Reaction of lithium ethynethiolate or ethyneselenolate, LiX¹CCPh (X¹= S or Se), with carbene complexes [M(CO)₅{C(OR¹)Ph}](M = Cr or W; R¹= Me or Et) afforded, by the formation of carbon-carbon bonds, a series of adducts, Li[M(CO)₅{C(OR¹)PhC(Ph)CX¹}]. The adducts reacted regioselectively with electrophiles: acidification with HCl gave α,β-unsaturated thio- and seleno-aldehyde complexes, [M(CO)₅{X¹CHC(Ph)C(OR¹)Ph}], alkylating agents [Et₃O][BF₄] or Mel gave α,β-unsaturated thio-or seleno-carbene complexes, [M(CO)₅{C(X¹R³)-C(Ph)C(OR¹)Ph](X¹R³SEt, SMe, or SeEt), which cleanly and spontaneously converted into sulphur- or selenium-co-ordinated indene derivatives, [M(CO)₅{(X¹R³)CH(C₆H₄-o)-C(OR¹)CPh}](X¹R³SEt or SeEt), and, finally, disulphides R⁴SSR⁴(R⁴Me, Et, or Ph) or the diselenide PhSeSePh yielded thio- or seleno-ester complexes[M(CO)₅{X₁C(X₂R₄)C(Ph)C(OR¹)Ph](X²R⁴SMe, SEt, SPh, or SePh). Four X-ray crystal-structure determinations established the stereochemical configurations of selected members of the various classes of acyclic products. The isomeric compound to one of the thioaldehyde complexes, [W(CO)₅{SC(Ph)CHC(OEt)Ph}], obtained from the adducts with X = S or Se and R¹= Et on acidification in the presence of CS₂ or CSe₂, was also characterized structurally by single-crystal X-ray diffraction.