A series of Al(III) and Sn(II) diiminophosphinate complexes have been synthesized. Reaction of Ph(ArCH2)P(=NBut)NHBut (Ar = Ph, 3; Ar = 8-quinolyl, 4) with AlR3 (R = Me, Et) gave aluminum complexes [R2Al{(NBut)(2)P(Ph)(CH2Ar)}] (R = Me, Ar = Ph, 5; R = Me, Ar = 8-quinolyl, 6; R = Et, Ar = Ph, 7; R = Et, Ar = quinolyl, 8). Lithiated 3 and 4 were treated with SnCl2 to afford tin(II) complexes [ClSn{(NBut)(2)P(Ph)(CH2Ar)}] (Ar = Ph, 9; Ar = 8-quinolyl, 10). Complex 9 was converted to [(F(Me3Si)(2)NSn{(NBut)(2)P(Ph)(CH2Ph)}] (11) by treatment with LiN(SiMe3)(2). Complex 11 was also obtained by reaction of 3 with [Sn{N(SiMe3)(2)}(2)], Complex 9 reacted with [LiOC6H4But-4] to yield [4-(BuC6H4OSn)-C-t{(NBut)(2)P(Ph)(CH2Ph)}] (12). Compounds 3-12 were characterized by NMR spectroscopy and elemental analysis. The structures of complexes 6, 10, and 11 were further characterized by single crystal X-ray diffraction techniques. The catalytic activity of complexes 5-8, 11, and 12 toward the ring-opening polymerization of e-caprolactone (CL) was studied. In the presence of BzOH, the complexes catalyzed the ring-opening polymerization of E-CL in the activity order of 5 > 7 approximate to 8 > 6 >> 11 > 12, giving polymers with narrow molecular weight distributions. The kinetic studies showed a first-order dependency on the monomer concentration in each case. (c) 2006 Wiley Periodicals, Inc.