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The solvatochromic indicator [Cu(acac)(tmen)(H 2O)].PF 6 ( 1.H 2O) has been synthesized and crystallographically characterized. 1.H 2O binds an H 2O molecule at the Cu(II) axial site, while the PF 6 (-) anion is coordination free. The binding properties of [Cu(PF 6)(acac)(tmen)] ( 1) and [Cu(BF 4)(acac)(tmen)] ( 2) have been investigated in solution and the solid state. The donor number of the PF 6 (-) anion (DN PF6) was determined from the UV-vis spectra of 1 in 1,2-dichloroethane. The value of DN PF6 of the PF 6 (-) anion is slightly larger than that of the tetraphenylborate anion (BPh 4 (-)), which is known as a noncoordinating anion. In the solid state, 1 and 2 reversibly bind and release H 2O molecules at the Cu(II) axial sites. The coordinated H 2O molecules in 2 are more easily removed than those in 1 because of the strong Lewis basicity of the BF 4 (-) anion compared to the PF 6 (-) ion. The lower melting point of 1 versus 2 is attributed to the loose binding of the PF 6 (-) anions to the Cu(II) centers, which induces the dynamic nature of the crystal.

The spectroscopy and photophysics of trans-[Cr(cyclam)(NCS)2]+ (where cyclam is 1,4,8,11-tetraazacyclotetradecane) were studied in a range of solvents. The cyclam NH stretching vibration [nu(NH)] wavenumber correlates with the Gutmann donor number, whereas the thiocyanate CN stretching vibration [nu(CN)] wavenumber correlates with the Snyder solvent strength (P') scale. These results signify that there is a difference in the solvent interactions with the two types of ligands. The energy of the ligand-to-metal charge transfer absorption maximum between 310 and 320 nm and the energy of the spin-forbidden (doublet-quartet) absorption and emission bands above 700 nm correlate with the nu(CN) wavenumber. This establishes the dominant role of solvent effects at the NCS- ligand in "tuning" the energy of these spectroscopic features. Quantum yields phirx for photosubstitution are0.02 at 54 degrees C and0.002 at 22 degrees C, demonstrating that photochemical reaction is a very minor pathway. The effects of solvent and temperature on the nonradiative decay of the doublet excited-state were investigated by observing the time-resolved phosphorescence between 700 and 750 nm. Below 30 degrees C, the lifetimes are relatively temperature-independent, whereas at higher temperatures, a strong Arrhenius-type dependence is observed. Values for the preexponential factor (A) and the activation energy (Ea) are solvent-dependent and follow a Barclay-Butler-type correlation. These observations are consistent with a dominant back-intersystem crossing pathway for nonradiative decay in the higher-temperature region. From trends observed between ln(A) and the nu(CN) frequency, it appears that solvent effects at the thiocyanate ligand play a dominant role in influencing the rate of nonradiative decay in the high-temperature region.

The title compound has been first synthesized and fully characterized as both tetraphenylborate and perchlorate salt. Its 300-900 nm absorption spectrum, recorded in nitromethane, water, and dimethyl sulfoxide, reveals the peculiar existence of two distinct bands whose intensities depend on the solvent donor number. This feature can be attributed to two separate metal-to-ligand charge-transfer transitions, in agreement with the theoretical predictions obtained by extensive configuration interaction calculations, which take into account the solvent effects. The calculation of the potential energy curves of the ground and excited states along the Ru-tetrazine coordinate allows the interpretation of the relative intensities of the observed bands, as well as the interpretation of their line-shape profiles.

Azobenzene-conjugated mononuclear and dinuclear terpyridyl complexes of Co(II), Co(III), and Fe(II) were synthesized, and their photoisomerization behavior was investigated. Co(II) and Co(III) complexes, [tpyCo(tpy-AB)]X(n) and [(Cotpy)(2)(tpy-AB-tpy)]X(n) (tpy-AB = C(15)N(3)H(10)-C(6)H(4)-N=NC(6)H(5), tpy-AB-tpy = C(15)N(3)H(10)-C(6)H(4)-N=NC(6)H(4)-C(15)N(3)H(10), X = PF(6) or BPh(4)), exhibit trans-to-cis photoisomerization by irradiation at 366 nm, and this behavior is dependent on solvents and counterions. For the Co(II) complexes, BPh(4) salts undergo cis-to-trans isomerization in propylene carbonate by both photoirradiation with visible light (435 nm) and heat, indicating that reversible trans-cis isomerization has occurred. [Co(tpy-AB)(2)](BPh(4))(2) shows a two-step trans-to-cis isomerization process. The trans-cis isomerization behavior of Co(III) complexes was observed only in the solvents with a low donor number such as 1,2-dichloroethane. Fe(II) complexes, [tpyFe(tpy-AB)]X(n) (X = PF(6) or BPh(4)), exhibit slight trans-to-cis photoisomerization due to the energy transfer from the azobenzene moiety to Fe(tpy)(2) moieties.

