Talk:Metal ions in aqueous solution

high solvation numbers of Ln and An
Some of the superheavies might reach that too – Rf appears to be [Rf(H2O)8]4+ (10.1016/j.nuclphysa.2015.07.013). Admittedly chemistry beyond Lr is quite recent and sketchy. Double sharp (talk) 16:08, 8 April 2017 (UTC)
 * BTW given the ready hydrolysis of ZrIV (surely HfIV behaves similarly), I am slightly skeptical that RfIV really forms such a cation without hydrolysis as ThIV can. The actinide contraction goes over the same number of elements as the lanthanide contraction and is helped by larger relativistic effects: this is why the +2 state is so much more important at the end of the actinide series than at the end of the lanthanide series (when only Yb shows it, and then it is readily oxidised; whereas Fm and Md show it and for No it is the most stable state). Since the lanthanide contraction cancels out the expected increase in size from the fifth to the sixth period almost exactly, I would not be surprised if Rf were actually smaller than Zr and Hf, and perhaps even showed some echoes of the chemistry of Ti along with predominant Zr-like behaviour. Double sharp (talk) 03:49, 1 June 2017 (UTC)
 * Retracting this; the An contraction appears to be smaller than the Ln contraction, so that Rf is slightly larger than Hf (and thus RfIV may well be basic where HfIV is amphoteric). Double sharp (talk) 07:05, 14 March 2018 (UTC)

FWIW, discussion below has led to the conclusion that we shouldn't discuss superheavies in this article. There we have only theory in the absence of experimentally clear confirmation (or in one case, Rf, just supposition based on similarity of the observed behaviour to the lighter homologue Hf). Also the predictions are very patchy, and calculations become pretty difficult because of need to take into account relativity, and the whole subject seems to be veering off from the focus of the article. Basically, it is a similar case to astatine. This sort of thing is probably better discussed in the individual element articles as almost totally predicted chemistry and indirect evidence. Double sharp (talk) 07:35, 16 December 2020 (UTC)

Periodic table
I have copied the discussion (below) here to open it up to a wider audience. The issue concerns the placement of Al in the list of metallic elements, shown in a partial long-form periodic table. I proposed the unconventional layout shown below as being more suitable for the subject matter of the article. Both the traditional and proposed layouts have advantages and disadvantages. The layout chosen depends in which criteria are regarded as being more important. What is undeniable is that the atomic number of Al is one more than that of Mg, which is a good reason for placing these elements next to each other. The discussion is opened out to see if a consensus can be reached.

The issue is: which layout should be used in this article for the table of metallic elements. Petergans (talk) 10:57, 18 May 2018 (UTC)
 * {| class=wikitable style=text-align:center


 * +The metallic elements
 * Li|| Be
 * Na|| Mg|| Al
 * K|| Ca|| Sc|| colspan=14|
 * Na|| Mg|| Al
 * K|| Ca|| Sc|| colspan=14|
 * K|| Ca|| Sc|| colspan=14|


 * Ti||V||Cr||Mn||Fe||Co||Ni||Cu
 * Zn||Ga||Ge
 * Rb|| Sr|| Y|| colspan=14|
 * Rb|| Sr|| Y|| colspan=14|
 * Zr||Nb||Mo||Tc||Ru||Rh||Pd||Ag
 * Cd||In||Sn||Sb
 * Cs|| Ba||La
 * Ce||Pr||Nd|| Pm ||Sm
 * Eu||Gd||Tb ||Dy ||Ho ||Er ||Tm ||Yb ||Lu
 * Hf||Ta||W||Re||Os||Ir||Pt||Au
 * Hg||Tl||Pb||Bi||Po||At
 * Fr||Ra||Ac||Th||Pa||U||Np||Pu
 * Am||Cm||Bk
 * Cf||Es||Fm||Md||No||Lr
 * Am||Cm||Bk
 * Cf||Es||Fm||Md||No||Lr

(Copied from User talk:Double sharp|talk)
 * Rf||Db||Sg||Bh||Hs||Mt||Ds||Rg||Cn||Nh||Fl||Mc||Lv||Ts||Og
 * }

