The reaction of the tetrameric compound [ Cp ∗ Ru(μ 3- Cl)] 4 ( 1; Cp ∗ = C 5 Me 5) with iPr 2PCH 2CO 2Me gives the monomeric species [ Cp ∗ RuCl{κ 2(P, O)- i Pr 2 PCH 2 CO 2 Me}] ( 2) which is an excellent starting material for the synthesis of neutral half-sandwich type ruthenium complexes containing substituted vinylidenes and allenylidenes as ligands. Compound 2 reacts with HCCCO 2Me to give [ Cp ∗ RuCl( C CHCO 2 Me)( i Pr 2 PCH 2 CO 2 Me)] ( 4), and the corresponding reaction of 2 with HCCCPh 2−OH affords [Cp ∗RuCl(CCHCPh 2OH)( iPr 2PCH 2CO 2Me)] ( 5) which on treatment with acidic alumina yields [Cp ∗RuCl(CCCPh 2)( iPr 2PCH 2PCH 2CO 2Me)] ( 6). In contrast, the reaction of 2 with Me 3SiCCCO 2Et gives, instead of an alkyne or vinylidene ruthenium derivative, the cyclobutadiene complex [ Cp ∗ RuCl{η 4- C 4( SiMe 3) 2( CO 2 Et) 2}] ( 7) which is accessible also from 1 and Me 3SiCCCO 2Et in almost quantitative yield. The crystal and molecular structures of 2 and 4 have been determined.