Patterns in the cyanide stretching frequencies have been examined in several series of monometal-and CN -bridged transition metal complexes. Metal-to-cyanide back-bonding can be identified as a major factor contributing to red shifts of ν CN in monometal complexes. This effect is complicated in cyanide-bridged complexes in two ways: (a) when both metals can back-bond to cyanide, the net interaction is repulsive and results in a blue shift of ν CN ; and (b) when a donor and acceptor are bridged, ν CN undergoes a substantial red shift (sometimes more than 60 cm -1 lower in energy than the parent monometal complex). These effects can be described by simple perturbational models for the electronic interactions. Monometal cyanide complexes and CN --bridged backbonding metals can be treated in terms of their perturbations of the CNπ and π* orbitals by using a simple, Hu ¨ckel-like, three-center perturbational treatment of electronic interactions. However, bridged donor-acceptor pairs are best described by a vibronic model in which it is assumed that the extent of electronic delocalization is in equilibrium with variations of some nuclear coordinates. Consistent with this approach, it is found that (a) the oscillator strength of the donor-acceptor charge transfer (DACT) absorption is roughly proportional to the red shift of ν CN and (b) there are strong symmetry constraints on the coupling. The latter point is demonstrated by a 10-fold larger red shift of the symmetrical than of the antisymmetrical combination of CNstretching frequencies in the centrosymmetric trans- ([14]aneN 4 )Cr(CNRu(NH 3 ) 5 ) 2 5+ complex ([14]aneN 4 ) 1,4,7,11-tetraazacyclotetradecane). The coupling of the metal dπ orbitals to CNπ and π* orbitals can be formulated in terms of ligandto-metal (LMCT) and metal-to-ligand (MCLT) charge transfer perturbations. The associated charge delocalizations provide a basis for the synergistic weakening of the C-N bond and D/A coupling.

The effects of donor-acceptor (D/A) electronic coupling, H DA , on the spectroscopic and electrochemical properties of several series of CN --bridged transition metal complexes have been examined. The complexes employed were formed by ruthenation of M(L)(CN) 2 n+ parent complexes (for n ) 0, M ) Ru(II) or Fe(II), and L ) bpy or phen; for n ) 1, M ) Cr(III), Rh(III), or Co(III), and L ) bpy, phen, or a tetraazamacrocyclic ligand). The observed half-wave potentials of the resulting CN --bridged D/A complexes spanned a 300-350 mV range in contrast to the range of about 80 mV expected on the basis of the oscillator strength, h DA , of the D/A charge-transfer MM′CT absorption band and the geometrical distance between donor and acceptor, r DA . Different series of complexes exhibit different correlations between E 1/2 and h DA . Several factors have been found to contribute to these differences: (a) symmetry effects; (b) solvational differences that arise when nonbridging ligands are changed; (c) solvational effects arising from differences in overall electrical charges; (d) partial delocalization of electron density along the D/A axis in such a way as to reduce the effective distance between centers of charge, r ge c . To take account of the effects of the solvational factors, systematic examination has been made of (a) the metal independent shifts of E 1/2 which occur when nonbridging ligands are changed; (b) the differences in E /12 that occur in closely related Ru(III)/Ru(II) couples which differ in charge; and (c) solvent peturbations of E 1/2 (Ru(NH 3 ) 5 3+,2+ ) and solvatochromic shifts of the central metal-to-ligand charge transfer (MLCT) and MM′CT absorbancies of (bpy) 2 (CN)Ru(CNRu(NH 3 ) 5 ) 3+ and (bpy) 2 -Ru(CNRu(NH 3 ) 5 ) 2 6+ . The experimental observations indicate that changes in the nonbridging ligand of the central metal can result in a range of about 90 mV variation in E 1/2 (Ru(NH 3 ) 5 3+,2+ ), the effect of a one unit increase in charge of the central metal is to increase E 1/2 by approximately 65 ( 15 mV, solvent perturbations of E 1/2 and the electron-transfer reorganizational energy, λ r , are approximately equal in magnitude, solvational corrections can be treated linearly, and the solvational contributions to E 1/2 that arise from charge delocalization are less than about 10 mV in these complexes. The complexes have a very rich charge-tansfer spectroscopy, and in some complexes as many as seven different CT transitions can be identified which depend on the oxidation state of the Ru(NH 3 ) 5 moiety. There is evidence for considerable mixing between these transitions. The mixed valence (Ru(NH 3 ) 5 2+ /Ru(NH 3 ) 5 3+ ), bisruthenates exhibit a unique Ru(NH 3 ) 5 /M MM′CT component in addition to the expected Ru(NH 3 ) 5 2+ f Ru(NH 3 ) 5 3+ CT; this relatively weak absorption tracks the dominant Ru(NH 3 ) 5 /central metal MM′CT absorption, and it is attributable to the different effects of local M c (CN -)Ru(NH 3 ) 5 electronic coupling in the mixed valence complex. Values of E 1/2 (obsd), co...

