M2(hpp)4Cl2 and M2(hpp)4, where M = Mo and W: preparations, structure and bonding, and comparisons with C2, C2H2, and C2Cl2 and the hypothetical molecules M2(hpp)4(H)2

The reaction between M(2)Cl(2)(NMe(2))(4), where M = Mo or W, and Hhpp (8 equiv) in a solid-state melt reaction at 150 degrees C yields the compounds M(2)(hpp)(4)Cl(2) 1a (M = Mo) and 1b (M = W), respectively, by the elimination of HNMe(2) [hpp is the anion derived from deprotonation of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, Hhpp]. Purification of 1a and 1b is achieved by sublimation of the excess Hhpp and subsequent recrystallization from either CH(2)Cl(2) or CHCl(3) (or CDCl(3)). By single-crystal X-ray crystallography, the structures of 1a and 1b are shown to contain a central paddlewheel-like M(2)(hpp)(4) core with Mo-Mo = 2.1708(8) A (from CH(2)Cl(2)), 2.1574(5) A (from CDCl(3)), W-W = 2.2328(2) A (from CDCl(3)), and M-N = 2.09(1) (av) A. The Cl ligands are axially ligated (linear Cl-M-M-Cl) with abnormally long M-Cl bond distances that, in turn, depend on the presence or absence of hydrogen bonding to chloroform. The quadruply bonded compounds M(2)(hpp)(4), 2a (M = Mo), and 2b (M = W), can be prepared from the reactions between 1,2-M(2)R(2)(NMe(2))(4) compounds, where R = (i)()Bu or p-tolyl, and Hhpp (4 equiv) in benzene by ligand replacement and reductive elimination. The compounds 2a and 2b are readily oxidized, and in chloroform they react to form 1a and 1b, respectively. The electronic structure and bonding in the compounds 1a, 1b, 2a, and 2b have been investigated using gradient corrected density functional theory employing Gaussian 98. The bonding in the M-M quadruply bonded compounds, 2a and 2b, reveals M-M delta(2) HOMOs and extensive mixing of M-M pi and nitrogen ligand lone-pair orbitals in a manner qualitatively similar to that of the M(2)(formamidinates)(4). The calculations indicate that in the chloride compounds, 1a and 1b, the HOMO is strongly M-Cl sigma antibonding and weakly M-M sigma bonding in character. Formally there is a M-M triple bond of configuration pi(4)sigma(2), and the LUMO is the M-M delta orbital. An interesting mixing of M-M and M-Cl pi interactions occurs, and an enlightening analogy emerges between these d(4)-d(4) and d(3)-d(3) dinuclear compounds and the bonding in C(2), C(2)H(2), and C(2)Cl(2), which is interrogated herein by simple theoretical calculations together with the potential bonding in axially ligated compounds where strongly covalent M-X bonds are present. The latter were represented by the model compounds M(2)(hpp)(4)(H)(2). On the basis of calculations, we estimate the reactions M(2)(hpp)(4) + X(2) to give M(2)(hpp)(4)X(2) to be enthalpically favorable for X = Cl but not for X = H. These results are discussed in terms of the recent work of Cotton and Murillo and our attempts to prepare parallel-linked oligomers of the type [[bridge]-[M(2)]-](n)().     

High yield syntheses of stable, singly bonded Pd2(6+) compounds

A general method for the syntheses of dipalladium compounds having a singly bonded Pd26+ core and the formula R,S-cis-Pd2(C6H4PPh2)2(O2CR)2Cl2 is described. When the alkyl group in the carboxylate ligands is an electron donating group, the compounds are stable and the yields high. The Pd-Pd distances for the diamagnetic compounds with R = CF3 and CMe3 are 2.5434(4) and 2.5241(9) A, respectively. Calculations at the DFT level suggest that the electronic configuration is sigma2pi4delta2delta*2pi*4. These represent rare examples of palladium(III) compounds.     

