What is the structure of [Co(NH3)5SO4]+?

What is the structure of $\ce{[Co(NH3)5SO4]+}$? If sulfate is a bidentate ligand, then the coordination number of cobalt would become 7. How is this possible?

• Are you sure that the complex is uncharged? – permeakra Jan 10 '16 at 11:45
• I am sorry. It has +1 charge. But then also how can the coordination number be 7? – rishabh gupta Jan 10 '16 at 15:16
• It can, but for Co(III) it is unlikely. So, indeed, c.n. is six. what would be your solution for the problem? – permeakra Jan 10 '16 at 15:44

There's a certain amount of misinformation here:

sulfate is a bidentate ligand

Sulfate is actually a flexidentate ligand, which is a term used to describe a polydentate ligand which is found to show different denticity in different complexes. Sulfate can act both as mono- or a bidentate ligand. Remember, just because it has two donor atoms doesn't make it a bidentate ligand.

The occurrence of complexes where sulfate acts as a bidentate ring would also be rare owing to the fact that four-membered rings are strained. This page lists $\ce{[Pt(SO4)(P(C6H5)3)2]}$ as one example of such an occurrence.

coordination number of cobalt would become 7. How is this possible?

If this implies that coordination numbers of 7 aren't possible, it isn't true. Coordination numbers of 3, 5, 7, 8-12, 14 are possible, albeit such compounds are quite rare. See this Wikipedia page for some examples.

Thus, the structure of the complex $\ce{[Co(NH3)5SO4]+}$ should be octahedral with cobalt showing a coordination number of 6.

Interestingly enough, it seems that crystal structure of $\ce{[Co(NH3)5(SO4)]+}$ has not yet been obtained. Probably it has something to do with trans-effect of $\ce{SO4^2-}$, which makes $\ce{NH3}$ ligand extremely labile. Also, C.N. 7 is also very common for cobalt and is often found in complexes like $\ce{[Co^{III}(Cp)L2]}$.

Anyway, to further illustrate Berry Holmes' great answer and show that sulfur oxoanions can not only be flexidentate, but also ambidentate (when denticity is 1), there are some examples how coordination abilities of various sulfur oxoanions deviate from each other in case of pentamminocobalt(III) complexes (only cationic parts are shown):

1. p-(Ammine-(sulfato-O)-tris(2-aminoethyl)amine-cobalt(III)) perchlorate $\ce{[Co(C6H21N5)(SO4)](ClO4)}$, CCDC #CULRIK [1]. $\angle(\ce{Co-O-S}) = 131.2^\circ$, $d(\ce{Co-O}) = \pu{1.958(3) Å}$.
$\color{#EEEEEE}{\Large\bullet}~\ce{H}$; $\color{#909090}{\Large\bullet}~\ce{C}$; $\color{#3050F8}{\Large\bullet}~\ce{N}$; $\color{#FF0D0D}{\Large\bullet}~\ce{O}$; $\color{#FFFF30}{\Large\bullet}~\ce{S}$; $\color{#F090A0}{\Large\bullet}~\ce{Co}$.

2. Pentaammine(sulfito-S)(IV)cobalt(III) chloride hydrate $\ce{[Co(NH3)5SO3](H2O)Cl}$, ICSD #37092 [2]. $\angle(\ce{Co-S-O}) = 110.0^\circ$, $d(\ce{Co-S}) = \pu{2.2138(8) Å}$.
$\color{#EEEEEE}{\Large\bullet}~\ce{H}$; $\color{#3050F8}{\Large\bullet}~\ce{N}$; $\color{#FF0D0D}{\Large\bullet}~\ce{O}$; $\color{#FFFF30}{\Large\bullet}~\ce{S}$; $\color{#F090A0}{\Large\bullet}~\ce{Co}$.

3. Pentaammine(thiosulfato-S)cobalt(III) chloride hydrate $\ce{[Co(NH3)5S2O3](H2O)Cl}$, ICSD #16144 [3]. $\angle(\ce{Co-S-S}) = 110.5^\circ$, $d(\ce{Co-S}) = \pu{2.286(6) Å}$.
$\color{#EEEEEE}{\Large\bullet}~\ce{H}$; $\color{#3050F8}{\Large\bullet}~\ce{N}$; $\color{#FF0D0D}{\Large\bullet}~\ce{O}$; $\color{#FFFF30}{\Large\bullet}~\ce{S}$; $\color{#F090A0}{\Large\bullet}~\ce{Co}$.

References

1. Chun, H.; Jackson, W. G.; McKeon, J. A.; Somoza, F. B.; Bernal, I. Eur. J. Inorg. Chem. 2000, 2000 (1), 189–193 DOI: 10.1002/(SICI)1099-0682(200001)2000:1<189::AID-EJIC189>3.0.CO;2-V.
2. Elder, R. C.; Heeg, M. J.; Payne, M. D.; Trkula, M.; Deutsch, E. Inorg. Chem. 1978, 17 (2), 431–440 DOI: 10.1021/ic50180a048.
3. Baggio, S. J. Chem. Soc. A 1970, 0 (0), 2384–2387 DOI: 10.1039/J19700002384.
• Are you sure that the second compound’s name is accurate? I would assume it to be called -sulfito-, not -sulfonito-. Also, there should be a kappa before the coordinating atom afaicr. – Jan Sep 27 '17 at 10:48
• @Jan sulfito- it is indeed, my bad. Thank you! – andselisk Sep 27 '17 at 10:52