A new member of the Mn12 family of single-molecule magnets (SMMs) has been prepared and found to be the first of this family to give a 3-D ferromagnetic network. [Mn12O12(O2CC6H4-p-F)(16)(H2O)(4)] (2) was prepared by carboxylate substitution on the acetate derivative with p-F-benzoic acid and crystallizes as 2 center dot 8MeCN in space group I (4) over bar 2m with extensive formation of intermolecular CH center dot center dot center dot F hydrogen-bonding. The latter leads to a combination of ferromagnetic (F) and antiferromagnetic (AF) interactions and an overall F network that gives a chi T-M value at low T that is abnormally high for an S = 10 ground state. 2 center dot 8MeCN undergoes solvent loss under vacuum to 2, with a decrease in unit-cell volume of 17%, primarily due to a 13% decrease in the c-axis. The chi T-M vs T plot for 2 indicates a switch to a net AF network. Exposure to air causes hydration to 2 center dot 3H(2)O, a concomitant increase in unit cell volume, and a switch back to a F network. The same conversion of 2 center dot 8MeCN to 2 center dot 3H(2)O can also be accomplished in one step rather than two steps, by leaving crystals of the former exposed to air at ambient temperature and pressure for 10 days, giving the same magnetic plots. Interestingly, the desolvation/solvation processes cause JahnTeller isomerism to occur, but the ratio of the faster-relaxing isomer to the normal slowly relaxing one does not change monotonically. Single-crystal micro-SQUID studies on 2 center dot 8MeCN show the expected magnetization hysteresis loops for a SMM and a small exchange-bias from the intermolecular interactions that is unexpectedly AF. Since the micro-SQUID study only identifies interactions along the easy-axis (z-axis) of the crystal, this is readily rationalized as due to the J(z) components of the intermolecular interactions in 2 center dot 8MeCN being net AF. The combined results offer useful insights into the degree of sensitivity of the magnetic properties to small environmental perturbations.