A number of zerovalent palladium and platinum M(NN)(alkene) complexes, containing the rigid bidentate nitrogen ligands bis(arylimino)acenaphthene (Ar-BIAN) and bis(phenylimino)camphane (Ph-BIC) have been synthesized and characterized. Stable complexes were obtained with electron poor alkenes, such as dimethyl fumarate, fumaronitrile, maleic anhydride, and tetracyanoethylene. Complexes bearing asymmetric Ar-BIAN or Ph-BIC ligands occurred as mixtures of isomers. From IR, UV, and H-1 and C-13 NMR spectroscopy and from substitution reactions it was concluded that the back-donation of electron density from the metal to the alkene is the major factor determining the stability of the complexes. Some of the complexes show fluxional behavior on the NMR time scale, which was explained by rotation of the alkene around the metal-alkene bond. The estimated rotation barriers of 50-69 kJ/mol for Pd and >72 kJ/mol for Pt are high as compared to reported values for other palladium and platinum alkene complexes. From a study of alkene substitution reactions of Pd(Ar-BIAN)(alkene) complexes it appeared that alkene substitution occurs via a fast associative mechanism for complexes containing Ph-BIAN or p-Tol-BIAN ligands and via a slower dissociative mechanism for complexes containing o,o’-i-Pr2C6H3-BIAN. Substitution of the p-Tol-BIAN ligand in Pd(p-Tol-BIAN)(alkene) by o,o’-i-Pr2C6H3-BIAN is also observed and is explained by initial dissociation of the alkene. Whereas the spectroscopic and crystal data point to M(II)-metallacyclopropane type structures, there activity points rather to M(O)-alkene type complexes. The X-ray crystal structure of Pd(o,o’-i-Pr2C6H3-BIAN)(maleic anhydride) has been determined. Crystals are triclinic, space group P1BAR, with a = 10.9931(9) angstrom, b = 11.7313(5) angstrom, c = 14.7906(13) angstrom, alpha = 86.276(5)degrees, beta = 81.936(7)degrees, gamma = 71.273(5)degrees, Z = 2, and final R = 0.055 for 3702 reflections with I > 2.5sigma(I).