This thesis details the synthesis and characterisation of alkaline earth metal complexes of phosphonium ylides and iminophosphoranes, together with some related phosphine oxide species. This thesis also contains the synthesis and structural characterisation of a number of novel phosphonium ylides and iminophosphoranes. Chapter 1 provides an introduction to the field of phosphonium ylides and related compounds, describing their preparation and subsequent reactivity. Chapter 2 contains an explanation of the experimental techniques and analytical tools used, whilst Chapter 3 details the syntheses and analyses for all of the complexes discussed in the thesis. Chapter 4 discusses novel alkaline earth metal phosphonium ylide complexes. The chapter contains the first structurally characterised neutral phosphonium ylide complexes of calcium, strontium and barium with the general formula (Ph(_3)PCH(_2))(_2) M[N(SiMe(_3))(_2)](_2), where M = Ca, Sr and Ba. The analogous iminophosphorane and phosphine oxide complexes are discussed in detail in chapter 5. Complexes of the general formula (Ph(_3)PNH)(_2) M[0C(_6)H2(Me)(^1)Bu(_2)](_2),(Ph(_3)P0)(_2) M[0C(_6)H(_2)(Me)’Bu(_2)](_2) and (Ph(_3)PO)(_2) M[N(SiMe(_3))(_2)](_2), are described where M =Ca, Sr and Ba. Chapter 6 details the synthesis of novel litbiated iminophosphoranes and related compounds as well as the synthesis of a novel dianion [Ph(_2)(PhCH')PN"] for use in synthetic organic chemistry. The final Chapter (7) details the synthesis and structural characterisation of a number of novel phosphonium ylides Ph(_2)R'PCHR" and iminophosphoranes Ph(_2)R’’’PNH and their corresponding salts e.g Ph(_2)R'PTH(_2)R"X and Ph(_2)R 'P+NH(_2)X .