The reaction of ethylenediamine (en) solutions of 2,2'- and 2,4'-bipyridine (bipy) with varying stoichiometric amounts of potassium and rubidium metal resulted in the isolation of compositionally pure solids containing the respective bipyridyl radical anions (2,2'- and 2,4'-bipy(*-)) and dianions (2,2'- and 2,4'-bipy(2-)). These species were structurally characterized by single-crystal X-ray diffraction in K(2,2'-bipy)(en) (1a), K(4)(2,2'-bipy)(4)(en)(4) (1b), Rb(2)(2,2'-bipy)(en)(2) (2), K(2,4'-bipy)(en) (3), K(4)(2,4'-bipy)(2)(en)(3.5) (4), and Rb(4)(2,4'-bipy)(2)(en)(3.5) (5). The crystallographic results obtained allow for interesting relationships to be drawn between the electronic structure of the bipyridyl moieties and metric structural data. Further characterization of the solids by means of powder X-ray diffraction, elemental analysis, electron paramagnetic resonance, and IR and Raman spectroscopy is also reported. These studies provide a comprehensive overview of the structural and spectroscopic properties of these often-cited yet elusive air- and moisture-sensitive species, helping to complement the existing data in the chemical literature.

The organometallic first-row transition-metal complexes [M(2,2'-bipy)(mes)2] (M = Cr (1), Mn (2), Co (4), Ni (5); 2,2'-bipy = 2,2'-bipyridine; mes = 2,4,6-Me3C6H2) were reacted with potassium and a suitable alkali-metal sequestering agent to yield salts of the anionic species [M(2,2'-bipy)(mes)2](-). The neutral parent compounds and their corresponding anionic congeners were characterized by single-crystal X-ray diffraction in [Cr(2,2'-bipy)(mes)2]·1.5C6H6, [Mn(2,2'-bipy)(mes)2], [Co(2,2'-bipy)(mes)2]·THF, [Ni(2,2'-bipy)(mes)2], [K(dibenzo-18-crown-6)·THF][Cr(2,2'-bipy)(mes)2]·2THF, [K(18-crown-6)][Mn(2,2'-bipy)(mes)2]·2THF, [K(18-crown-6)][Mn(2,2'-bipy)(mes)2]·0.67py·0.67tol, [K(2,2,2-crypt)][Co(2,2'-bipy)(mes)2], and [K(2,2,2-crypt)][Ni(2,2'-bipy)(mes)2]. These species, along with the previously reported neutral and anionic iron complexes [Fe(2,2'-bipy)(mes)2](0/-) (3/3(-)), form a homologous series of compounds which allow for an in-depth study of the interactions between metals and ligands. Single-crystal X-ray diffraction data, DFT calculations, and various spectroscopic and magnetic measurements indicate that the anionic complexes (1(-)-5(-)) can be best formulated as M(II) complexes of the 2,2'-bipyridyl radical anion. These findings complement recent studies which indicate that bond metric data from single-crystal X-ray diffraction may be employed as an important diagnostic tool in determining the oxidation states of bipyridyl ligands in transition-metal complexes.

Evidence is presented of significant covalency in the ytterbium 4f shell of tris-cyclopentadienyl ytterbium (YbCp(3)) in its electronic ground state, that can be represented by the superposition of an ionic configuration Yb(III):4f(13)(Cp(3)) and a charge-transfer configuration Yb(II):4f(14)(Cp(3))(-1). The relative weights of these configurations were determined from (i) the difference in their 4f photoionization cross sections, (ii) the accumulation of spin-density centered on the (13)C atoms of the Cp ring, as measured by a pulsed EPR (HYSCORE) experiment, (iii) the reduction in the spin-density in the 4f shell, manifest in the (171)Yb hyperfine interaction, and (iv) the principal values of the g-tensor, obtained from the EPR spectrum of a frozen glass solution at 5.4 K. Each of these methods finds that the spin density attributable to the charge transfer configuration is in the range 12-17%. The presence of configuration interaction (CI) also accounts for the highly anomalous energies, intensities, and vibronic structure in the "f-f" region of the optical spectrum, as well as the strict adherence of the magnetic susceptibility to the Curie law in the range 30-300 K.

Addition of potassium metal and 2,2,2-crypt (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) to a tetrahydrofuran (THF) solution of Fe(2,2'-bipyridine)(mes)(2) (1; mes = 2,4,6-Me(3)C(6)H(2)) yielded the anionic complex [Fe(2,2'-bipyridine)(mes)(2)](-) which was isolated as [K(2,2,2-crypt)][Fe(2,2'-bipyridine)(mes)(2)] (2) alongside the side-product [K(2,2,2-crypt)][Fe(mes)(3)] x C(6)H(12) (3). A compositionally pure sample of 2 was obtained by dissolving a mixture of 2 and 3 in dry pyridine and layering the resulting solution with toluene. Solid state magnetic susceptibility measurements on 1 reveal Curie-Weiss paramagnetic behavior with a molar magnetic moment of 5.12(1) mu(B) between 20 and 300 K, a value which is in line with the expected iron(II) spin-only value of 4.90 mu(B). The magnetic measurements carried out on 2 reveal more complex temperature dependent behavior consistent with intramolecular antiferromagnetic coupling (J = -46 cm(-1)) between the unpaired electrons of the iron(II) ion (S(Fe) = 2) and a pi* orbital of the bipyridyl radical (S(bipy) = 1/2). Structural data, Mossbauer and electron paramagnetic resonance (EPR) spectroscopic measurements, and density functional theory (DFT) calculations are all consistent with this model of the electronic structure. To the best of our knowledge, species 2 represents the first crystallographically characterized transition metal complex of the 2,2'-bipyridyl ligand for which magnetic, spectroscopic, and computational data indicate the presence of an unpaired electron in the pi* antibonding orbital.

The reaction of ethylenediamine solutions of 4,4'-bipyridine with varying stoichiometric amounts of sodium resulted in the isolation of the 4,4'-bipyridyl radical anion (44bipy(*-)) and the unprecedented 4,4'-bipyridyl dianion (44bipy(2-)). The radical was characterized by single-crystal X-ray diffraction in Na(44bipy)(en) (1) and Na2(44bipy)2(en)2 (2) and the dianion in Na 2(44bipy)(en)2 (3), allowing for interesting correlations to be drawn between electronic structure and metric structural data. Further characterization of the solids by means of powder X-ray diffraction, electron paramagnetic resonance, and/or elemental analysis is also reported.