Two contrasting ethynyl hydroboration pathways in the formation of a novel tris-hydroboration product from reaction of dimesitylborane with 2,5-diethynylpyridineElectronic supplementary information (ESI) available: NMR data for 4, rotatable 3-D molecular structure diagrams of optimised geometries in CHIME format and energy data for optimised geometries. See http://www.rsc.org/suppdata/cc/b3/b316250h/