title: Electronic structure of a paramagnetic {MNO}6 complex: MnNO 5,5-tropocoronand.
abstract: 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 (approximately 1.7 A) 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 d(xz)(2)d(yz)(2)d(xy)(1)d(x(2)-z(2))(1) electronic configuration, where the z axis is identified with the nearly linear MnNO axis. The d(y(2)) 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 d(xz) and d(yz) orbitals are in actuality d(pi)(Fe)-pi*(NO)-based pi-bonding molecular orbitals, the alpha and beta "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.
authors.affiliation: Department of Chemistry and Center for Theoretical and Computational Chemistry, University of Tromsø, N-9037 Tromsø, Norway.
authors.forename: Espen
authors.initials: E
authors.lastname: Tangen
authors.forename: Jeanet
authors.initials: J
authors.lastname: Conradie
authors.forename: Katherine
authors.initials: K
authors.lastname: Franz
authors.forename: Simone
authors.initials: S
authors.lastname: Friedle
authors.forename: Joshua
authors.initials: J
authors.lastname: Telser
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Lippard
authors.forename: Abhik
authors.initials: A
authors.lastname: Ghosh
mesh_terms: D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D017895:Manganese Compounds / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D010087:Oxides / Q000737:chemistry*
chemical_list: D017895:Manganese Compounds; D010087:Oxides; C048856:permanganic acid