A homogeneous series of heterobimetallic complexes of [R-Fc(4-py)Ru(NH3)5](PF6)2 (R = H, Et, Br, acetyl; Fc(4-py) = 4-ferrocenylpyridine) have been prepared and characterized. The mixed-valence species generated in situ using ferrocenium hexafluorophosphate as the oxidant show class II behavior, and the oxidized sites are ruthenium centered. deltaE(1/2), E(1/2)(Fe(III)/Fe(II)) - E(1/2)(Ru(III)/Ru(II)), an upper limit for deltaGo that is an energetic difference between the donor and acceptor sites, changes sharply and linearly with Gutmann solvent donor number (DN) and Hammett substituent constants (sigma). The solvent-dependent and substituent-dependent intervalence transfer bands were found to vary almost exclusively with deltaE(1/2). The activation energy for the optical electron transfer versus deltaE(1/2) plot yields a common nuclear reorganization energy (lambda) of 0.74 +/- 0.04 eV for this series. The equation that allows one to incorporate the effect of both solvent donicity and substituents on optical electron transfer is Eop = lambda + deltaGo, where deltaGo = (deltaGo)intrinsic + (deltaGo)solvent donicity + (deltaGo)substituent effect (deltaGo )intinnsic with a numerical value of 0.083 +/- 0.045 eV was obtained from the intercept of the deltaE(1/2) of [H-Fc(4-py)Ru(NH3)5]2+,3+,4+ versus DN plot. (deltaGo)solvent donicity was obtained from the average slopes of the deltaE(1/2) of [R-Fc-(4-py)Ru(NH3)5]2+,3+,4+ versus DN plot, and (deltaGo)substituent effect was obtained from the average slopes of the corresponding deltaE(1/2) versus sigma plot. The empirical equation allows one to finely tune Eop of this series to Eop = 0.82 + 0.019(DN) + 0.44sigma eV at 298 K, and the discrepancy between the calculated and experimental data is less than 6%.

The relative shift of the Eu(III) 7F0−5D0 transition toward lower energies due to the formation of inner-sphere Eu(III) complexes is shown to be linearly proportional to the total ligand donor number of the ligand bound to Eu(III) for 40 organic ligand complexed species in both water and DMSO solvents. Such a correlation allows estimation of the donor number, CNL, of organic ligands coordinated to Eu(III) from the relative frequency, Δν, of the Eu(III) 7F0−5D0 transition by the equation of CNL = 0.237Δν + 0.628. The validity of the method has been confirmed by using shift data of other Eu(III) complex systems from the literature. The nature of the shift of 7F0−5D0 band upon complexation is discussed as possibly due to a small degree of covalency in the Eu−L bond of the complexes with organic ligands.

The visible absorption and emission spectra of the trans-diammine(1,4,8,11-tetraazacyclotetradecane)chromium(III) ion, [Cr(cyclam)(NH[sub 3])[sub 2]][sup 3+], have been studied along with the emission lifetimes and quantum yields in the liquid phase for dimethylformamide, dimethyl sulfoxide, hexamethylphosphoramide, and aqueous solvents. The measured quantum yields for emission from the photochemically unreactive doublet excited state are small (1-4 [times] 10[sup [minus]3]) and they vary in an approximately linearly manner with changes in the observed lifetimes. The reciprocal lifetimes [tau][sup [minus]1] in the different media are a function of temperature, and this dependence is described by a sum of two exponential terms. The term expressing the lifetime behavior at lower temperatures is characterized by low apparent activation energies ([le] 6 kJ mol[sup [minus]1]) and relatively small preexponential factors ([le] 6 [times] 10[sup 4] s[sup [minus]1]) whereas in the high-temperature region the activation energies and preexponential factors are much larger, ranging from 64 to 78 kJ mol[sup [minus]1] and from 2.2 [times] 10[sup 15] to 1.7 [times] 10[sup 16] s[sup [minus]1], respectively. For aqueous solution and dimethylformamide, the volumes of activation for [tau][sup [minus]1] in the low-temperature region are nearly zero; by contrast, those describing the effects of pressure in the high-temperature region are near +7 cm[supmore » 3] mol[sup [minus]1]. The nature of the solvent has notable effects on the visible electronic spectra and on the vibrational bands observed in the emission and infrared spectra, and the calculated rate constants describing the decay of the doublet excited state in the high-temperature region exhibit a semilogarithmic relationship to Gutmann donor numbers (and solvent scales that are proportional to the donor number scale). The natures of the nonradiative processes and the influence of solvent as a mediating agent are discussed.« less

