High-nuclear 3d metal coordination clusters have received much attention in recent years due to increasing interest in the construction and development of better and more effective materials for potential applications in information storage and processing as well as in molecular spintronics. The reaction of [Mn6O2(O2CCHMe2)10(EtOH)(hmta)(H2O)2]·EtOH (1) (where hmta = hexamethylenetetramine) with Dy(NO3)3·6(H2O) and 2,4,6-tris-(2-pyridyl)-s-triazine led to new hexanuclear [Mn6O2(O2CCHMe2)10 (MeCN)2(H2O)2].(H2O) (2) and decanuclear [Mn10O2(O2CCHMe2)18(H2O)2]·0.33(CH2Cl2) (3) clusters. Single-crystal X-ray diffraction shows that compound 3 crystallizes in the triclinic P-1 space group: a = 13.8258(9), b = 14.0550(9), c = 30.9761(18) Å, α = 87.893(5), β = 82.419(5), γ = 61.726(7)°, V = 5252.6(6) Å3, Z = 2. Cluster 3 (Fig. 1) consists of a [MnII 8MnIII 2O2(O2CCHMe2)18] core that comprises a central hexanuclear [MnII 4MnIII 2(μ 4-O2)]10+ unit. Isobutyrate groups extend this hexanuclear unit connecting four peripheral MnII centres. Water molecules coordinate to two outer MnII centers and complete their coordination sphere. All manganese atoms are six-coordinated and have distorted octahedral surrounding. The MnIII-O bond distances are in the range of 1.872(5) – 2.243(6) Å and the majority of MnII-O bond distances are in the range 2.066(7) – 2.416(6) Å, except two long MnII-O distances 2.549 and 2.673 Å for two peripheral MnII atoms. This long distance is related with oxygen atom of isobutyrate ligand, which coordinates simultaneously to three neighboring MnII atoms.