text: The halides of low-valent early metals often are clusters with extensive M-M bonding. The situation contrasts with the higher halides of these metals and virtually all halides of the late transition metals, where metal-halide bonding is replete.
  Transition metal halide clusters are prevalent for the heavier metals: Zr, Hf, Nb, Ta, Mo, W, and Re.  For the earliest metals Zr and Hf, interstitial carbide ligands are also common.  One example is ZrCCl. One structure type features six terminal halides and 12 edge-bridging halides.  This motif is exemplified by tungsten(III) chloride,  [TaCl],   Another common structure has six terminal halides and 8 bridging halides, e.g. MoCl.
  Many of the early metal clusters can only be prepared when they incorporate interstitial atoms.
  In terms of history, Linus Pauling showed that "MoCl" consisted of Mo octahedra. F. Albert Cotton established that "ReCl" in fact features subunits of the cluster ReCl, which could be converted to a host of adducts without breaking the Re-Re bonds.  Because this compound is diamagnetic and not paramagnetic the rhenium bonds are double bonds and not single bonds. In the solid state further bridging occurs between neighbours and when this compound is dissolved in hydrochloric acid a ReCl complex forms. An example of a tetranuclear complex is hexadecamethoxytetratungsten W(OCH) with tungsten single bonds. A related group of clusters with the general formula MMoX such as PbMoS.  These sulfido clusters are called Chevrel phases.