= Solution
\b[Solid $\mathrm{MoCl_2}$ contains octahedral $\mathrm{Mo_6}$ clusters, while $\alpha$-$\mathrm{MoCl_3}$ has paired <molybdenum> centres in a layered <chloride> lattice.] The first material is better represented by $\mathrm{Mo_6Cl_{12}}$ than by isolated bent or linear $\mathrm{MoCl_2}$ molecules. Eight inner chlorides cap the triangular faces of an octahedral metal core, giving $[\mathrm{Mo_6(\mu_3-Cl)_8}]^{4+}$. Outer <chloride> coordination completes the solid: one useful connectivity notation is $\mathrm{Mo_6Cl_8Cl_2Cl_{4/2}}$, with two terminal outer chlorides and four intercluster-shared outer chlorides per octahedron. The empirical total is $8+2+4/2=12$ chlorides for six metals.
Each Mo(II) is formally $d^4$, giving 24 metal-based electrons. These support extensive Mo–Mo bonding along the 12 octahedral edges; they are not localized as four free spins on each <molybdenum>. In $\alpha$-$\mathrm{MoCl_3}$, Mo(III) is $d^3$ in edge-sharing $\mathrm{MoCl_6}$ octahedral layers. The metal honeycomb distorts into short Mo–Mo pairs. A localized dimer picture uses the six d electrons to fill one sigma and two pi bonding combinations, producing a formal triple bond and pairing the spins. The bulk crystal remains an extended <chloride> lattice, rather than a collection of unconnected $\mathrm{Mo_2Cl_6}$ molecules.
By comparison, $\mathrm{CrCl_2}$ has predominantly localized high-spin Cr(II), $d^4$, in Jahn–Teller-distorted <chloride> octahedra; $\mathrm{CrCl_3}$ has layered Cr(III), $d^3$, with three locally unpaired electrons. Their magnetic exchange is not evidence for the same strong short multiple bonds as in the <molybdenum> solids. The greater radial extent of Mo $4d$ orbitals enables larger metal–metal overlap than the more contracted Cr $3d$ orbitals. Real structure and magnetism still depend on ligand geometry and phase, so the orbital comparison is a trend rather than a prohibition of <chromium>–<chromium> bonds in other compounds. Diffraction and magnetic/spectroscopic studies directly distinguish the dimerized $\alpha$-$\mathrm{MoCl_3}$ lattice from $\mathrm{CrCl_3}$.<Structural and Magnetic Transitions Caused by Dimer Formation in the CrCl3–MoCl3 Solid Solution by Samuel Froeschke and others>[Introduction and spectroscopic discussion: paired Mo centres, localized Cr spins and diffraction/magnetic evidence]{ref}
\Image[/past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-1-molybdenum-chlorides.png]
{title=Original Mo6 octahedral-core and paired-layer schematics; selected chloride connectivity is explained in the text}
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