I placed Al in group 3 because, in this context of ions in solution it makes more sense. This is not the normal layout convention, but then, neither is the long-form periodic table. Further, there is an explanation in the article, copied below, as to why Al may be considered together with Sc rather than with Ga in terms of periodicity.
 * "By convention aluminium is placed in group 13, with gallium, indium and thallium. Nevertheless, a comparison of aluminium and scandium aqua ions illustrates a trend between the congeners Na+/K+, Mg2+/Ca2+ and Al3+/Sc3+ which depends on the increase in size on going from rows 3 to 4 in the periodic table. Shannon radii for 6-coordinate Al3+ and Sc3+ are 54 and 74.5 pm. Ga3+ has a Shannon radius of 62 pm, only about 13% larger than that of Al3+. This is due to the presence of the ten elements between scandium and gallium, which make an additional contribution to the general decrease in size across all rows of the periodic table."

I won't undo your edit; please do so if you can accept the motivation for putting Al in group 3 rather than in group 13. Petergans (talk) 09:13, 15 May 2018 (UTC)
 * I confess I don't find this argument convincing. Indeed, Na+/K+ have Shannon radii 1.02 and 1.38 Å respectively (ratio 1.35), Mg2+/Ca2+ have Shannon radii 0.72 and 1.00 Å respectively (ratio 1.39), Al3+/Sc3+ have Shannon radii 0.535 and 0.745 Å respectively (ratio 1.39), while Al3+/Ga3+ have Shannon radii 0.535 and 0.62 Å respectively (1.16). This is, as you say, because of the d-block insertion between Sc and Ga. But look at the fifth and sixth periods. Rb+/Cs+ have Shannon radii 1.52 and 1.67 Å respectively (ratio 1.10), Sr2+/Ba2+ have Shannon radii 1.18 and 1.35 Å respectively (ratio 1.14), Y3+/La3+ have Shannon radii 0.90 and 1.032 Å respectively (ratio 1.15), Zr4+/Ce4+ have Shannon radii 0.72 and 0.87 Å respectively (ratio 1.21), while Zr4+/Hf4+ have Shannon radii 0.72 and 0.71 Å respectively (ratio 0.99). Again, this is because of the f-block insertion between Ce and Hf. So it seems to me that this argument suggests that we consider Zr with Ce instead of Hf in terms of periodicity, and if that's undesirable then I don't see why its case for putting Al in group 3 should be any better. Also, since Sc3+, Y3+, and La3+ are not actually hexacoordinate in aqueous solution, whereas Al3+, Ga3+, In3+, and Tl3+ are, I think Al fits better due to size with its usual group 13 congeners, while noting that it has the hardness of a group 3 ion. Double sharp (talk) 12:19, 15 May 2018 (UTC)
 * This is not a periodic table! In this context, the unconventional layout for the metallic elements is better suited to the subject matter of the articlec. Petergans (talk) 08:01, 16 May 2018 (UTC)
 * And that's what I'm disputing. First of all, the aqueous Al3+ ion has a structure more similar to Ga3+, In3+, and Tl3+ precisely because the group 13 ions are smaller than the group 3 ions and can only coordinate six water molecules unlike the bigger Sc3+, Y3+, and La3+ ions. As a result, it is questionable at best if moving Al to group 3 is an improvement even given the subject matter. Second of all, in periods 5 and 6, the lanthanide contraction is so big that Hf4+ is smaller than Zr4+, but no one is advocating putting Ce4+ under Zr4+ instead; at least Ga3+ is still bigger than Al3+. I don't see why we should make an exception from the effects of inserting the d- and f-blocks just for aluminium. That, to my mind, is not better suited to the subject matter, particularly when it is not and cannot be carried out consistently, and when the similarities of Al3+ to Ga3+ are not appreciably weaker than those of Al3+ to Sc3+. Double sharp (talk) 11:38, 16 May 2018 (UTC)
 * Furthermore, I disagree that this is not a periodic table. If it were not, it would not be formatted like one. A list of metallic elements to be discussed would look like:
 * lithium
 * beryllium
 * sodium
 * oganesson
 * and not be any more useful at organising anything than an actual periodic table. A normal periodic table is useful as always, as suggested immediately by the group-based organisation of "Solvation numbers and structures" (except for putting Al in group 3, which in this context is even stranger because the Al(H2O)63+ solvation number and structure is the same as that of Ga3+ through Tl3+, not Sc3+ through La3+). Frankly, given this and the other question about what this argument would do to the case for putting Ce and Th in group 4, I think moving aluminium around raises more questions than it actually answers. Double sharp (talk) 14:41, 16 May 2018 (UTC)
 * and not be any more useful at organising anything than an actual periodic table. A normal periodic table is useful as always, as suggested immediately by the group-based organisation of "Solvation numbers and structures" (except for putting Al in group 3, which in this context is even stranger because the Al(H2O)63+ solvation number and structure is the same as that of Ga3+ through Tl3+, not Sc3+ through La3+). Frankly, given this and the other question about what this argument would do to the case for putting Ce and Th in group 4, I think moving aluminium around raises more questions than it actually answers. Double sharp (talk) 14:41, 16 May 2018 (UTC)

