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During lectures, we were told that 3d transition metal fluorides generally prefer a rutile structure. Chlorides prefer a $\ce{CdCl2}$ structure where $\ce{Cl-}$ adopt a CCP lattice and the metal cations occupy octahedral voids within alternate sheets. Bromides and iodides prefer a $\ce{CdI2}$ structure which is the same structure as $\ce{CdCl2}$ but iodide anions form a HCP lattice.

The difference between fluorides and the other halides is rationalised easily by the lack of covalency/directionality within the bonds. However, it is not immediately obvious why larger halides prefer to adopt a HCP lattice while chlorides prefer a CCP lattice.

My guess is that from a pure structure perspective, the metal-metal bond distance on average decreases from rutile > cadmium chloride > cadmium iodide. Comparing rutile and the cadmium salts, each octahedra in rutile is edge sharing with 2 other octahedra but corner sharing with 8 other octahedra while for the cadmium salts, each octahedra is edge sharing with 6 octahedra. The more edge sharing octahedra for the cadmium salts implies the metal-metal bond distances are short which can only be stabilised by covalency within ionic bonding. The ability to stabilise such structures and the directionality of covalent interactions makes is stable for 3d chlorides to iodides to adopt the structure of the cadmium salts. Between cadmium chloride and iodide, the metal ions are stacked on top of each other for the iodide salt resulting in shorter distances. The large size of an iodide salt allows for this structure to happen.

Above is my best guess at what is happening. The problem with the argument is that it does not explain why the cadmium iodide structure is more stable than the cadmium chloride structure for larger halides. Additionally, it does not appear that this small repulsion would have much effect on the relative stability between a CCP and HCP arrangement of anions. From an entropic point of view, there does not appear to be much of a difference in entropy between the two arrangements. I am very confused by this trend in nature and would like some help clearing up this confusion.

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    $\begingroup$ Often it is hard to figure out why some elements are FCC and some others are HCP (or, stranger, DHCP) - the stacking is a little different that is all. But that stacking makes the difference... $\endgroup$
    – Jon Custer
    Commented Dec 5, 2023 at 21:44
  • $\begingroup$ chemistry.stackexchange.com/questions/178137/…\chem{CdCl_2}$-and-$\chem{CdI_2}$ $\endgroup$
    – user142643
    Commented Jan 15 at 7:05
  • $\begingroup$ chemistry.stackexchange.com/questions/178137/… $\endgroup$
    – user142960
    Commented Jan 25 at 13:25
  • $\begingroup$ Dear @JonCuster, You have to edit my comment which I made on this website on Jan 15 at 7:05 AM from its current state itself to chemistry.stackexchange.com/questions/178137/chemistry_of_rutile_$\ce{CdCl_2}$−and− $\ce{CdI_2}$ prior to deleting the recent comments which I am now posting you as that is urgent $\endgroup$
    – user142960
    Commented Jan 25 at 13:40
  • $\begingroup$ @user142960 - I don’t have to do anything, much less make some unclear edit to a question I did not ask. Sorry, not my problem. $\endgroup$
    – Jon Custer
    Commented Jan 26 at 1:42

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