= Solution
The average Pt <oxidation state> is $+3$, since six <bromides> balance the two <metal> atoms. A natural <mixed valence> assignment is \b[alternating Pt(II), $d^8$, and Pt(IV), $d^6$], rather than two equivalent Pt(III) sites. Use neutral square-planar $\mathrm{Pt^{II}(NH_3)_2Br_2}$ units and octahedral $\mathrm{Pt^{IV}(NH_3)_2Br_4}$ units. At every site place two <ammonia> and two <bromide> ligands in an equatorial plane; the Pt(IV) sites have two additional axial <bromides> pointing toward the adjacent Pt(II) sites.
Along the chain the connectivity is
$$
\cdots\mathrm{Pt^{II}\ \cdots Br{-}Pt^{IV}{-}Br\cdots\ Pt^{II}\ \cdots Br{-}Pt^{IV}{-}Br}\cdots.
$$
Every Pt–Br–Pt bridge is \b[asymmetric: short to Pt(IV), long to Pt(II)]. The latter contact is a secondary axial interaction, consistent with the preference of Pt(II) for a square-planar primary <coordination sphere>. The Pt(IV) site is smaller and more strongly attracts the bridging <bromides>. A drawing with all axial distances identical would obscure the distinct local environments.
There are four equatorial <bromides> and two axial bridging <bromides> per two-Pt repeat, giving precisely $\mathrm{Pt_2(NH_3)_4Br_6}$. A direct Pt–Pt bond is not required. This is a proposed structure consistent with the stated composition and two environments; the formula alone does not determine every equatorial cis/trans arrangement or numerical bond length.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2023/paper-2-coordination.png]
{title=Proposed asymmetric platinum chain and octahedral hydration-isomer connectivities for nickel}
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