I again do not think the new layout is an improvement. Firstly, all of these groupings of elements are already apparent on a normal periodic table. Secondly, the transactinides are omitted despite there having already been some aqueous chemistry done on Rf, Db, Sg, and Hs. Thirdly, I remain unconvinced that the changes are actually going to help understanding. For example, if we are going to duplicate La and Ac, then why not Y which behaves like a heavy lanthanide? And again, putting Al on top of Sc neglects the greater similarity of the structure and solvation of the Al3+ cation to the Ga3+, In3+, and Tl3+ cations rather than to the Sc3+, Y3+, and La3+ cations. I completely disagree that Al having an atomic number one more than Mg is a good reason to put them next to each other; otherwise one could use the same reasoning to put Ti and Zr over Ce, as I have previously noted. Pending further discussion I have restored the previous standard 18-column layout, which seems to have remained mostly constant for almost seven years since June 2011 when an IP editor moved Al to its usual position above Ga.

Perhaps we might find a larger audience for this discussion at WT:CHEM? Double sharp (talk) 15:34, 20 May 2018 (UTC)

Petergans has changed the periodic table picture shown back to his preferred layout; I have moved Al back to being on top of Ga (this time more consistently), because I'm still not convinced that moving it to above Sc is more useful in this context – not the least because the 6-coordinate structure of Al3+ is similar to that of Ga3+ and In3+, but not that of Sc3+ and Y3+. Even papers specifically focused on the topic of metal ions in aqueous solution do not make this change (e.g. 10.1351/PAC-CON-09-10-22). Double sharp (talk) 12:12, 24 October 2018 (UTC)

It is a fact that the atomic number of Al is one more than the atomic number of Mg. In the present context, it makes sense for these two elements to be shown next to each other and for Al to be above Sc. If this was a periodic table, I would accept the conventional layout, but it is not; the elements in the top right corner are missing from this table as they are not metallic. Petergans (talk) 13:01, 24 October 2018 (UTC)
 * The atomic number of Zr is also one more than that of Y, but we're not putting Y over Lu, are we? That is actually rather easier to justify than putting Al over Sc. This may not be a full periodic table, but it certainly is a partial one. All the nonmetallic elements in the top right corner might not be shown, but they could in principle be put in their locations without disturbing the format of the table at all (as evidenced by how I put a few metalloids back in a while ago). Double sharp (talk) 13:29, 24 October 2018 (UTC)


 * Do you know how many angels can dance on a pin-head? Petergans (talk) 10:26, 25 October 2018 (UTC)

Static vs dynamic methods
I see that labels of static vs dynamic are used in connection to the methods of determination. I think that a more explanation is required for these qualifiers. Perhaps dynamic is intended as a synonym for kinetic or transport properties, from what it can be noticed.--5.2.200.163 (talk) 14:07, 20 June 2018 (UTC)

Relation between radial distribution function and hydration number
I think that the article should mention the explicit expression of the hydration number as a function of radial distribution function, which I see mentioned in the article.--5.2.200.163 (talk) 14:23, 20 June 2018 (UTC)

Determination of the residence time of solvent
I think it would be necessary to add details about the determination procedure of the residence time of solvent in the first solvation shell.--109.166.139.84 (talk) 21:59, 24 October 2019 (UTC)

