Oxidation number

[deleted799625]

The oxidation and coordination numbers of cobalt in the compound [Co(en)2(ox)]Cl2 are respectively
2,3
2,5
3,6
4,8
None of the above

I'm getting oxidation number = 4 and coordination number = 6 but I am not sure about the oxidation number :/ I've searched on google but all the oxidation numbers for Co are 2 and 3 only


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[BerkReviewTeach]

Ethylenediamine is a neutral bidentate ligand and chloride is a negative mono dentate ligand. So two ens and two chlorides equates to a coordination number of 6 (as you stated), but cobalt needs to be +2 in terms of formal charge for the overall compound to be neutral. I believe the author of the question was probably referring to the formal charge on cobalt as being +2.

 

[aldol16]

Ethylenediamine is a neutral bidentate ligand and chloride is a negative mono dentate ligand. So two ens and two chlorides equates to a coordination number of 6 (as you stated), but cobalt needs to be +2 in terms of formal charge for the overall compound to be neutral. I believe the author of the question was probably referring to the formal charge on cobalt as being +2.

Oxidation state is also +2 in this case. In organometallic chemistry, we refer to chlorides as "X-type" ligands and ethylenediamine as two "L-type" ligands. X-type ligands formally oxidize the metal center by 1 unit, as per convention. In fact, you can do an EPR experiment on Co(II) compounds and see that it's actually oxidized by two units.

 

[BerkReviewTeach]

Oxidation state is also +2 in this case. In organometallic chemistry, we refer to chlorides as "X-type" ligands and ethylenediamine as two "L-type" ligands. X-type ligands formally oxidize the metal center by 1 unit, as per convention. In fact, you can do an EPR experiment on Co(II) compounds and see that it's actually oxidized by two units.

In organometallic chemistry, as well as inorganic chemistry, it is far more common to say Cobalt is d7. Your ESR (EPR) results can confirm high-spin versus low-spin as well as a formal charge of +2 versus +3. But it is safe to say that this is beyond the scope of the MCAT. While the term oxidation state applies, it is not the standard jargon of chemists, so I would tend to say that this question should be retooled. Not to mention having five choices, one of which is "none of the above", makes it unrealistic practice for the MCAT.

 

[deleted799625]

In organometallic chemistry, as well as inorganic chemistry, it is far more common to say Cobalt is d7. Your ESR (EPR) results can confirm high-spin versus low-spin as well as a formal charge of +2 versus +3. But it is safe to say that this is beyond the scope of the MCAT. While the term oxidation state applies, it is not the standard jargon of chemists, so I would tend to say that this question should be retooled. Not to mention having five choices, one of which is "none of the above", makes it unrealistic practice for the MCAT.

Doesn't it have charge of +2 since its Cl2? Why is it neutral?

 

[Altius Premier Tutor]

Question #38 on AAMC PE2 asks about both coordination number and oxidation number and gives one a good insight into how the AAMC authors approach coordination complexes. I think this is important for students to keep clear because you'll see more of it on the new exam form; given that heme, many enzyme active sites, etc., include coordinated metal ions.

 

[aldol16]

In organometallic chemistry, as well as inorganic chemistry, it is far more common to say Cobalt is d7. Your ESR (EPR) results can confirm high-spin versus low-spin as well as a formal charge of +2 versus +3. But it is safe to say that this is beyond the scope of the MCAT. While the term oxidation state applies, it is not the standard jargon of chemists, so I would tend to say that this question should be retooled. Not to mention having five choices, one of which is "none of the above", makes it unrealistic practice for the MCAT.

We actually never refer to Co as d7 because in all coordination complexes, the s-orbitals are used for ligation of the Lewis base-like ligands - they provide better overlap than d-orbitals. So the two s-electrons are "bumped up" into the d-shell and we term cobalt as a d9 metal. This, of course, is beyond the scope of the MCAT, but I assure you as a PhD chemist in organometallics, we do not refer to Co as a d7 metal. As a direct example, see: High-resolution EPR spectroscopic investigations of a homologous set of d9-cobalt(0), d9-rhodium(0), and d9-iridium(0) complexes. - PubMed - NCBI. Perhaps you are referring to Co(II)? In that case, I would agree that it's d7. We also refer to "oxidation state" and not "formal charge." If you read most papers in organometallics, we will refer to metals complexed with various ligands with oxidation states - I do not see formal charge mentioned much at all.