Paramagnetism at ambient temperature, diamagnetism at low temperature in a Ru2(6+) core: structural evidence for zero-field splitting

Variable temperature magnetic studies of the Ru(2)(6+) guanidinate compounds Ru(2)(hpp)(4)Cl(2) (1) and Ru(2)(hpp)(4)(CF(3)SO(3))(2) (2) show that they are paramagnetic with two unpaired electrons at room temperature and that they appear essentially diamagnetic at 2 K. In neither compound do the Ru-Ru distances vary by more than 0.008(1) A from 27 to 296 K. This argues strongly that the ground state electronic configuration remains constant over this temperature range and that the decrease in magnetism as the temperature is lowered must be attributable to zero-field splitting of the (3)A(2g) ground state arising from the electronic configuration sigma(2)pi(4)delta(2)pi(2). The Ru-Ru distance in 1 is about 0.04-0.05 A longer than that in 2 which indicates that the Ru(2)(hpp)(4)(2+) core is quite sensitive to the nature of the axial ligands. The electronic spectra show three absorption bands for each compound.     

Metal-metal bonding in molecular actinide compounds: electronic structure of [M2X8](2-) (M = U, Np, Pu; X = Cl, Br, I) complexes and comparison with d-block analogues

Density functional and multiconfigurational (ab initio) calculations have been performed on [M(2)X(8)](2-) (X = Cl, Br, I) complexes of 4d (Mo, Tc, Ru), 5d (W, Re, Os), and 5f (U, Np, Pu) metals in order to investigate general trends, similarities and differences in the electronic structure and metal-metal bonding between f-block and d-block elements. Multiple metal-metal bonds consisting of a combination of sigma and pi interactions have been found in all species investigated, with delta-like interactions also occurring in the complexes of Tc, Re, Np, Ru, Os, and Pu. The molecular orbital analysis indicates that these metal-metal interactions possess predominantly d(z2) (sigma), d(xz) and d(yz) (pi), or d(xy) and d(x2-y2) (delta) character in the d-block species, and f(z3) (sigma), f(z2x) and f(z2y) (pi), or f(xyz) and f(z) (delta) character in the actinide systems. In the latter, all three (sigma, pi, delta) types of interaction exhibit bonding character, irrespective of whether the molecular symmetry is D(4h) or D(4d). By contrast, although the nature and properties of the sigma and pi bonds are largely similar for the D(4h) and D(4d) forms of the d-block complexes, the two most relevant metal-metal delta-like orbitals occur as a bonding and antibonding combination in D(4h) symmetry but as a nonbonding level in D(4d) symmetry. Multiconfigurational calculations have been performed on a subset of the actinide complexes, and show that a single electronic configuration plays a dominant role and corresponds to the lowest-energy configuration obtained using density functional theory.     

The extraordinary ability of guanidinate derivatives to stabilize higher oxidation numbers in dimetal units by modification of redox potentials: structures of Mo(2)(5+) and Mo(2)(6+) compounds

Full characterization of the first homologous series of dimolybdenum paddlewheel compounds having electronic configurations of the types sigma(2)pi(4)delta(x), x = 2, 1, 0, and Mo-Mo bond orders of 4, 3.5, and 3, respectively, has been accomplished with the guanidinate-type ligand hpp (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). Essentially quantitative oxidation of Mo(2)(hpp)(4), 1, by CH(2)Cl(2) gives Mo(2)(hpp)(4)Cl, 2. The halide in 2 can be replaced by reaction with TlBF(4) to produce Mo(2)(hpp)(4)(BF(4)), 3. Further oxidation of 2 by AgBF(4) produces Mo(2)(hpp)(4)ClBF(4), 4. The change from bond order 4 (in 1) to 3.5 in Mo(2)(hpp)(4)Cl is accompanied by an increase in the Mo-Mo bond length of 0.061 to 2.1280(4) A. A further increase of 0.044 A in the Mo-Mo distance to 2.172(1) A is observed as the bond order decreases to 3 in 4. At the same time, the Mo-N distances decrease smoothly as the oxidation state of the Mo atoms increases. Electrochemical studies have shown two chemically reversible processes at very negative potentials, E(1)(1/2)= -0.444 V and E(2)(1/2)= -1.271 V versus Ag/AgCl. These correspond to the processes Mo(2)(6+/5+) and Mo(2)(5+/4+), respectively. The latter potential is displaced by over 1.5 V relative to those of the Mo(2)(formamidinate)(4) compounds and the first one has never been observed in such complexes. Thus, in surprising contrast to previously observed behavior of the dimolybdenum unit, when it is surrounded by the very basic guanidinate ligand hpp, there is an extraordinary stabilization of the higher oxidation numbers of the molybdenum atoms.     