The newly added book inserted as Further reading seems to contain such info.--109.166.139.84 (talk) 00:32, 25 October 2019 (UTC)

Kinetic parameters for water exchange
I see there are some specs re water exchange kinetics, which deserve some questions/clarifications. How is the aqua ion concentration determined?--109.166.139.12 (talk) 21:53, 18 November 2019 (UTC)


 * $$ \mathrm {rate} = - \left( \frac{d[A]}{dt} \right) _T = k[A]$$

Also how is the constant k determined as first-order rate constant? By using a graph concentration of aqua ion [A] vs time, [A](t) which is supposed to be (approximately) a linear function?--109.166.139.12 (talk) 22:09, 18 November 2019 (UTC)

There is another aspect needing clarification, two inconsistent statements about the dimension of rate constant k:

"The unit of the rate constant for water exchange is usually taken as mol dm−3s−1. The half-life for this reaction is equal to loge2 / k. This measure is useful because it is independent of concentration. It has the dimension of time. The quantity 1/k, equal to the half life divided by 0.6932, is known as the residence time or time constant."

--109.166.139.12 (talk) 22:42, 18 November 2019 (UTC)

The inconsistency is between the dimension amount (of substance)/(volume*time) of k and the dimension time for 1/k.--109.166.139.12 (talk) 23:00, 18 November 2019 (UTC)

In the first sentence "rate constant" should be replaced with "reaction rate".--109.166.139.12 (talk) 00:53, 19 November 2019 (UTC)
 * Please register a user name for Wikipedia. Let me know when you have done so. I will answer your questions there. Petergans (talk) 10:04, 19 November 2019 (UTC)
 * I haven chosen a name for this IP range 109.... and future account - Electrochemical Diffus-n.--109.166.139.12 (talk) 21:17, 19 November 2019 (UTC)

Ion transfer number for determining hydration number
I see in the article some statement re the determination of hydration number of metal ions by using ion transport number. Some more details are needed.--109.166.139.12 (talk) 21:11, 19 November 2019 (UTC)