Intense near-infrared (NIR) absorption bands have been found in mixed-valence Ru(NH3)5(2+,3+) complexes bridged by trans-Ru(py)4(CN)2 and cis-Os(bpy)2(CN)2, epsilonmax approximately 1.5 x 10(3) cm(-1) and deltav1/2 approximately 5 x 10(3) cm(-1) for bands at 1,000 and 1,300 nm, respectively. The NIR transitions implicate substantial comproportionation constants (64 and 175, respectively) characteristic of moderately strong electronic coupling in the mixed-valence complexes. This stands in contrast to the weakly forbidden electronic coupling of Ru(NH3)5(2+,3+) couples bridged by M(MCL)(CN)2+ complexes (MCL = a tetraazamacrocyclic ligand) (Macatangay; et al. J. Phys. Chem. 1998, 102, 7537). A straightforward perturbation theory argument is used to account for this contrasting behavior. The electronic coupling between a cyanide-bridged, donor-acceptor pair, D-(CN-)-A, alters the properties of the bridging ligand. Such systems are described by a "vibronic" model in which the electronic matrix element, HDA, is a function of the nuclear coordinates, QN, of the bridging ligand: HDA = HDA degrees + bQN. Electronic coupling in the dicyano-complex-bridged, D-[(NC)M(CN)]-A, systems is treated as the consequence of the perturbational mixing of the "local", D(NC)M and M(CN)A, vibronic interactions. If M is an electron-transfer acceptor, then the nuclear coordinates are assumed to be configured so that bQN is larger for D(NC)M but very small (bQN approximately 0) for M(CN)A. When the vertical energies of the corresponding charge-transfer transitions, EDM and EDA, differ significantly, a perturbation theory treatment results in HDA = HDAHAM/Eave independent of M and consistent with the earlier report. When EDM approximately equals EDA, configurational mixing of the excited states leads to HDA proportional to HDM, consistent with the relatively intense intervalence bands reported in this paper. Some implications of the model are discussed.

A series of covalently linked, transition-metal donor/acceptor complexes are described in which the net donor−acceptor coupling matrix element, H DA, is independent of the extent of coupling between the donor and the bridging ligand. The bridging ligand in these complexes is a transition-metal dicyano complex with a tetraaza aliphatic nonbridging ligand, cis- or trans-M(MCL)(CN)2 + for M = Rh(III), Co(III), or Cr(III), donor = Ru(NH3)5 2+, and the acceptor = Ru(NH3)5 3+. The electronic coupling (and electron delocalization) between the donor and the central atom (M) of the bridging ligand varies from H DL ≈ 103 to ∼3 × 103 cm-1 through the series of M(MCL)(CN)2 +-bridged complexes, and this variation has an effect on the energy of the Ru(II)/Ru(III) CT absorption maximum, which is expected from perturbational mixing of these electronic states. However, the usually correlated superexchange contribution to H DA is not observed and appears to be less than about 10% of the contribution predicted. This is in contrast to observations on related complexes with pyridyl-type bridging ligands. The unusual behavior can be a consequence of the dependence of D/A electronic coupling on the CN- vibrational distortions and the mixing of the two Ru(II)/Ru(III) electron-transfer states with the BL state promoted by in-phase and out-of-phase combinations of CN- stretches. Such an approach predicts very little superexchange coupling when there is little electron delocalization onto the bridging ligand and requires that H DA be a strong function of the electron-transfer coordinates.

Patterns of the shifts in bridging cyanide-stretching frequencies have been examined in several fully saturated, &mgr;-cyano, bi- or trimetallic transition-metal donor-acceptor (D/A) complexes. An earlier (Watzky, M. A.; et al.Inorg. Chem. 1996, 35, 3463) inference that the bridging ligand nuclear and the D/A electronic coordinates are entangled is unequivocally demonstrated by the 32 cm(-)(l) lower frequency of nu(CN) for (NH(3))(5)Cr(CNRu(NH(3))(5))(4+) than for the cyanopentaamminechromium(III) parent. This contrasts to the 41 cm(-)(1) increase in nu(CN) upon ruthenation of (NH(3))(5)RhCN(2+). More complex behavior has been found for cis and trans trimetallic, donor-acceptor complexes. The symmetric combination of CN(-) stretching frequencies in trans-Cr(III)(MCL)(CNRu(II)(NH(3))(5))(2)(5+) complexes (MCL = a tetraazamacrocyclic ligand) shifts 100-140 cm(-)(1) to lower frequency, and the antisymmetric combination shifts less than about 30 cm(-)(1). This contrast in the shifts of the symmetric and the antisymmetric combinations of the CN stretches persists even in a trans complex with no center of symmetry. Two CN stretches have also been resolved in an analogous cis complex, and both shift to lower frequency by about 60 cm(-)(1). The net shift, summed over all the CN-stretching frequencies, is about the same for the bis-ruthenates of related dicyano complexes. A simple, symmetry-adapted perturbation theory treatment of the coupled vibrations is employed to deal with the opposing effects of the "kinematic" shifts (delta) of nu(CN) to higher frequency, expected in the absence of D/A coupling, and shifts ( f ) of nu(CN) to lower frequency that occur when D/A coupling is large. The Rh(III)- and Cr(III)-centered complexes correspond to different limits of this model: delta > f and delta < f, respectively. When referenced by means of this model to complexes with Rh(III) acceptors, the shifts in trimetallic complexes, summed over the symmetric and antisymmetric combinations of CN stretches, are about twice those of bimetallic complexes. Similarly referenced and summed over all bridging CN frequencies, the shifts of nu(CN) to lower energies are proportional to the oscillator strength of the electronic, donor-acceptor charge-transfer transition. The simplest interpretation of this correlation is that the donor-acceptor coupling in these systems is a function of the nuclear coordinates of the bridging ligand. This behavior of these complexes is semiquantitatively consistent with expectation for CN(-)-mediated vibronic (pseudo-Jahn-Teller) coupling of neighboring donors and acceptors, and the observed Ru(II)/CN(-) CT absorption parameters can be used in a simple, semiclassical vibronic model to predict shifts in nu(CN) that are in reasonable agreement with those observed.

