The reactions of a phosphavinyl Grignard reagent, [CyP C(Bu t )MgCl(OEt 2 )] Cy=cyclohexyl, with a variety of main group 13, 14 and 16 mono-halide compounds have been investigated. When the Grignard reagent is reacted with bromocatecholborane the terminal phosphavinyl complex, [(C 6 H 4 O 2 )B{C(Bu t ) PCy}], is formed. Related terminal phosphavinyl tin and gallium complexes, [R 3 Sn{C(Bu t ) PCy}], R=Me or Bu n and [IGa{C(Bu t ) PCy} 2 ], have been prepared by similar routes. The reaction of the Grignard reagent with PhSeCl has afforded a new λ 5 ,λ 5 -diphosphete, [{(Bu t )CP(Cy)(SePh)} 2 ], the mechanism of formation of which is discussed. The preparation of a phosphavinyl selenium compound, [(C 8 H 4 O 2 N)P C(Bu t )(SePh)], is also described. All compounds have been spectroscopically characterised and several have been crystallographically authenticated.