The reaction of a hydrated cobalt(II) perchlorate salt with the rigid tripodal ligand, tris(3,5-dimethylpyrazolyl) phosphine oxide, (O)P(3,5-DMPz)3, results in selective in situ hydrolysis of a P‒N bond affording a neutral mononuclear Co(II) complex, [Co{(O)P(O)(3,5-DMP)2}2] (1Co(II)). The X-ray crystal structure of 1Co(II) shows that it is formed by the coordination of two N2O-tripodal (O)P(O)(3,5-DMPz)2 ligands, leading to a CoIIN4O2 coordination sphere with an elongated pseudo-trigonal antiprismatic geometry. The analysis of the dc magnetic data revealed that this compound shows a strong easy-axis magnetic anisotropy with D = ‒45.3 cm-1 and |E |= 10.2 cm-1. Ab initio theoretical studies further supported these values and therefore confirmed the axiality in the ground state, with the magnetic anisotropy axis lying along the P‒Co(II)‒P direction. Dynamic magnetic measurements confirmed slow relaxation of magnetization under an applied dc field of 0.15 T. The magnetic relaxation does not take place through an Orbach process but through a combination of direct and Raman processes and it is much faster than that observed for other trigonal antiprismatic Co(II) complexes. This is probably due to the comparatively smaller magnetic anisotropy of 1Co(II), as a result of the larger distortion of its geometry generated by the non-symmetrical (O)P(O)(3,5-DMP)2 ligand.