Bonding in complexes of bis(pentalene)di-titanium, Ti2(C8H6)2

Bonding in the bis(pentalene)di-titanium ‘double-sandwich’ species Ti2Pn2 (Pn = C8H6) and its interaction with other fragments have been investigated by xdensity functional calculations and fragment analysis. Ti2Pn2 with C2v symmetry has two metal-metal bonds and a low-lying metal based empty orbital, all three frontier orbitals having a1 symmetry. The latter may be regarded as being derived by symmetric combinations of the classic three frontier orbitals of two bent bis(cyclopentadienyl) metal fragments. Electrochemical studies on Ti2Pn†2 (Pn† = C8H4{SiiPr3-1,4}2) reveal a one-electron oxidation, and the formally mixed-valence Ti(II)-Ti(III) cationic complex [Ti2Pn†2][B(C6F5)4] has been structurally characterised. Theory indicates an S = ½ ground state electronic configuration for the latter, confirmed by EPR spectroscopy and SQUID magnetometry. Carbon dioxide binds symmetrically to Ti2Pn2 preserving C2v symmetry, as does carbon disulfide. The dominant interaction in Ti2Pn2CO2 is σ donation into the LUMO of bent CO2 and donation from the O atoms to Ti2Pn2 is minimal, whereas in Ti2Pn2CS2 there is significant interaction with the S atoms. The bridging O atom in the mono(oxo) species Ti2Pn2O, however, employs all three O 2p orbitals in binding and competes strongly with Pn, leading to weaker binding of the carbocyclic ligand, and the sulfur analog Ti2Pn2S behaves similarly. Ti2Pn2 is also capable of binding one, two and three molecules of carbon monoxide. The bonding demands of a single CO molecule are incompatible with symmetric binding and an asymmetric structure is found. The dicarbonyl adduct Ti2Pn2(CO)2 has Cs symmetry with the Ti2Pn2 unit acting as two MCp2 fragments. Synthetic studies show, that in the presence of excess CO a tricarbonyl complex Ti2Pn†2(CO)3 is formed, which optimises to an asymmetric structure with two terminal CO ligands and one semi-bridging. Low temperature 13C NMR spectroscopy reveals a rapid dynamic exchange between the two bound CO sites and free CO

Targeted ferromagnetic coupling in a trinuclear copper(II) complex: Analysis of the S-t-3/2 spin ground state

Glaser T, Heidemeier M, Grimme S, Bill E. Targeted ferromagnetic coupling in a trinuclear copper(II) complex: Analysis of the S-t-3/2 spin ground state. Inorganic Chemistry. 2004;43(17):5192-5194.The trinuclear Cu-II complex [(talen)Cu(II)3] (1) using the new triplesalen ligand H(6)talen has been synthesized and structurally characterized. The three Cull ions are bridged in a m-phenylene linkage by the phloroglucinol backbone of the ligand. This m-phenylene bridging mode results in ferromagnetic couplings with an S-t = 3/2 spin ground state, which has been analyzed by means of EPR spectroscopy and DFT calculations. The EPR spectrum exhibits an unprecedented pattern of 10 hyperfine lines due to the coupling of three Cull ions (I = 3/2). Resonances around g = 4 in both perpendicular and parallel mode EPR spectra demonstrate a zero-field splitting of D similar to 74 x 10(-4) cm(-1) arising from an isotropic/antisymmetric exchange interactions. The DFT calculations show an alteration in the sign of the spin densities of the central benzene ring corroborating the spin-polarization mechanism as origin for the ferromagnetic coupling