This thesis details the synthesis and characterisation of s-block metal (lithium, sodium and magnesium) phosphonium ylide, R(_3)PCHR', and iminophosphorane complexes, R(_3)PNR', together with some related phosphine oxide, R(_3)PO, and sulfide, R(_3)PS, species, where R= Ph or Me(_2)N and R' = H, Me or Ph. The first three chapters of the thesis provide an introduction to the topic, with details of the experimental methods employed and results obtained. Chapter 4 presents a systematic study of Lewis base complexed s-block metal aryloxides, (ArOM L)(_2) where Ar = C(_6)H(_3)Ph(_2) or MeC(_6)H(_2)((^1)Bu)(_2), L = R(_3)PCHR', R(_3)PNR', R(_3)PO or R(_3)PS, and M = Li, Na, and (ArO)(_2)Mg(L)(_2) where Ar = MeC(_6)H(_2)(^1)Bu)(_2) and L = R(_3)PCHR’ or R(_3)PNR'. This includes a discussion of the single crystal XRD structures of the eight neutral ligands used, L, and ten new complexes containing either lithium or sodium. Chapter 5 describes the application of some related s-block metal ylide complexes, e.g. Ph(_3)PCHMe-LiN(CH(_2)Ph(_2))(_2) and Ph(_3)PCHMe-NaN(SiMe(_3))(_2), to the Wittig reaction, together with some solution-state and solid-state discussions (two single crystal XRD structures) of s-block metal amide-phosphonium ylide complexes, R(_3)PCHR'.MNR'(_2) where R = Ph or Me(_2)N, R’ = H or Me, and R" = SiMe(_3) or CH(_2)Ph Chapter 6 details a range of N-s-block metallated iminophosphorane complexes (e.g. Ph(_3)PNLi-LiBr.2thf and R(_3)PNMgX-L where R = Ph or Me(_2)N and L = Lewis base) their application to transmetallation reactions with copper(I) compounds, solution-state and solid-state NMR studies and six new single- crystal XRD structures. Chapter 7 describes some very recent work involving alkyldiphenylphosphonium ylides, MePh(_2)PCHPh, and imines, MePh(_2)PNPh, including a single crystal XRD structural study of the latter and three of its lithium derivatives, e.g. [CH(_2)LiPh(_2)PNPh](_4). Chapter 8 is concerned with a number of unexpected results, including the synthesis, solid-state structure and proposed mechanism of a novel A^-phosphino- iminophosphorane, Ph(_2)(C(_3)H(_4)Ph)PNP(C(_3)H(_5))Ph, and two aminophosphonium salts, [R(_3)PNH(_2)](^+)[OC(_6)H(_3)Ph(_2)]" where R = Ph or Me(_2)N.