Olefin metathesis has had a large impact on modern organic chemistry, but important shortcomings remain: for example, the lack of efficient processes that can be used to generate acyclic alkenyl halides. Halo-substituted ruthenium carbene complexes decompose rapidly or deliver low activity and/or minimal stereoselectivity, and our understanding of the corresponding high-oxidation-state systems is limited. Here we show that previously unknown halo-substituted molybdenum alkylidene species are exceptionally reactive and are able to participate in high-yielding olefin metathesis reactions that afford acyclic 1,2-disubstituted Z-Alkenyl halides.

See the recent publication by Richard Schrock and Amir Hoveyda:
Direct Synthesis of Z-Alkenyl Halides through Catalytic Cross-Metathesis

(M. J. Koh, T. T. Nguyen, H. Zhang, R. R. Schrock, A. H. Hoveyda, Nature, 2016, 531, 459.)