title: Syntheses, structures and electronic properties of zwitterionic iron(II) and cobalt(II) complexes featuring ambidentate tris(pyrazolyl)methanide ligands.
abstract: By deprotonation of the corresponding tris(pyrazolyl)methane ((Me)Tpm) precursor complexes [M((Me)Tpm)(2)][(OTf)(2)] (1 and 2), the zwitterionic tris(pyrazolyl)methanide iron(II) (3) and cobalt(II) (4) "sandwich" complexes of the general formula [M((Me)Tpmd)(2)] are easily accessible. The structurally characterised complexes 3 and 4 are the first such homoleptic 3d transition metal species to feature two "naked", outward pointing pyramidal carbanions. Density functional theory calculations show metal-centred HOMOs and LUMOs with the destabilised carbanion orbitals in the energy region of the filled transition-metal-centred frontier orbitals. The electronic structures of these complexes have been investigated in detail by various spectroscopic techniques such as NMR, EPR, SQUID, Mössbauer, etc. Both complexes adopt a high-spin (HS) configuration at room temperature in solution and in the solid state. A thermally induced high-spin to low-spin (HS-LS) transition is observed for the iron(II) complex 3. The HS-LS transition temperature of 3 in solution differs from that in the solid state, which in turn strongly depends on the amount of solvent molecules in the crystal lattice. Electrochemical studies on the corresponding cobalt(II) complex 4 provide evidence for a HS-Co(II) <==> LS-Co(III) transition upon oxidation, which was confirmed by preliminary synthetic oxidation reactions. Overall, it can be concluded that the related kappa(3)N-donor ligands tris(pyrazolyl)hydroborates (Tp(R)) and (R)Tpmd ligands have similar bonding properties and that the metal cations experience more or less the same ligand environment.
authors.affiliation: Institut für Anorganische Chemie, Universität Karlsruhe (TH), Engesserstr. 15, 76131 Karlsruhe, Germany.
authors.forename: Istemi
authors.initials: I
authors.lastname: Kuzu
authors.forename: Ivo
authors.initials: I
authors.lastname: Krummenacher
authors.forename: Ian J
authors.initials: IJ
authors.lastname: Hewitt
authors.forename: Yanhua
authors.initials: Y
authors.lastname: Lan
authors.forename: Valeriu
authors.initials: V
authors.lastname: Mereacre
authors.forename: Annie K
authors.initials: AK
authors.lastname: Powell
authors.forename: Peter
authors.initials: P
authors.lastname: Höfer
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Harmer
authors.forename: Frank
authors.initials: F
authors.lastname: Breher