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Two-dimensional (2D) coordination polymer (CP) [Co III (CN) 6 ] 2 [Co II (TODA)] 3 •7H 2 O (TODA = 1,4,10-trioxa-7,13diazacyclopentadecane, Co-TODA) was reported earlier to show field-induced slow magnetic relaxation, displaying single-ion magnet (SIM) behaviors. Most SIMs are molecular compounds with fewer adopting coordination polymer (CP) or metal−organic framework (MOF) structures. In the current work, magnetic and phonon properties of Co-TODA have been studied by advanced spectroscopies and computations. The combined use of far-IR magneto-spectroscopy (FIRMS) and variable-temperature (VT) high-frequency and -field electron paramagnetic resonance (HFEPR) gives spin Hamiltonian (SH) parameters: Axial zerofield splitting (ZFS) parameter D as +38.0(1.0) ≤ D ≪ +40.2(1.0) cm −1 and rhombic ZFS parameter E as 0 ≪ |E| ≤ 7.3(1.0) cm −1 , showing that Co-TODA has the easy-plane magnetic anisotropy. Two Co II centers in the CP, as determined by synchrotron single-crystal X-ray diffraction at 15(2) K, show similar magnetic properties indistinguishable in FIRMS at 5.3(3) K or in HFEPR at 5−150 K. Ab initio calculations explore the origin of the magnetic anisotropy and magnetostructural correlations. VT inelastic neutron scattering (INS) spectra of Co-TODA have been obtained to show the phonon properties of the CP. Density functional theory (DFT) calculations, giving both a calculated INS spectrum and spin distributions in Co-TODA, demonstrate that, compared with other high-spin Co II complexes, the larger the spin density on a metal ion, the larger the ZFS in the complex. Pulsed X-band EPR studies probe relaxations of the Co II ions from the M S = +1/2 to −1/2 state in the ground Kramers doublet (KD), yielding spin−lattice (T 1 ) and spin−spin relaxation (T 2 ) times. The work reported here highlights the versatility and power of the spectroscopic techniques and computations in the characterization of magnetic and phonon properties of a CP and the understanding of its magnetic anisotropy.