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A cobalt complex with composition CoBr2Cl * 4NH3 gives two ions per formula unit in solution. Adding silver nitrate solution gives an immediate white precipitate of AgCl but no precipitate with Br-. What is the structural (coordination) formula of this complex?

1. [CoBrCl(NH3)4]Br
2. [CoCl(NH3)4]Br2
3. [CoBr2Cl(NH3)4]
4. [CoBr2(NH3)4]Cl

Correct answer: [CoBr2(NH3)4]Cl

Solution

Two ions per formula unit means the complex dissociates into a complex cation and one counter-ion. Since AgNO3 immediately precipitates Cl- (as AgCl) but not Br-, Cl- is the outer-sphere (free) ion. Both Br- ions must be in the inner coordination sphere (bonded to Co). The complex cation is [CoBr2(NH3)4]+, Co(III) with charge +3 balanced by two Br- (-2) and four NH3 (0) = net +1. The structural formula is [CoBr2(NH3)4]Cl.

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