The monomeric tetrakis(2,4-pentanedionato) complexes of zirconium(IV) and hafnium(IV) have been shown to react in acetone/water (95:5) mixtures with 1,4,7-triazacyclononane (L) affording upon addition of NaClO4 colorless crystals of [{LZr(acac)2}2O](ClO4)2 and [{LHf(acac)2}2O](ClO4)2, respectively. Crystal data for the zirconium species (and in brackets for the corresponding hafnium complex): monoclinic space group: P21/n (P21/n), a=10.731(3) (10.701(5)), b=36.60(1) (36.58(1)), c=11.562(3) (11.552(3)) Å, β=96.13(2)° (96.16(3)°). Each metal(IV) ion is coordinated to two didentate 2,4-pentanedionato ligands and a tridentate macrocyclic amine. Two such units are linked by a bent μ-oxo bridge (Zr(1)OZr(2) 150.4(2)°; Hf(1)O1Hf(2) 151.3(3)°) which completes the eight- coordination at each metal centre. The MOoxo bonds are quite short (ZrOoxo 1.961(4), 1.975(4) Å; HfOoxo 1.950(6), 1.976(6) Å).