No abstract

The physical and photophysical properties of a series of monometallic, [Ru(bpy)(2)(dmb)](2+), [Ru(bpy)(2)(BPY)](2+), [Ru(bpy)(Obpy)](2+) and [Ru(bpy)(2)(Obpy)](2+), and bimetallic, [{Ru(bpy)(2)}(2)(BPY)](4+) and [{Ru(bpy)(2)}(2)(Obpy)](4+), complexes are examined, where bpy is 2,2'-bipyridine, dmb is 4,4'-dimethyl-2,2'-bipyridine, BPY is 1,2-bis(4-methyl-2,2'-bipyridin-4'-yl)ethane, and Obpy is 1,2-bis(2,2'-bipyridin-6-yl)ethane. The complexes display metal-to-ligand charge transfer transitions in the 450 nm region, intraligand pi --> pi transitions at energies greater than 300 nm, a reversible oxidation of the ruthenium(II) center in the 1.25-1.40 V vs SSCE region, a series of three reductions associated with each coordinated ligand commencing at -1.3 V and ending at approximately -1.9 V, and emission from a (3)MLCT state having energy maxima between 598 and 610 nm. The Ru(III)/Ru(II) oxidation of the two bimetallic complexes is a single, two one-electron process. Relative to [Ru(bpy)(2)(BPY)](2+), the Ru(III)/Ru(II) potential for [Ru(bpy)(2)(Obpy)](2+) increases from 1.24 to 1.35 V, the room temperature emission lifetime decreases from 740 to 3 ns, and the emission quantum yield decreases from 0.078 to 0.000 23. Similarly, relative to [{Ru(bpy)(2)}(2)(BPY)](4+), the Ru(III)/Ru(II) potential for [{Ru(bpy)(2)}(2)(Obpy)](4+) increases from 1.28 to 1.32 V, the room temperature emission lifetime decreases from 770 to 3 ns, and the room temperature emission quantum yield decreases from 0.079 to 0.000 26. Emission lifetimes measured in 4:1 ethanol:methanol were temperature dependent over 90-360 K. In the fluid environment, emission lifetimes display a biexponential energy dependence ranging from 100 to 241 cm(-)(1) for the first energy of activation and 2300-4300 cm(-)(1) for the second one. The smaller energy is attributed to changes in the local matrix of the chromophores and the larger energy of activation to population of a higher energy dd state. Explanations for the variations in physical properties are based on molecular mechanics calculations which reveal that the Ru-N bond distance increases from 2.05 Å (from Ru(II) to bpy and BPY) to 2.08 Å (from Ru(II) to Obpy) and that the metal-to-metal distance increases from approximately 7.5 Å for [{Ru(bpy)(2)}(2)(Obpy)](4+) to approximately 14 Å for [{Ru(bpy)(2)}(2)(BPY)](4+).

The development of a direct analysis in real time-mass spectrometry (DART-MS) method and first prototype vaporizer for the detection of low molecular weight (∼30-100 Da) contaminants representative of those detected in water samples from the International Space Station is reported. A temperature-programmable, electro-thermal vaporizer (ETV) was designed, constructed, and evaluated as a sampling interface for DART-MS. The ETV facilitates analysis of water samples with minimum user intervention while maximizing analytical sensitivity and sample throughput. The integrated DART-ETV-MS methodology was evaluated in both positive and negative ion modes to (1) determine experimental conditions suitable for coupling DART with ETV as a sample inlet and ionization platform for time-of-flight MS, (2) to identify analyte response ions, (3) to determine the detection limit and dynamic range for target analyte measurement, and (4) to determine the reproducibility of measurements made with the method when using manual sample introduction into the vaporizer. Nitrogen was used as the DART working gas, and the target analytes chosen for the study were ethyl acetate, acetone, acetaldehyde, ethanol, ethylene glycol, dimethylsilanediol, formaldehyde, isopropanol, methanol, methylethyl ketone, methylsulfone, propylene glycol, and trimethylsilanol.