The [Cp'M(NO)₂]BF₄ (M = Cr, Mo or W; Cp' = Cp or Cp*; except CpW) cations effect the condensation of methylpropiolate and 2-methyl-2-butene within their coordination spheres. The isolated product ratios of the reaction vary with the formation of the six-membered ring [Cp'M(NO)₂-C=C(H)C(Me)(H)C(Me)₂OC(=OCH₃)]BF₄ complexes being favoured over the five-membered ring [Cp'M(NO)₂- C=C(H)C(Me)(C(H)(Me)₂)OC(=OCH₃)]BF₄ complexes. Furthermore, the isolated product ratios vary from metal to metal with the percentage of six-membered ring product increasing from Cr < Mo < W. The connectivity extant in the molecular structures of the cationic complexes was determined by single crystal X-ray crystallographic analysis of a representative example of this class, [Cp*Cr(NO)₂-C=C(H)C(Me)(H)C(Me)₂OC(=OCH₃)]BF₄. These cationic lactone complexes may be demethylated by reacting their THF solutions with Nal to afford the neutral lactone complexes, Cp'M(NO)₂-C=C(H)C(Me)(H)C(Me)₂OC(=O) and Cp'M(NO)₂-C=C(H)C(Me)(C(H)(Me)₂)OC(=O). 2-Methyl-2-pentene and methyl propiolate are condensed in the presence of [Cp*Mo(NO)₂]BF₄ to generate the six-membered ring complex as the only isolable product. The condensation of methylpropiolate and diphenylacetylene or 1-phenyl-1-propyne by [Cp*Mo(NO)₂]BF₄ results in an unprecedented formation of the cationic α-pyrone complexes [Cp*Mo(NO)₂-C=C(H)C(R)=C(Ph)OC(=OCH₃)]BF₄ (R = CH₃ or Ph). These complexes may be demethylated to produce the neutral a-pyrone complexes, Cp*Mo(NO)₂-C==C(H)C(R)==C(Ph)OC(=0) (R = CH₃ or Ph). Only one regioisomer is formed with the regiochemistry of the 1-phenyl-1-propyne complex being confirmed by a single crystal X-ray crystallographic analysis of the neutral complex, Cp*Mo(NO)₂-C=C(H)C(CH₃)=C(Ph)OC(=O). / Science, Faculty of / Chemistry, Department of / Graduate