New Group 3 metal complexes of the type [LM(CH2SiMe3)(2)(THF)(n)] supported by tridentate phosphido-diphosphine ligands [ (o-C6H4PR2)(2)PH; L1-H: R = iPr; L2-H: R = Ph] have been synthesized by reaction of L1-H and L2-H with [M(CH2SiMe3)(3)(THF)(2))] (M = Y and Sc). All the new complexes [(o-C6H4PR2)(2)PM(CH2SiMe3)(2)(THF)(n)] [M = Y, R = iPr (1), R = Ph (2); M = Sc, R = iPr (3), R = Ph (4)] were studied as initiators for the ring opening polymerization of lactide. The yttrium complexes (1 and 2) exhibited high activity and good polymerization control, shown by the linear fits in the plot of number-averaged molecular weight (M-n) versus the percentage conversion and versus the monomer/initiator ratio and by the low polydispersity index values. Interestingly, very good molar-mass control was observed even when L-Lactide was polymerized in the absence of solvent at 130 degrees C. A good molar-mass control but lower activities were observed in the polymerization reaction of lactide promoted by the analogous scandium complexes 3 and 4. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1374-1382, 2010