The crystal structure of the new phase BaCo2II((As3O6)-O-III)(2)center dot 2(H2O) is built from the stacking of infinite [BaCo2(As3O6)(2)center dot H2O] sheets containing (infinity)[(CoO4)-O-II](6-) chains interconnected by perpendicular (infinity)[(AsO2)-O-III](-) chains. It shows a metamagnetic transition below similar to 9 K at a critical field of similar to 0.11 T, leading to a moment value of 70% of the expected saturation, related to the spin flip between individual robust canted ferromagnetic chains. We propose a field-dependent scenario with magnetic moments lying in the (CoO6)-O-II octahedral basal planes, hilly compatible with our experimental results. Magnetic measurements under ac-field show slow spin dynamics with an intrinsic single-chain magnet (SCM)-like component slightly modified in the field-aligned regime. The characteristic relaxation time and energy barrier are about tau(o) = 5.1 x 10(-10) s and Delta(tau) = 35.3 K at H-dc = 0, respectively, which falls close to values found for other (but organometallic) SCM Co-II chains. This magnetic behavior is unique in the field of pure-inorganic compounds.