The cationic mononuclear h 3 -allenyl/propargyl complexes exhibit new non-classical organometallic bonding of s,p-mode and remarkable activity of potent carbon electrophiles. The organic moieties of the title species, general formula R%R¦CCCR, use all three adjacent unsaturated carbons to bond with metal in a unique coplanar manner. The C 3 skeletons thus are severely distorted from the normal linear configuration to 150 9 5°. The R%R¦C C distances are in the range of long double bonds; whereas C CR are between double and triple bonds, indicating the resonance structures of allenyl and propargyl. The cationic complexes with h 3 -allenyl/propargyl ligands can undergo feasible reactions of regioselective nucleophilic addition which provide a pragmatic synthetic access to new organometallic species of central-carbon-substituted h 3 -allyl, h 3 -oxa-and h 3 -aza-trimethylenemethane. The broad spectrum of C Nu bond formation encompasses hydroxylation, alkoxylation, phenoxylation, acyloxylation, thioxylation, thio-and seleno-phenoxylation, amination, amidation, phosphination, etc. As to carbation, the h 3 -allenyl/propargyl complexes react with carbanions to form h 3 -trimethylenemethane complexes. The electrophilicity of h 3 -C 3 H 3 allows us to conduct regioselective carbon carbon coupling with the electron-rich aromatics to achieve aromatic electrophilic substitution. Although both h 3 -allenyl/propargyl and h 1 -allenyl complexes are essentially subject to similar nucleophilic addition, the former are distinctly more reactive and versatile. From the mechanistic viewpoint, the cationic h 3 -allenyl/propargyl complexes prefer to undergo external nucleophilic attack at the central carbon. In contrast, nucleophilic addition to the h 1 -allenyl complexes requires preceding coordination of the nucleophile to a metal.