The structures and magnetic properties of [K(18-crown-6)](+) (1) and [K(18-crown-6)(THF)(2)](+) (2) salts of the eta(8)-cyclooctatetraenide sandwich complex [Er(COT)(2)](-) (COT2- = cyclooctatetraene dianion) are reported. Despite slight differences in symmetry, both compounds exhibit slow magnetic relaxation under zero applied dc field with relaxation barriers of similar to 150 cm(-1) and waist-restricted magnetic hysteresis. Dc relaxation and dilution studies suggest that the drop in the magnetic hysteresis near zero field is influenced by a bulk magnetic avalanche effect coupled with tunneling of the magnetization. Through dilution with [K(18-crown-6)(THF)(2)][Y-(COT)(2)] (3), these phenomena are substantially quenched, resulting in an open hysteresis loop to 10 K. Importantly, this represents the highest blocking temperature yet observed for a mononuclear complex and the second highest for any single-molecule magnet. A comprehensive comparative analysis of the magnetism of [K(18-crown-6)][Ln(COT)(2)] (Ln = Sm, Tb, Dy, Ho, Yb) reveals slow relaxation only for [K(18-crown-6)][Dy(COT)(2)] (4) with weak temperature dependence. Collectively, these results highlight the utility of an equatorial ligand field for facilitating slow magnetic relaxation in the prolate Er-III ion.