Electronic Structure of a Paramagnetic {MNO}⁶ Complex: MnNO 5,5-Tropocoronand

Using density functional theory (OLYP/STO-TZP) calculations, we have investigated the electronic structure of [Mn(5,5-tropocoronand)(NO)], a rare paramagnetic {MNO}6 complex. Experimental methods, including magnetic susceptibility measurements and high-field electron paramagnetic resonance spectroscopy, have not provided an unambiguous spin state assignment for this complex. In other respects, however, the compound was fully characterized, including by means of single-crystal X-ray structure determination. The optimized S = 1 OLYP geometry reproduced all key aspects of the trigonal-bipyramidal molecular structure, including a short Mn−N(O) distance (∼1.7 Å) and an essentially linear MnNO angle. In contrast, the S = 0 and S = 2 optimized structures disagreed with the crystal structure in critical respects. Moreover, three different exchange-correlation functionals (OLYP, B3LYP, and B3LYP*) indicated an S = 1 ground state by a clear margin of energy. An examination of the Kohn−Sham MOs of this state indicated a primarily dxz2dyz2dxy1dx2−z21 electronic configuration, where the z axis is identified with the nearly linear MnNO axis. The dy2 orbital is formally unoccupied in this state, interacting, as it does, head-on with two tropocoronand nitrogens lying along the y axis, the pseudo-3-fold axis of the trigonal bipyramid. The doubly occupied dxz and dyz orbitals are in actuality dπ(Fe)−π*(NO)-based π-bonding molecular orbitals, the α and β “components” of which are significantly offset spatially. This offset results in excess minority spin density on the NO unit. Thus, the OLYP/TZP atomic spin populations are Mn, 2.85; N(O), −0.52; and O, −0.35.