SPIN-DENSITY MAPS FOR AN OXAMIDO-BRIDGED MN(II)CU(II) BINUCLEAR COMPOUND - POLARIZED NEUTRON-DIFFRACTION AND THEORETICAL-STUDIES

This paper is devoted to the determination of the spin density in the S = 2 ground state of [Mn(Me(6)-[14]ane-N-4)Cu(oxpn)](CF3SO3)(2) with Me(6)-[14]ane-N-4 = 14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane and oxpn = N,N'-bis(3-aminopropyl)oxamido. The crystal structure, pre viously determined at room temperature through X-ray diffraction and a t 40 K through unpolarized neutron diffraction, consists of oxamido-br idged [Mn(Me(6)-[14]ane-N-4)Cu(oxpn)](2+) cations and non-coordinated triflate anions. Within the cation the S-Mn = 5/2 and S-Cu = 1/2 local ground states are antiferromagnetically coupled, which gives rise to a ground S = 2 and an excited S = 3 pair states, with a quintet-septet energy gap of -93.3 cm(-1). The experimental spin density has been de duced from polarized neutron diffraction data recorded at 2 K under 50 kOe. Positive spin populations were observed on the manganese side, a nd negative spin populations on the copper side. The delocalization of the spin density from the metal center toward the terminal and bridgi ng ligands was found to be more pronounced on the copper side than on the manganese side, and the nodal surface (of zero spin density) is cl oser to the manganese than to the copper atom. The experimental data h ave been compared to the results of several theoretical approaches, co rresponding to different levels of sophistication. These approaches ar e as follows: (i) the pure Heitler-London description of the S = 2 gro und state; (ii) the incorporation of the spin delocalization in the He itler-London scheme, using the concept of magnetic orbitals in the ext ended Huckel formalism; and (iii) two types of density functional theo ry methods. In cases ii and iii, the experimental spin populations hav e been fairly well reproduced. The DPT approaches have provided some i mportant insights on both spin-delocalization and spin-polarization ef fects.