Al in list of metallic elements in aqueous solution (copied from User talk:Double sharp)
If this were a periodic table, I would agree with you, but it is not: it is a table showing which elements are metallic, as its title specified. In this context, placing Al, on its own, above Ga looks odd, and there is nothing in the article text to explain why it is there. At User:Petergans/sandbox I have toyed with the idea of using a standard layout and colouring the non-metallic elements red, but everything I tried was unsatisfactory in one way or another. Petergans (talk) 11:37, 14 December 2020 (UTC)
 * , the thing is that every other element there is placed where a periodic table would put it. So, when looking at the table I'd mentally read it as "aha, this is a PT with the nonmetals erased". And then putting Al in an unusual place makes me wonder what's going on. That being said, if it causes confusion, maybe the best thing to do would be to just make it a normal PT and mark the nonmetals in some other colour – similar to what you did in your sandbox, but with the usual gap so that Ne and Ar go above Kr. Such a table would still list the metallic elements, but it wouldn't cause this conflict of interpretations, I think. Double sharp (talk) 12:46, 14 December 2020 (UTC)
 * Thanks for the helpful comments. A revised PT is in my sandbox. There is no mention an aqua-ion of Sb3+ in my inorganic text-books (G&E, C&W, H&S), but that's not conclusive. Petergans (talk) 17:02, 14 December 2020 (UTC)
 * Yes, I like what you have. It agrees pretty well with the article's topic, that is the aqua cations rather than the metals. I decided I liked your sandbox table enough to just put it in.
 * Having said that, I made two changes: the H/He row is now included (all red of course), and some elements near the boundaries. At+ seems accepted from recent studies: solvation number not really confirmed but seems very low from what I gathered elsewhere. H/He just completes the table (I added a line about how transactinoids are excluded because there's little to no experimental data), and they're red anyway.
 * Ge2+ and Sb3+ should exist in water according to theoretical studies. See and 10.1021/ic901737y for Sb3+, see 10.1002/jcc.21315 for Ge2+, they study the hydration structure. Of course, strongly distorted due to the lone pair, and basically on the edge of hydrolysis and only stable at very low pH, that well explains why most textbook sources hesitate to mention them. Experiments in perchloric acid media (weakly complexing) confirm Ge2+ by production of germanous perchlorate: see 10.1002/jccs.196400020. As for Sb3+, solutions of antimony perchlorate can similarly be prepared, and we know that metal salts there are mostly present as aqua cations (well, here cites it to Gmelin and mentions Sb and Bi explicitly). Both cations seem about equally unstable and near the edge, but real and within the norm for p block cations with the lone pair and very prone to hydrolysis (cf. Sn2+). Au contraire, Ge4+, Sn4+, Pb4+, As3+ were investigated (first link, for As3+ also 10.1016/j.cplett.2009.03.011), and it turns out those hydrolyse immediately without being given a chance to exist. I am OK if they are removed as too far away, but given that W3+ and such are probably even more unstable due to tendency towards polycation formation, I think it will rather hurt less to just be as open as possible for the lede graphic. After all: not every element on that table is discussed later, and it is probably easier to just be accepting in the lede graphic and not worry about whether to include or exclude borderlines when they will be justly be given little screen time later anyway. But if you don't agree, I won't mind too much if Ge, Sb, and At go red. It is not an important issue; at this stage it comes close to angels dancing on the head of a pin. Double sharp (talk) 05:01, 15 December 2020 (UTC)
 * The Germanium dichloride dioxane complex is stable and available commercially. What happens when it is dissolved in water, perhaps at low temperature? Is there a reference for Ge2+ (aq) that can be cited? Is there any evidence for At+ in aqueous solution? Petergans (talk) 11:05, 15 December 2020 (UTC)
 * Germanium: as mentioned above studies Ge(II) perchlorate, and on the grounds of the weakly complexing nature of perchlorate attributes the presence of a Ge(H2O)n2+ cation that is then steadily hydrolysed. The Aqueous Chemistry of the Elements by Schweitzer and Pesterfield (OUP 2010) states (p. 190) that Ge2+ (aq) forms when GeO is dissolved in dilute acid, although both species are unstable (presumably to oxidation to the more stable +4 state). Sorry, I could not find what happens to GeCl2, but I guess it would be similar. Coordination number of Ge2+ according to theoretical study should be between 3–4 per . For astatine evidence is of course spottier and mostly indirect. That At(I) is At+ in aqueous solution was suspected by 2010, see, because it would explain the behaviour quite well (it reacts with halogens to form AtXmn−). Calculations there though are apparently for the other species, not for At(I). This being said, Gmelin volume in 1989 says it is really At(OH2)+ and is generated by protonation of AtOH. Currently, two proposed structures are either At(OH2)+ and At(H2O)2+, with positive charge on the oxygen(s) rather than the astatine. I am sceptical that you can really call this an aqua cation if the positive charge is not on the At, though. Since the evidence is apparently just an inference without a calculation to back it up, I removed astatine as a bit too unsettled to include. Well, that makes what's drawn match the usual diagonal stairstep, so maybe it's not bad as just something to hang up. Double sharp (talk) 12:07, 15 December 2020 (UTC)
 * Great! The article is good as it stands. Perhaps you might like to create a new section on theoretical studies? I'm not at all familiar with that subject area. Petergans (talk) 14:43, 15 December 2020 (UTC)
 * I added a paragraph to the section on the p block metals covering the cations that are only studied well by theory (Ge(II), Sb(III), Po(IV)). Double sharp (talk) 07:25, 16 December 2020 (UTC)

