies a strong negative charge on the terminal sulfur. $\ce{S=O}$ bonds are shorter implying a primarily double bond character. Several pentahydrate and anhydrous isoforms are known. The sodium atom is interacting with several neighboring anions. This is a common situation in salt crystals. For example, in NaCl each $\ce{Na+}$ interacts with $\ce{6 Cl-}$ ions. In the case of the crystalline form, none of the above.

$\ce{NaS2O3-}$ was probably studied in a gas phase, but I haven't seen papers supporting this. All the forms are probably present in the gas phase, but I don't know the exact distribution. Given that the $\ce{S-S}$ bond in the crystal structure has a single bond character, structure 2 should be more commonly found in the gas phase.

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    $\begingroup$ 'NaS2O3- was probably studied in a gas phase, but I haven't seen papers about it. In gas phase - probably all forms are present but I don't know the exact distribution. ' Could we try and stick to facts, hypothesising about a gas phase structure that doesn't exist really isn't helping the original poster who asked the question. $\endgroup$
    – NotEvans.
    Jul 17, 2016 at 18:28
  • $\begingroup$ Apart from what @NotEvans. wrote, structure 2 is utterly wrong. If there were donor-acceptor bonds, they would most likely be reversed to what is shown. $\endgroup$ Jul 19, 2017 at 4:02

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