Solvent effects on the electrochemistry and spectroelectrochemistry of diruthenium complexes. Studies of Ru2(L)4Cl where L=2-CH3ap, 2-Fap, and 2,4,6-F3ap, and ap is the 2-anilinopyridinate anion

Three Ru2(5+) diruthenium complexes, (4,0) Ru2(2-CH3ap)4Cl, (3,1) Ru2(2-Fap)4Cl, and (3,1) Ru2(2,4,6-F3ap)4Cl where ap is the 2-anilinopyridinate anion, were examined as to their electrochemical and spectroelectrochemical properties in five different nonaqueous solvents (CH2Cl2, THF, PhCN, DMF, and DMSO). Each compound undergoes a single one-electron metal-centered oxidation in THF, DMF, and DMSO and two one-electron metal-centered oxidations in CH2Cl2 and PhCN. The three diruthenium complexes also undergo two reductions in each solvent except for CH2Cl2, and these electrode processes are assigned as Ru2(5+/4+) and Ru2(4+/3+). Each neutral, singly reduced, and singly oxidized species was characterized by UV-vis thin-layer spectroelectrochemistry, and the data are discussed in terms of the most probable electronic configuration of the compound in solution. The three neutral complexes contain three unpaired electrons as indicated by magnetic susceptibility measurements using the Evans method (3.91-3.95 muB), and the electronic configuration is assigned as sigma2pi4delta2pi(*2)delta, independent of the solvent. The three singly oxidized compounds have two unpaired electrons in CD2Cl2, DMSO-d6, or CD3CN (2.65-3.03 muB), and the electronic configuration is here assigned as sigma2pi4delta2pi(*2). The singly reduced compound also has two unpaired electrons (2.70-2.80 muB) in all three solvents, consistent with the electronic configuration sigma2pi4delta2pi(*2)delta(*2) or sigma2pi4delta2pi(*3)delta*. Finally, the overall effect of solvent on the number of observed redox processes is discussed in terms of solvent binding, and several formation constants were calculated.     

Facilitating access to the most easily ionized molecule: an improved synthesis of the key intermediate, W2(hpp)4Cl2, and related compounds

A far superior synthesis is reported for W(2)(hpp)(4)Cl(2), a key intermediate in the synthesis of the most easily ionized closed-shell molecule W(2)(hpp)(4) (hpp = the anion of the bicyclic guanidine compound 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). At 200 degrees C, the one-pot reaction of the air-stable and commercially available compounds W(CO)(6) and Hhpp in o-dichlorobenzene produces W(2)(hpp)(4)Cl(2) in multigram quantities with isolated yields of over 90%. At lower temperatures, the reaction can lead to other compounds such as W(Hhpp)(2)(CO)(4) or W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2), which are isolable in good purity depending upon the specific conditions employed. These compounds provide insight into the reaction pathway to W(2)(hpp)(4)Cl(2) and W(2)(hpp)(4). Two additional derivatives, W(2)(hpp)(4)X(2) where X is PF(6)(-) or the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), have also been synthesized and structurally characterized. A comparison of the electrode potentials of W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2) and the di-p-anisylformamidinate analogue shows that oxidation of the hpp compound is significantly displaced (1.12 V) and shows that the bicyclic guanidinate ligand is considerably better than the formamidinate anion at stabilizing high oxidation states. A differential pulse voltammogram of W(2)(hpp)(4)(TFPB)(2) in THF shows two reduction processes with an E(1/2) of -0.97 V for the first and -1.81 V (vs Ag/AgCl) for the second. DFT calculations on the W(2)(hpp)(4)(2+) units in W(2)(hpp)(4)X(2) compounds show that the metal-metal bonding orbitals are destabilized by the axial ligands, which accounts for significant variations in the W-W distances. The low-energy gas-phase ionizations of W(2)(hpp)(4) are also reported and discussed.     

