The catalytic activity of a new family of aluminum bis(iminophosphorano)methanide and methandiide complexes (1-5) toward olefin polymerization has been evaluated. The complexes (1-3) are monomeric methanide complexes of the form [kappaN,kappaN'-{Me3SiN=P(Ph)(2))(2)CH}Al(Cl)R] (R = Et) (1) or [kappaNkappaN' -{ (Me3SiN=P(Ph)(2))(2)CH}AlR2} (R = Me, Bu) (2, 3) which have a structure reminiscent of R2Al(acac) complexes. These systems are moderately effective as transition metal-free ethylene polymerization catalysts when activated by trityl tetrafluoroborate yielding slurry polymerization activities in the range of 3-68 g polymer/(mmol Al h) (normalized to 1 atm) under normal industrial conditions. A remarkable bis(iminophosphorano)methandiide complex [kappaC,kappaC,kappaN,kappaN' -{Me3SiN=P(Ph)(2))(2)C}(R2Al)(2)] (R = Me) (4) which is based on a spirocyclic carbon center subtended by two AlMe2 units (each of which is further coordinated with an imine group) gave, under similar catalytic activation and reaction conditions, very high polymerization activity (2 x 103 g polymer/(mmol Al h)) for ethylene. Solution polymerization activity was also high. This system also co-polymerized ethylene and 1-octene. The analog 5 wherein R = Bu was not so effective. These latter complexes indicate that dialuminum species may provide effective transition metal-free olefin polymerization systems. (C) 2002 Elsevier Science B.V. All rights reserved.