4d and 5d metal cations
Do you think we should go a little bit further about what happens for these metals? Currently we have just a short uncited remark. My understanding is that in some cases (e.g. W) simple aqua cations are hard to find, even though of course W is chemically clearly a metal, because of tendencies towards cluster formation in the lower oxidation state where cations are likely. FWIW the Persson paper on structures lists Nb, Tc, Ta, W, and Re as lacking aqueous cationic chemistry (for Nb and Ta, I guess it is because to a first approximation these metals have only the +5 oxidation state as the usual one). Unfortunately, I haven't found theoretical studies for this region like I have for the ones near the metal-nonmetal dividing line. Double sharp (talk) 07:31, 16 December 2020 (UTC)
 * The issue here is that the elements in groups 4 to 10 and periods 2 and 3 are generally found in oxidation states greater than 3, where they may form partially hydrolysed, oligomeric species. For example, species of Zr(IV) of this type are well characterised (Greenwood & Earnshaw, p966). G&E also state that Zr(III) reduces water, meaning the aqua ion is not a stable species. Similarly, the lower oxidation states of the elements in groups 5-8 are not stable in aqueous solution. Is [Pd(H20)4]2+ square planar? What is the structure Ag(aq)+ in solution?. On the other hand, there is no mention of Pt(II) or Au(I) in this article and this gap should be filled if there is published data on their aqua-ions. Petergans (talk) 14:19, 16 December 2020 (UTC)
 * Article I linked claims: group 4, Zr(IV) and Hf(IV) square antiprismatic (but extremely prone to hydrolysis); nothing for heavy group 5; group 6, Mo(III) octahedral but strongly hydrolysed; nothing for heavy group 7; group 8, Ru(II), Ru(III), Os(II) all octahedral; group 9, Rh(III), Ir(III) octahedral; group 10, Pd(II) and Pt(II) not square planar but strongly tetragonally elongated octahedral; group 11, Ag(I) distorted tetrahedral, Au(I) linear, Au(III) is mentioned but not made clear (seems some kind of distorted octahedron). Of course I guess many are not exactly stable in the listed oxidation states. I've added these to the section.
 * Well, to avoid quibbles, I changed the PT header to "the metallic elements" to avoid quibbles about rhenium and suchlike. We already have the standard diagonal line between metals and nonmetals there after all. Double sharp (talk) 15:15, 16 December 2020 (UTC)
 * P.S. Used the more recent Persson review to update the info from Richens' book about the f elements. And also grabbed hold of a recent study for Ac(III). I think now every element for which something is known should be covered (unless I miss something more recent). Double sharp (talk) 15:43, 16 December 2020 (UTC)
 * Excellent work! This is an important topic and deserves the comprehensive treatment that it now has. Petergans (talk) 17:33, 16 December 2020 (UTC)

Lanthanides and actinides
Surely there should be a section on lanthanides and actinides? Conjecture should be avoided, so perhaps the heavier actinides might be omitted from the table of elements that form metal ions. Petergans (talk) 21:32, 3 February 2021 (UTC)
 * They are currently covered with scandium and yttrium from group 3 (which the Ln are quite related to anyway, so maybe not a bad thing). According to Persson their structures are only known to Cf in the actinides (Ac study came after his paper). Beyond there, I only know of calculations ( summarises some). Seems the expectation is that similar to lanthanoids, coordination numbers for heavy actinoids(III) should fall from 9 to 8 across Bk-Lr, but it is only conjecture. It seems uncontroversial that these ions exist (some discussion of Fm to Lr in ), but nothing firm is known of their structure. Though, I hesitate to say "cut them out" because by that logic francium is shaky, which seems a bit pedantic since I don't think anyone seriously disputes that it would form an aqua cation while it lasts. (And then there are those very easily hydrolysed species near the metal-nonmetal border.) So, current version seems OK to me, but I don't mind stopping to show at californium. Double sharp (talk) 13:40, 10 April 2021 (UTC)

Periodic table of aqua-ions
The table has been re-organised to long form; this removes possible controversy over the position of lutetium etc.. Early transition metals in rows 2 and 3 are marked as not forming aqua-ions.

