Four coordinate molybdenum alkene complexes of the type Mo(NR) 2 (PMe 3 ) (CH 2 CH 2 R ) [R = Bu , R = H (2a) ; R = Me (2b) ; R = adamantyl, R = H (2c)] have been synthesised by treatment of Mo(NR) 2 Cl 2 (dme) [R = Bu (1a); R = adamantyl (1b) with two equivalents of RMgCl (R = Et, n-Pr) in diethylether. The molecular structure of2b is reported [triclinic, space group Pl, a = 9.312(2), b = 9.723(2), c = 11.203(2) , α = 78.85(2), β = 88.23(2), γ = 85.94(2)°, U = 992.5(4) 3 , Z = 2, T = 293 K]. The alkyne derivatives Mo(NR) 2 (PMe 3 )(R C CR ) [R = Bu t , R = R = Ph (3a); R = Bu t , R = Ph, R = H (3b); R = adamantyl, R = R = Ph (3c)] are prepared from 2a and 2c by prolonged warming at 60-70°C in heptane in the presence of one equivalent of the alkyne. The molecular structure of 3a [orthorhombic, space group Pbca (No. 61), a = 13.980(3),b = 15.333(6), c = 24.989(7) , U = 5356(3) 3 , Z = 8, T = 120 K] reveals an analogous pseudo-tetrahedral geometry to that found for 2b. The relationship of these complexes to known zirconocene derivatives is discussed.