In fact, if you are interested in how we determine oxidation state of compounds like this, we use either ionic ligand formalism or neutral ligand formalism. Each determines, again, the oxidation state. See here: The Organometallic HyperTextBook: Electron Counting. This link also provides a good refresher on electron counting since the "s" electrons are really bumped up to the d-shell in metal complexes - this would be a good resource for you and for interested students.

EPR also would not detect Co(III) in the low-spin state.

 

[aldol16]

Doesn't it have charge of +2 since its Cl2? Why is it neutral?

The cobalt-ethylenediamine complex has a +2 charge because it has to balance out the two chlorides so that the overall complex (including the chlorides) is neutral.

 

[rabbott1971]

Aldol16, I'm relieved to see you say you have a phD in chemistry because now I don't feel as bad for barely being able to follow your train of thought.

 

[aldol16]

Aldol16, I'm relieved to see you say you have a phD in chemistry because now I don't feel as bad for barely being able to follow your train of thought.

Ah, I apologize - my explanation was directed at the other poster, because I believe he or she incorrectly asserted that we organometallic chemists refer to Co as d7 (we do not) and that we talk about formal charge instead of oxidation state (we also do not - at least of all the organometallics papers I read).

For a general chemistry student, a good way to go about solving this problem is to look first at ligands whose charges you know, like chloride. For your purposes, chloride will always be negatively charged. This means there are two negative charges that have to be balanced by the rest of the complex (since overall, it's neutral). That means the cobalt + "en" ligands must have a net positive 2 charge. Then you need to know that en is neutral (perhaps this part is in the passage since most general chemistry students shouldn't be expected to know that). If en is neutral, then the +2 must reside solely on the metal center. Therefore, in order to get to a +2 state, it's oxidized by two electrons.

In organometallics, we actually have different ways of figuring out the oxidation state (with the same result). These are known as ionic ligand formalism or neutral ligand formalism. If you're interested, here's a link: The Organometallic HyperTextBook: Electron Counting

 

[rabbott1971]

Never apologize for unlocking boss level at chemistry, my friend. I'm sure you worked hard to get there. Thanks for the analysis.

 

[BerkReviewTeach]

Aldol, I appreciate this being cordial (thank you) and engaging. I believe there is a generation gap (or partial one) at work here. You are not the only one to have been through graduate school in organometallic chemistry. It's been a while, but I'm confident eg, t2g, d7, high-spin, low-spin, etc... all still exist in the terminology. Having just looked through Cotton and Wilkinson (the standard for graduate level inorganic chemistry back in the late 90s), the terms d5, d6, d7, etc... are commonplace. It is the standard vernacular of inorganic chemistry.

I clicked on the link you had for the paper on planar vs. distorted planar vs. tetrahedral coordination about the central metals using bulky ligands in tetradentate species. I assume that 2002 paper has it's roots in the trans-effect papers that preceded it by ten to fifteen years.

As for low spin d6 cobalt, (III state), ESR would confirm it being diamagnetic, thus distinguishing it from paramagnetic versions of cobalt (such as high spin d6, and all d7s). That is my point in the comparison before. While spectroscopy is most often used to confirm the existence of a structural feature, it can also be applied to confirm the absence of a feature.

I would greatly recommend you get a copy of Cotton and Wilkinson if you don't currently have it, as it is an amazing desk reference. Tenable-Sugano diagrams, d-splitting, and ESR spectroscopy are extensive and extremely clear in that book. It's a great book (hence the nickname "the bible of inorganic chemistry").

I don't think there's anything worth further exchange in this thread, so I'm tapping out. I appreciate your insights. I look forward to crossing paths in future threads.

 

[aldol16]

Aldol, I appreciate this being cordial (thank you) and engaging. I believe there is a generation gap (or partial one) at work here. You are not the only one to have been through graduate school in organometallic chemistry. It's been a while, but I'm confident eg, t2g, d7, high-spin, low-spin, etc... all still exist in the terminology. Having just looked through Cotton and Wilkinson (the standard for graduate level inorganic chemistry back in the late 90s), the terms d5, d6, d7, etc... are commonplace. It is the standard vernacular of inorganic chemistry.

I clicked on the link you had for the paper on planar vs. distorted planar vs. tetrahedral coordination about the central metals using bulky ligands in tetradentate species. I assume that 2002 paper has it's roots in the trans-effect papers that preceded it by ten to fifteen years.

