Four mononuclear cobalt(II) complexes with pseudo tetrahedral geometry were isolated with different counteranions; their structure solution reveals the molecular formula as [Co(L-1)(4)]X-2 [where L-1 = thiourea (NH2CSNH2) and X = NO3 (1), Br (2), and I (3)] and [Co(L-1)(4)](SiF6) (4). The detailed analysis of direct-current (dc) magnetic data reveals a zero-field splitting (ZFS; D) with m(s) = +/-(3)/2 as the ground levels (D < 0) for the four complexes. The magnitude of the ZFS parameter is larger, in absolute value, for 1 (D = -61.7 cm(-1)) than the other three complexes (-5.4, -5.1, and -12.2 cm(-1) for 2-4, respectively). The sign of D for 1, 2, and 4 was unambiguously determined by X-band electron paramagnetic resonance (EPR) spectroscopy of the diluted samples (10%) at 5 K. For 3, the sign of D was naturally endorsed from the frequency-dependent out-of-phase signal (chi(M)'') observed in the absence of an external dc magnetic field and confirmed by high-frequency EPR (70-600 GHz) experiments performed on a representative pure polycrystalline 3, which gave a quantitative D value of -5.10(7) cm(-1). Further, the drastic changes in the spin Hamiltonian parameters and their related relaxation dynamics phenomena (of 2-4 compared to 1) were rationalized using ab initio complete-active-space self-consistent field/n-electron valence perturbation theory calculations. Calculations disclose that the anion-induced structural distortion observed in 2-4 leads to a nonfavorable overlap between the pi orbital of cobalt(II) and the pi* orbital of the sulfur atom that reduces the overall vertical bar D vertical bar value in these complexes compared to 1. The present study demonstrates that not only the first but also the second coordination sphere significantly influences the magnitude of the ZFS parameters. Particularly, a reduction of D of up to similar to 90% occurs (in 2-4 compared to 1) upon a simple variation of the counteranions and offers a viable approach to modulate ZFS in transition-metal-containing single-molecule magnets.