Bis(eta(3)-allyl)nickel reacts with pentane-2,4-dione derivatives having alkenyl substituents in the 3-position to give Ni(3-R-pentane-2,4-dionato)(2) compounds which have been shown by X-ray diffraction and NMR spectroscopy to be monomeric [R=5-hexenyl, C22H34NiO4, a=13.713(3) Angstrom, b=4.962(1) Angstrom, c=16.643(3) Angstrom, beta=92.46(3)degrees, (3)degrees, space group P2(1)/n, Z=2; R=7-octenyl, C26H42NiO4, a=4.8449(5) Angstrom, b=8.3202(8) Angstrom, c=16.792(1) Angstrom, alpha=89.480(4)degrees, beta=88.240(4)degrees, gamma=77.30(1)degrees; R=9-decenyl, C30H50NiO4, a=4.8067(2) Angstrom, b=8.2809(3) Angstrom, c=19.4014;(7) Angstrom, alpha=90.974(4)degrees, beta=91.907(4)degrees, gamma=102.495(4)degrees, space group , Z=1; R=methyl, C12H18NiO4, a=4.9447(4) Angstrom, b=8.0649(7) Angstrom, c=9.0633(6) Angstrom, alpha=109.377(6)degrees, beta=93.706(7)degrees, gamma=106.283(7)degrees, space group , Z=1; R=phenylethano, C26H30NiO4, a=12.792(1) Angstrom, b=6.503(1) Angstrom, c=14.933(1) Angstrom, beta=109.025(7)degrees, space group P2(1)/n, Z=2] in the crystal and in solution at room temperature. No inter- or intramolecular interaction of the alkenyl substituent with the 16e metal atom is observed. The corresponding Ni compounds containing the 3-phenyl-substituted dione have been shown to adopt both monomeric [C22H22NiO4, a=10.2261(5) Angstrom, b=6.6439(3) Angstrom, c=13.7862(7) Angstrom, beta=93.677(6)degrees, space group P2(1)/n, Z=2] and trimeric [(C22H22NiO4)(3), a=10.5917(7) Angstrom, b=12.3092(8) Angstrom, c=12.5926(5) Angstrom, alpha=103.937(4)degrees, beta=96.388(3)degrees, gamma=96.227(5)degrees, space group , Z=1] forms in the crystal while the compound formed by tert-butyl acetoacetate [(C16H26NiO6)(3), a=9.713(1) Angstrom, b=17.550(1) Angstrom, c=17.391(1) Angstrom beta=100.272(5)degrees, space group P2(1)/n, Z=2] is a trimer in which the central Ni atom interacts solely with bridging acetyl groups. The crystal structure of 3-phenyl-2,4-pentanedione [C11H12O2, a=19.2961(6) Angstrom, b=6.8561(2) Angstrom, c=7.5935(2) Angstrom,space group Pnma, Z=4] has also been determined.