Do germanium, arsenic, antimony and bismuth in low oxidation states form aqua-ions? Sources available to me do not give any information. Petergans (talk) 11:21, 13 April 2021 (UTC)
 * In long form, position of Sc and Y becomes the controversy. I have restored the Sc/Y/Lu/Lr grouping per the January 2021 IUPAC preliminary report on this, which was decided to be a default convention for WP at Wikipedia_talk:WikiProject_Elements/Archive_59 shortly after its publication. FWIW, the Oxford Dictionary of Chemistry uses this grouping.
 * As we discussed earlier on this talk page, there is some experimental evidence for germanium and antimony aqua ions in low oxidation states, but computer modelling was necessary to determine the structures because they hydrolyse too easily. So Ge2+, Sb3+, and Bi3+ are already discussed in the article (with the cites). Computer modelling suggests there is no As3+ (instant hydrolysis). Astatine was hypothesised in the past, but evidence seems against it. I added some text about As and At (the latter, I supposed, necessitated changing "Group 13-16" to "Group 13-17"). Persson does not consider Ge and Sb, but he doesn't consider Po either. He does consider Bi. I suppose germanium is not a real metal.
 * Zr, Hf, Mo, and Os have been turned back to blue per Persson.
 * Are there actually any studies on the structures of aqua cations of Fr and Ra? I can't find any. Such things must exist of course, so maybe they should be asterisked. Double sharp (talk) 15:25, 13 April 2021 (UTC)
 * Mostly evidence for Ge and Sb cations comes from perchlorates (because it is weakly complexing, so the metal should appear as an aqua complex), substantiated by simulations that suggest they should exist. But I still haven't found direct evidence. Some authors use them, while others don't. Double sharp (talk) 05:18, 15 April 2021 (UTC)

Asterisks given to Ge, Sb, Po at the dividing line, as well as strong radioactives Fr, Ra, and Es-Lr.

As for transactinoids, aqueous chemistry has only been done for Rf, Db, Sg, and Hs. Given the congeners, though, one should not really expect to see aqua ions for these, and indeed such were not studied in detail. So, actinoids seems a good place to stop. Double sharp (talk) 08:24, 15 April 2021 (UTC)

See Talk:List of ions in aqueous chemistry for further discussion relating aqua-ions. Petergans (talk) 11:39, 25 July 2021 (UTC)​

Hydrogen
I wonder if hydronium ought to be explained briefly, given how most introductory chemistry books cannot resist writing "H+".Double sharp (talk) 04:12, 14 April 2021 (UTC)
 * Hmm, very interesting. Seems to have made it to this secondary source, although most textbooks have probably not changed as anticipated. I guess, better leave it out for now (sticking to standard stuff), with the excuse that H is really too far from being physically a metal. Double sharp (talk) 12:30, 14 April 2021 (UTC)
 * I question the assignment in {Hmm,}. The fundamental vibrations for O-H strething for liquid water occur around 3500 cm-1. Features at 1747cm-1 and below will be overtones and/or combination bands. The principal effect of hydrogen bonding on the ir spectrum is to broaden the absorption bands. This is an extensively researched topic which includes studies of D/T isotopomers and overtones and combination bands in the near-infrared. See Electromagnetic absorption by water for some details. Petergans (talk) 19:56, 14 April 2021 (UTC)


 * And the next day I found this later IR spectroscopy study. Mostly assigns a Zundel-like H5O2+ structure and doubts the H13O6+ structure of the studies I found yesterday "This does not rule out more delocalized structures; however, when considering the picosecond kinetics of proton transfer recently measured for this species, we believe that any waters outside this core would be very dynamic and rapidly exchanging". So, I guess I was right to leave it out.
 * I suppose the characterisation in the hydronium article would be the best way to put it, then: simply to say that "H+" in water is solvated somehow, but that the "how" is not fully characterised, and that several structures have been proposed. That is if we think H+ is worth mentioning here at all. Double sharp (talk) 05:00, 15 April 2021 (UTC)
 * Well, added a paragraph on hydrogen. My excuse: Richens includes it. :) Double sharp (talk) 07:50, 15 April 2021 (UTC)

Metal ions in aqueous solution
This article mainly discusses metal cations containing water as the ligand. What about anions and metal cations with different ligands? Some of them are also stable in aqueous solution. --Leiem (talk) 06:37, 17 August 2021 (UTC)

Solvation and pH
Electrochemical potentials vary significantly on pH, especially the case where an ionic aquo species (-OH or H3O+) is part of the equation. Solvation in water also affected by pH.

This article glosses over Anion Solvation. Sort of important in e.g. Battery chemistry or Electroplating that Anion mobility plays a part. TaylorLeem (talk) 01:33, 29 December 2021 (UTC)

A new review from this year
Here. Maybe some updates should follow. Double sharp (talk) 15:28, 28 November 2022 (UTC)
 * ✅ Double sharp (talk) 17:07, 28 November 2022 (UTC)