2-Propene- and 2-methyl-2-propeneselenolato complexes of molybdenum(II) and tungsten(II) have been prepared via insertion of gray selenium into alkali-metal-molybdenum and -tungsten bonds and subsequent reaction with allyl and beta-methallyl chlorides; The complexes are monomeric in the solid state. The (beta-methallyl)molybdenum compound cp(CO)(3)MoSeCH2C(CH3)=CH2 (2) decomposes slowly in solution with CO loss and formation of the corresponding dimer [cp(CO)(2)MoSeCH2C(CH3)=CH2](2) (3), whose crystal structure has been determined (C22H24Mo2O4Se2, monoclinic, P2(1)/c, a = 11.151(2) Angstrom, b = 13.436(3) Angstrom, c = 16.872(3) Angstrom, beta = 102.42(2)degrees, Z = 4). A monomeric structure has been found for cp(CO)(3)WSeCH2C(CH3)=CH2 (5) (C12H12O3SeW, triclinic, , a 7.592(3) Angstrom, b = 7.700(3) Angstrom, c = 12.296(5) Angstrom, alpha = 98.31(4)degrees, beta = 104.24(2)degrees, gamma = 108.49(3)degrees, Z = 2). Additionally, the complexing properties of the selenolato complexes have been investigated. The reactions of the molybdenum complexes cp(CO)(3)MoSeCH2CH=CH2 (1) and (2) with the metal(0)-carbonyl complexes W(CO)(5)(THF), (CH3CN)(3)Mo(CO)(3), and (eta(6)-C7H8)Mo(CO)(3) (C7H8 = cycloheptatriene) led to the formation of [cp(CO)(3)Mo(mu-SeCH2C(CH3)=CH2)M(CO)(5)] (M = W (6), Mo (7)) and [cp(CO)(3)Mo(mu-SeCH2CH=CH2)Mo(CO)(5)] (8), respectively. 6 and 7 are isostructural and crystallize in the monoclinic space group P2(1)/n (6, C17H12MoO8SeW, a = 14.708(2) Angstrom, b = 9.837(2) Angstrom, c = 14.958(2) Angstrom, beta = 103.030(10)degrees, Z = 4; 7, C17H12Mo2O8Se, a = 14.712(2) Angstrom, b = 9.8670(10) Angstrom, c = 14.982(2) Angstrom, beta = 103.100(10)degrees, Z = 4), whereas [cp(CO)(3)Mo(mu-SeCH2CH=CH2)Mo(CO)(5)] (8) crystallizes in the noncentrosymmetric orthorhombic space group Pna2(1) (C16H10Mo2O8Se, a = 18.638(4) Angstrom, b = 9.707(2) Angstrom, c = 11.169(3) Angstrom, Z = 4). Furthermore, Se-77-NMR spectra displayed chemical shifts consistent with earlier results on related complexes.