We have demonstrated the effect of a solvent at the second coordination sphere on slow relaxation of magnetization for hepta-coordinated cobalt(II) complexes with the formulas [Co(H4L)(DMF)(H2O)](NO3)(2)center dot(DMF) (1), [Co(H4L)(MeOH)(H2O)](NO3)(2)center dot(MeOH) (2), and [Co(H4L)(DEF)(H2O)](NO3)(2) (3) (H4L = 2,2'-(pyridine-2,6-diylbis(ethan-1-yl-1-ylidene))bis(N-phenylhydrazinecarboxamide). Structural analysis reveals that the presence of lattice solvent molecule in 1 and 2 dramatically changes the crystal packing and noncovalent interactions as compared to 3 where no solvent molecule is present in the crystal lattice. The dc and ac magnetic susceptibility measurements reveal the presence of easy-plane magnetic anisotropy for all the complexes, and field induced slow relaxation behavior has been observed above 2 K for 1 and 2 in contrast to 3 due to the availability of the solvent molecules in the crystal lattice. The ab initio calculations further support the sign of D and the negligible effect of the first co-ordination sphere, as almost similar D values were obtained for all the complexes. The field and temperature dependence of relaxation time confirm that quantum tunnelling of magnetization (QTM) plays a major role in slow magnetic relaxation, and thermal dependence like an optical or acoustic Raman pathway is also important. To further analyze the effect of dipole dipole interaction on slow magnetic relaxation, a dilution experiment has been performed.