To date, almost all Single Molecule Magnets (SMM) contain metal ions with strongly quenched orbital angular momenta in the ground states. In these SMMs the barrier for the magnetization reversal arises from the combination of the high-spin ground S-state of the molecule and a sufficiently large negative zero-field splitting parameter DS. Recently a new family of SMMs comprising ions with unquenched orbital angular momenta has been reported. The conventional theoretical approach fails to explain the magnetic and relaxation characteristics of such kind systems. The present contribution is aimed at the elaboration of the model of magnetic properties and magnetic relaxation in the cyano-bridged pentanuclear Mn(III)2Mn(II)3 SMMs containing low-spin Mn(III) ions with ground orbitally degenerate states. The developed model takes into account the spin-orbit interaction and the trigonal component of the crystal field acting on the ground-state cubic 3T1(t2 4) terms of the apical Mn(III) ions as well as the isotropic contribution to the exchange interaction between Mn(III) and Mn(II) ions. When the trigonal field splitting 3T1 → 3A1 + 3E stabilizes the 3E term, the combination of the named interactions leads to the ground state of the cluster with the total angular momentum projection MJ =15/2; the energies of the low-lying levels obtained from this treatment increase with decreasing MJ values, a situation that leads to a barrier for the reversal of magnetization. The proposed model provides a satisfactory agreement between the observed and calculated dc magnetic susceptibilities. The interaction of the orbitally degenerate Mn(III)-ions with the acoustic phonons of the host crystal is supposed to be responsible for magnetic relaxation in the Mn(III)2Mn(II)3 clusters because the gaps between the calculated MJ levels are of the order of 1−10 cm−1 . The monophonon transitions between the states MJ and MJ ±1 , MJ ± 2 induced by electron-vibrational interaction are shown to be allowed . The process of magnetic relaxation is presented as a cascade : at the beginning, all clusters are in the MJ = 15 2 state, first they climb to the states MJ −1 and MJ − 2 , then to the states MJ − 2 , MJ − 3 and MJ − 4 , etc., until they reach the states MJ= ±1/2, and whereupon they climb down to MJ = −15 2 . For the calculation of the temperature dependence of the relaxation time of magnetization the set of master equations for the populations nM (t) J of the MJ states of the Mn(III)2Mn(II)3 clusters is solved. The relaxation time of magnetization is estimated as the time at which the populations of the states with MJ =15/2 and MJ = −15/2 become values of the same order of magnitude. For the trigonal crystal field parameter | Δ | ( (Δ < 0) the obtained values of the relaxation time are in a qualitative agreement with the temperature dependence of the ac susceptibilities measured for the[Mn(III)(CN)6]2[Mn(II)(tmphen)2]3 (tmphen = 3, 4, 7, 8 – tetramethyl–1, 10–phenanthroline) cluster. In order to reveal the possibility of enhancing the barrier for magnetization reversal and the relaxation time of magnetization in the family of clusters Mn(III)2Mn(II)3 we vary the trigonal crystal field parameter | Δ | ( (Δ < 0) and demonstrate that increase in | Δ | (le(aΔds< t0o) a considerable growth of both characteristics. This result is expected to be useful for the controllable design of new cyano-bridged SMMs with extremely long relaxation times of magnetization and high blocking temperatures. The variation of the crystal field can be achieved by substitution of the terminal ligands in a controllable way with the aid of cyanide chemistry. Acknowledgements. Financial support of CRDF (Award No.MOC2-2611-CH-04) and MRDA/CRDF (Award No. MTFP-04-07 Follow-On Award) is highly appreciated.