The synthesis and structural characterization of four related heterometallic complexes of formulas [(Dy2Co2III)-Co-III(OMe)(2)(teaH)(2)(O(2)Cph)(4)(MeOH)(4)](NO3)(2)center dot MeOH center dot H2O (1a) and [(Dy2Co2III)-Co-III(OMe)(2)-(teaH)(2)(O2CPh)(4)(MeOH)(2)(NO3)(2)]center dot MeOH center dot H2O (1b), [(Dy2Co2III)-Co-III(OMe)(2)(dea)(2)(O2CPh)(4)(MeOH)(4)](NO3)(2) (2), [(Dy2CO2III)-C-III(OMe)(2)(mdea)(2)(O2CPh)(4)(NO3)(2)] (3), and [(Dy2Co2III)-Co-III(OMe)(2)(bdea)(2)(O2CPh)(4)(MeOH)(4)](NO3)(2)center dot 0.5MeOH center dot H2O (4a) [(Dy2Co2III)-Co-III(OMe)(2)(bdea)(2)-(O2CPh)(4)(MeOH)(2)(NO3)(2)]center dot MeOH center dot 1.5H(2)O (4b) are reported (teaH(3) = triethanolamine, deaH(2) = diethanolamine, mdeaH(2) = N-methyldiethanolamine, and bdeaH(2) = N-n-butyldiethanolamine). Compounds 1 (equivalent to la and lb) and 4 (equivalent to 4a and 4h) both display two unique molecules within the same crystal and all compounds display a butterfly type core, with the Dy-III ions occupying the central body positions and the diamagnetic Co" ions the outer wing-tip sites. Compounds 1-4 were investigated via direct current and alternating current magnetic susceptibility measurements, and it was found that each complex displayed single-molecule magnet (SMM) behavior. All four compounds display unique coordination and geometric environments around the Dy-III ions and it was found that each displays a different anisotropy barrier. Ab initio calculations were performed on 1-4 and these determined the low lying electronic structure of each Dy-III ion and the magnetic interactions for each cluster. It was found that there was a strong correlation between the calculated energy gap between the ground and first excited states of the single-ion ligand-field split Dy-III levels and the experimentally observed anisotropy barrier. Furthermore, the transverse g factors found for the Dy-III ions, defining the tunnelling rates within the ground Kramers doublets, are largest for 1, which agrees with the experimental observation of the shortest relaxation time in the high-temperature domain for this complex. The magnetic exchange between the Dym ions revealed overall antiferromagnetic interactions for each compound, derived from the dominant dipolar exchange resulting in nonmagnetic ground states for 1-4. The diamagnetic ground states coupled with small tunneling gaps resulted in quantum tunneling time scales at zero field of between 0.1 and >1.5 s.