I think you misunderstand my point. I am not asserting that the terminology you use is wrong - I am asserting that you have applied it incorrectly to cobalt. Co(0), which presumably is what we were talking about, is not d7 - it's d9. That's my point. I'm not sure what counterpoint you're trying to make here. The paper I cite as well as the link I posted was to make the point that we chemists refer to Co(0) as a d9 complex, not d7. This has been the standard in organometallics since the very beginning - never has Co(0) been referred to as d7 because organometallic chemists have always understood that the energy of the d-orbitals relative to the s-orbitals changes upon ligation.

Again, I'm not sure why you bring up the trans-effect - this is beyond the scope of what undergraduate chemistry students need to know. That paper was solely to illustrate how to apply electron counting as we do it in organometallics - as we've done it for decades. Cobalt(0) is a d9 complex. That's it.

Since you correctly identify Co(III) as a d6 complex, then I must assume that you erroneously identified cobalt as a d7 complex above: "In organometallic chemistry, as well as inorganic chemistry, it is far more common to say Cobalt is d7." Co(II) may be d7 but elemental cobalt is d9.

I would greatly recommend you get a copy of Cotton and Wilkinson if you don't currently have it, as it is an amazing desk reference. Tenable-Sugano diagrams, d-splitting, and ESR spectroscopy are extensive and extremely clear in that book. It's a great book (hence the nickname "the bible of inorganic chemistry").

For something as simple as electron counting, I prefer pointing students towards Shriver & Atkins' Inorganic Chemistry. Some educators don't like it, but I find it gets the job done in the right, concise way.

 

[BerkReviewTeach]

The original question posed by the OP is a Co(II) complex, making it d7. At the time of my first post, no one had mentioned anything about Co(0), only Co2+. That is why I mentioned d7 to describe Co2+ in my post where I quoted you ("Oxidation state is also +2 in this case. In organometallic chemistry, we refer to chlorides as "X-type" ligands and ethylenediamine as two "L-type" ligands. X-type ligands formally oxidize the metal center by 1 unit, as per convention. In fact, you can do an EPR experiment on Co(II) compounds and see that it's actually oxidized by two units."). You introduced Co(0) complexes after that saying they are not d7, which is very true but was irrelevant to the thread at that point. We had been talking about Co(II) at that time, so I described as d7. But I am very glad you linked that paper because Co(0) complexes (as well as Ir(0), and so on) were not hot research topics when I was in the lab. So it was interesting to see the path research took. When I left lab, although I very much enjoyed my time there, I never looked back. So in terms of research trends, I'm out of date.

As for the trans-effect, my point was directly to you and has nothing to do with the OP's question. I was simply saying I enjoyed the paper you posted with Co(0) complexes and I believe that such a research study was the natural progression in the research march from the trans-effect associated with square planar complexes, which was a popular topic in my research days.

 

[aldol16]

The original question posed by the OP is a Co(II) complex, making it d7. At the time of my first post, no one had mentioned anything about Co(0), only Co2+. That is why I mentioned d7 to describe Co2+ in my post where I quoted you ("Oxidation state is also +2 in this case. In organometallic chemistry, we refer to chlorides as "X-type" ligands and ethylenediamine as two "L-type" ligands. X-type ligands formally oxidize the metal center by 1 unit, as per convention. In fact, you can do an EPR experiment on Co(II) compounds and see that it's actually oxidized by two units."). You introduced Co(0) complexes after that saying they are not d7, which is very true but was irrelevant to the thread at that point. We had been talking about Co(II) at that time, so I described as d7. But I am very glad you linked that paper because Co(0) complexes (as well as Ir(0), and so on) were not hot research topics when I was in the lab. So it was interesting to see the path research took. When I left lab, although I very much enjoyed my time there, I never looked back. So in terms of research trends, I'm out of date.

Ah, I see my confusion then. I took your statement in isolation:

In organometallic chemistry, as well as inorganic chemistry, it is far more common to say Cobalt is d7.

Cobalt(II) is d7, cobalt itself is d9. Cleared up. I am curious as to what you mean that it's "more common" to say cobalt(II) is d7. "More common" relative to what? d7 and Co(II) are the same thing - both terms imply oxidation of cobalt metal (which is d9), by two units.

As for the trans-effect, my point was directly to you and has nothing to do with the OP's question. I was simply saying I enjoyed the paper you posted with Co(0) complexes and I believe that such a research study was the natural progression in the research march from the trans-effect associated with square planar complexes, which was a popular topic in my research days.

Ah, I see. We don't worry as much about trans influence/effect nowadays - we do invoke it to explain certain phenomena but we don't do the kind of detailed kinetic measurements that groups like Crabtree's used to do (at least not in my part of organometallics).