The species MoCl feature Mo(II) (d) centers.  Six Mo(II) centers gives rise to a total of 24 valence electrons, or 2e/Mo-Mo vector.  More electron-deficient derivatives such as TaCl have fewer d-electrons.  For example, the naked cluster Ta, the core of TaCl would have 5(6) - 14 = 16 valence electrons.  Fewer d-electrons result in weakened M-M bonding and the extended Ta---Ta distances accommodate doubly bridging halides.