A study of the effect of the nature of the solvent on the b 1 *←b 1 LMCT oscillator strength of the complex [(NH 3 ) 5 Ru((2,3-dichlorophenyl)cyanamide(1−))][ClO 4 ] 2 is reported. A plot of LMCT oscillator strength vs donor number gives a linear relationship with intercept 0.186, slope −0.00294/DN, and R=0.998. Weak and strong coupling approaches to donor and acceptor interactions are used to evaluate this novel effect

Phosphines are traditionally considered as Lewis bases or ligands in transition metal and main group complexes. Despite their electron-rich (lone pair-bearing) nature, an extensive coordination chemistry for Lewis acidic phosphorus centers is being developed; such chemistry provides a new synthetic approach for phosphorus-element bond formation, leading to new types of structures and modes of bonding. Complexes of Ph2P+ with a variety of donor elements (P, N, C) give experimentally short donor-acceptor bond lengths, when compared to other cationic phosphorus Lewis acid complexes. We have calculated that the energy of the lowest unoccupied molecular orbital (LUMO) in Ph2P+ is lower than that of (Me2N)2P+, which partially explains the greater exothermicity of reactions of donors with the diaryl acceptor. Furthermore, the energies required to distort the diphenylphosphenium cation from its ground-state geometry are significantly smaller than those of the diamido cations and, thus, enhance the exothermicity of donor coordination. These computational data, in conjunction with evidence from experimental solid-state structures, indicate that Ph2P+ is a significantly better Lewis acid relative to the more common diaminophosphenium analogues (R2N)2P+ and are used to elucidate the nature of the bonding in donor-phosphenium complexes.

The THF adduct of uranyl hexafluoroacetylacetonate undergoes fast reversible displacement reactions with other bases in chloroform solutions. The NMR shift of the ..cap alpha..-CH/sub 2/ group is found to be a useful monitor of the equilibrium established and has been used to measure the relative strength of oxygenated bases. The values correlate well with Gutmann's donor number (DN), the heat of reaction of the bases with SbCl/sub 5/.

Near-IR-vis spectra and cyclic voltammetric parameters are reported for the title compounds in the solvent series nitromethane, acetonitrile, propylene carbonate, N,N-dimethylformamide, and dimethyl sulfoxide (over which Gutmann donor number (DN) ranges from 2.7 to 29.8). As has been found in related systems, the E/sup 0/ of (NH/sub 3/)/sub 5/M/sup III/II/ decreases with donor number, with the magnitude of the response to solvent being quantitatively similar (0.025 +/- 0.002 V/DN) for all of the couples examined. (Consequently the stability of (Ru(NH/sub 3/)/sub 5/)/sub 2/pz/sup 5 +/ with respect to disproportionation is solvent independent within experimental error.) By contrast, the vis nu/sub h/) and near-IR (nu/sub 1/) absorption bands exhibit very different solvent dependences. For Ru(NH/sub 3/)/sub 5/pzCH/sub 3//sup 3 +/, the position of nu/sub h/ is solvent independent, but nu/sub 1/ exhibits a typical MLCT solvent dependence. For Os(NH/sub 3/)/sub 5/pzCH/sub 3//sup 3 +/, nu/sub 1/ shifts to longer wavelength with increasing donor number (MLCT character) while nu/sub h/ exhibits the opposite behavior. Both vis and near-IR bands of (Ru(NH/sub 3/)/sub 5/)/sub 2/pz/sup 5 +/ shift only very slightly with solvent, but the vis band of (Ru(NH/sub 3/)/sub 5/)/sub 2/pz/sup 4 +/ is more solvent dependent - about 80% as muchmore » as Ru(NH/sub 3/)/sub 5/pz/sup 2 +/. The spectral shifts are discussed in terms of a simplified molecular orbital scheme for the M-pz back-bonding interaction, and the nature of the solvent sensitivity is found to be related to the metal and pyrazine character of the MO's involved in the transition. 51 references, 5 figures, 4 tables.« less