Structural and magnetic evidence concerning spin crossover in formamidinate compounds with ru2 5+ cores

The magnetic and structural properties of two Ru2(DArF)4Cl compounds, where DArF is the anion of a diaryl formamidine, are presented here. The compounds with Ar = p-anisyl and m-anisyl both show temperature dependence of chiT (chi = molar magnetic susceptibility), but for different reasons. For the para compound, there is a Boltzmann distribution between a pi*3 ground state and a delta*pi*2 upper state, and this is confirmed by a temperature dependence of the Ru-Ru bond length: 2.4471(5) A at 23 K and 2.3968(5) A at 300 K. For the meta compound, a delta*pi*2 configuration persists over the range of 23-300 K as shown by an invariant Ru-Ru bond length, but the chiT drops with decreasing temperature owing to zero-field splitting of a 4B2u ground state.     

Syntheses, structural determination, and electrochemistry of Ru(2)(Fap)(4)Cl and Ru(2)(Fap)(4)(NO)Cl

Ru(2)(Fap)(4)Cl and Ru(2)(Fap)(4)(NO)Cl, where Fap is the 2-(2-fluoroanilino)pyridinate anion, were synthesized, and their structural, electrochemical, and spectroscopic properties were characterized. Ru(2)(Fap)(4)Cl, which was obtained by reaction between Ru(2)(O(2)CCH(3))(4)Cl and molten HFap, crystallizes in the monoclinic space group P2(1)/c, with a = 11.2365(4) A, b = 19.9298(8) A, c = 19.0368(7) A, beta = 90.905(1) degrees, and Z = 4. The presence of three unpaired electrons on the Ru(2)(5+) core and the 2.2862(3) A Ru-Ru bond length for Ru(2)(Fap)(4)Cl are consistent with the electronic configuration (sigma)(2)(pi)(4)(delta)(2)(pi*)(2)(delta*)(1). The reaction between Ru(2)(Fap)(4)Cl and NO gas yields Ru(2)(Fap)(4)(NO)Cl, which crystallizes in the orthorhombic space group Pbca, with a = 10.0468(6) A, b = 18.8091(10) A, c = 41.7615(23) A, and Z = 8. The Ru-Ru bond length of Ru(2)(Fap)(4)(NO)Cl is 2.4203(8) A, while its N-O bond length and Ru-N-O bond angle are 1.164(8) A and 155.8(6) degrees, respectively. Ru(2)(Fap)(4)(NO)Cl can be formulated as a formal Ru(2)(II,II)(NO(+)) complex with a linear Ru-N-O group, and the proposed electronic configuration for this compound is (sigma)(2)(pi)(4)(delta)(2)(pi*)(3)(delta*)(1). The binding of NO to Ru(2)(Fap)(4)Cl leads to some structural changes of the Ru(2)(Fap)(4) framework and a stabilization of the lower oxidation states of the diruthenium unit. Also, IR spectroelectrochemical studies of Ru(2)(Fap)(4)(NO)Cl show that NO remains bound to the complex upon reduction and that the first reduction involves the addition of an electron on the diruthenium core and not on the NO axial ligand.     

Better understanding of the species with the shortest Re2(6+) bonds and related Re2)(7+) species with tetraguanidinate paddlewheel structures

A series of compounds has been made containing quadruply bonded Re2(hpp)4X2 species (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine), where X is CF3SO3 (1), CF3CO2 (2), and F (3). The distances of 2.1562(7), 2.1711(5), and 2.1959(4) A for 1-3 show significant effects of the sigma and pi electron donating ability of the axial ligands on the metal-metal distance. With the weakly coordinating triflate ligand the Re-Re distance is the shortest for any quadruple bonded species known. In addition to examining the effects of axial ligands on the Re2(hpp)42+ core, our study of the Re2(hpp)43+ core is being extended beyond the preliminary results previously reported in only one compound [Re2(hpp)4Cl2]PF6 (Dalton Trans. 2003, 1218). We now report the structural characterization by both X-ray and neutron diffraction of the compound [Re2(hpp)4F](TFPB)2, 4 (TFPB = the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), and a detailed study by EPR spectroscopy of [Re2(hpp)4Cl2]PF6 at 9.5, 34.5, and 95 GHz frequencies, using dilute fluid solutions, frozen glass, and neat powder, show that the unpaired electron in the [Re2(hpp)4Cl2]+ ion is in an MO of predominant metal character with little mixing from the guanidinate ligands.     

