Question comparing # of d-electrons in central atom (transition metal) from TBR Gen Chem book

[arc5005]

Which of the following pairs of molecules does not have the central atom (transition metal) with the same number of d-electrons in both compounds?

A) Fe(NH3)63+ and FeCl63-
B) Co(H2O)63+ and MnCl64-
C) Cr(NH3)63+ and MoCl63-
D) Os(NH3)42+ and RhCl3(Pr3)3

Answer & their reasoning:
B) Co(H2O)63+ and MnCl64-

To determine the # of electrons on the central metal, the formal charge of the metal first must be determined. From the charge, the electronic configuration is found, so the d-electron count is found.

Choice A is eliminated:
Fe carries a +3 charge in both Fe(NH3)63+ and FeCl63-. Is this actually true? I'm really confused by this one.

Choice B is correct:
Co carries a +3 charge in Co(H2O)63+, and Mn carries a +2 charge in MnCl64-. Neutral Co is [Ar]4s23d7, so Co3+ has a d-electron count of 6 (3d6). Neutral Mn is [Ar]4s23d5, so Mn2+ has a d-electron count of 5 (3d5).

Choice C is eliminated:
Cr carries a +3 charge in Cr(NH3)63+, and Mo carries a +3 charge in MoCl63-. Neutral Cr is [Ar]4s13d5, so Cr3+ has a d-electron count of 3 (3d3). Neutral Mo is [Kr]5s14d5, so Mo3+ has a d-electron count of 3 (4d3). The d-electron counts are equal.

Choice D is eliminated:
Os carries a +2 charge in Os(NH3)42, and Rh carries a +3 charge in RhCl3(Pr3)3. Neutral Os is [Xe]6s24f145d6, so Os2+ has a d-electron count of 6 (5d6). Neutral Rh is [Kr]5s24d7, so Rh3+ has a d-electron count of 6 (4d6).

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[Pulling17]

Im no expert, but yes both have +3 ox state for option A. I think theres many ways of figuring out why Fe(NH3)6 3+ has a Fe(III) but the way I think of it is that you can just simply calculate the oxidation state of the Fe, or since we know that NH3 exists as a neutral molecule, then the charge of Fe must be +3 to get have a net charge of 3+ on the whole compound. I hope that was somewhat of a decent explanation to that

 

[arc5005]

Thank you. I think just that little bit helped me realize what was going on in this problem, and now I understand it.

EXCEPT...I do not remember anything about how/what this looks like: RhCl3(PR3)3.

 

[aldol16]

Fe carries a +3 charge in both Fe(NH3)63+ and FeCl63-. Is this actually true? I'm really confused by this one.

NH3 is a neutral ligand - therefore, if the whole complex is 3+, then you know that it must be the iron that was oxidized by 3 electrons. Similarly, Cl- has a -1 charge so just counting Cl-, you would expect -6 overall. Since it's -3, you know that another component of the complex must be contributing +3 and the only other component is Fe.

 

[arc5005]

Is Pr3 a neutral ligand????

 

[aldol16]

Is Pr3 a neutral ligand????

What is the structure of "Pr3"? That could be a few things and it's ambiguous what it's referring to.