A new family of five-coordinate lanthanide single-molecule magnets (Ln SMMs) [Dy(Mes*O) 2 (THF) 2 X] (Mes* = 2,4,6-tri-tert-butylphenyl; X= Cl, 1; Br, 2; I, 3) is reported with energy barriers to magnetic reversal > 1200 K. The five-coordinate Dy III ions have distorted square pyra-midal geometries, with halide anions on the apex, and two Mes*O ligands mutually trans-to each other, and the two THF molecules forming the second trans-pair. These geometrical features lead to a large magnetic anisotropy in these complexes along the trans-Mes*O direction. QTM and Raman relaxation times are enhanced by varying the apex halide from Cl to Br to I, or by dilution in a diamag-netic yttrium analogue. Single-molecule magnets (SMMs) show slow relaxation of mag-netisation under certain conditions; these have received huge interest in the last 25 years due to their potential applications in quantum computing, molecular spintronics and ultra-high-density storage. [1, 2] For technological applications, it is important to determine the highest temperature at which a SMM can retain its magnetisation, [1] and to understand the multiple relaxation mechanisms, which can be involved in their magnetic dynamics. Lanthanide (Ln) SMMs have provided some promising candidates since their discovery in 2003. [3, 4] The design criteria to synthesise Ln SMMs, [5] with a single Dy III centre in a highly axial ligand field environment to generate large magnetic anisotropy and to stabilise the highest m J = ⌃ 15/2 spin state of Dy III as the ground state, has led to Ln SMMs with magnetisation dynamics dominated by an Orbach-like (ther-mally activated) relaxation process, with very high values of the activation energy (D). [6-10] Two of the recent high-performing SMMs, [Dy(OtBu) 2 (py) 5 ][BPh 4 ] (4) [6a] and [Dy(Cp ttt) 2 ][B(C 6 F 5) 4 ] (5, Cp ttt , C 5 H 2 tBu 3-1,2,4) [7] have similar D values (1815 K and 1760 K, respectively) but very different T B 100s (12 K in 4 and 53 K in 5; here we use T B 100s to define the temperature at which the magnetic relaxation time is 100 s in zero field). The reasons behind the significant difference in T B 100s , despite having comparable energy barriers, can be attributed to the differences in the other relaxation processes involved, for example , Raman and quantum tunnelling of magnetization (QTM), and therefore it is necessary to understand these mechanisms in detail to design higher-performing SMMs. [11-16] Therefore , systematic studies on a series of compounds from a given family are essential. In particular, a series of complexes with fine control of only one structural characteristic should be ideal to see its influence on their dynamic properties. To this point, only two large families of Dy-based SMMs with D/k B > 1000 K are known: the pentagonal bipyramidal [Dy(Solv) 5 (L 1)(L 2)] (Solv = THF, Py; L 1 = OtBu, Cl, Br, PhO; L 2 = OtBu, Cl, Br, PhO) complexes [6] and the metallocene [Dy(Cp R1)(Cp R2)] + cations (R 1 = H, Me, tBu, iPr; R 2 = H, Me, tBu, iPr). [7-10] Here we present a family of five-coordinate Dy SMMs, [Dy(Mes*O) 2 (THF) 2 X] (Mes* = 2,4,6-tri-tert-butylphenyl) (X = Cl, 1; Br, 2; I, 3), designed such that the Dy coordination sphere can be selectively varied at a single position to study its influence on the relaxation dynamics. In this system, the sterically demanding aryloxide ligand was employed to reduce the coordination number at the Dy centre. Alkoxide and aryloxide-based ligands have been widely used in Ln chemistry, [17] and in synthesising Ln SMMs in recent years. [6, 14, 18-20] The [Dy(Mes*O) 2 (THF) 2 X] complexes were prepared directly by the salt metathesis reactions of two equivalents of NaOMes* with the parent halide in THF (Scheme 1). Similarly, the diamagnetic Y(III) analogous compound, 1-Y and a 5% doped sample 5 %Dy@1-Y were synthesised to perform complementary NMR spectroscopy and dilution experiments, respectively. [a] V.