The metal-NO interaction in the redox systems [Cl5Os(NO)]n-, n = 1-3, and cis-[(bpy)2ClOs(NO)]2+/+: calculations, structural, electrochemical, and spectroscopic results

Experimental and computational results for the two-step redox system [Cl5Os(NO)]n- (n = 1-3) are reported and discussed in comparison to the related one-step redox systems [Cl5Ru(NO)]n- and [Cl5Ir(NO)]n- (n = 1, 2). The osmium system exhibits remarkably low oxidation and reduction potentials. The structure of the precursor (PPh4)2[Cl5Os(NO)] is established as an {MNO}6 species with almost linear OsNO arrangement at 178.1 degrees. Density-functional theory (DFT) calculations confirm this result, and a comparison of structures calculated for several oxidation states reveals an increased labilization of the trans-positioned M-Cl bond on reduction in the order M = Ir < Os < Ru. Accordingly, the intact reduced form [Cl5Os(NO)]3- could not be observed in fluid solution even on electrolysis at -70 degrees C in n-butyronitrile solution, as confirmed both by DFT calculations and by comparison with the electron paramagnetic resonance and infrared spectroelectrochemically characterized redox pairs cis-[(bpy)2ClOs(NO)]2+/+ and [(CN)5Os(NO)]2-/3-. The DFT calculations indicate that the oxidation of [Cl5Os(NO)]2- occurs largely on the metal, the highest occupied molecular orbital (HOMO) of the precursor being composed of Os 5d (58%) and Cl(eq) 3p orbitals (41%). As for the related [(CN)5Os(NO)]2-, the reduction is largely NO centered, the lowest unoccupied molecular orbital (LUMO) of [Cl5Os(NO)]2- has 61% pi*(NO) character with significant 5d Os contributions (34%). A rather large degree of metal-NO back-donation is estimated to occur in the {OsNO}7 configuration of [Cl5Os(NO)]3- which leads to an unusual low value of 1513 cm(-1) calculated for nu(NO), signifying contributions from an Os(III)(NO-) formulation. Detailed analyses of the conformational dependence of the g anisotropy suggest that the different reduced species reported previously for [Cl5Os(NO)]3- in AgCl host lattices may be distinct in terms of eclipsed or staggered conformations of the bent NO. axial ligand relative to the Os(II)Cl4 equatorial plane. The staggered form is calculated to be more stable by 105 cm(-1). The weak absorptions of [Cl5Os(NO)]2- at 573, 495, and 437 nm are assigned as MLCT/LLCT transitions to the doubly degenerate pi*(NO) LUMO. The oxidized form [Cl5Os(NO)]- contains Os(III) in an {OsNO}5 configuration with a spin density of 0.711 on Os. In all three states of [Cl5Os(NO)]n-, the N bonded form is vastly preferred over the NO-side-on bonded alternative.     

The first structurally confirmed paddlewheel compound with an M(2)(7+) core: [Os(2)(hpp)(4)Cl(2)](PF(6))

Oxidation of Os(2)(hpp)(4)Cl(2), 1 (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), with (FeCp(2))PF(6) produces air-stable [Os(2)(hpp)(4)Cl(2)]PF(6), 2. This is the first structurally confirmed metal-metal bonded paddlewheel compound having an M(2)(7+) core. The Os-Os distances for two crystalline forms, 2.2acetone and 2.hexane, are 2.3309(4) and 2.3290(6) A, respectively. EPR, (1)H NMR, and magnetization data indicate that 2 has an unpaired electron and an exceptionally low g value of 0.791 +/- 0.037. An electrochemical study shows that there is a quasireversible wave corresponding to a more highly oxidized species with an unprecedented Os(2)(8+) core.     

Oxalate-bridged dinuclear M2 units: dimers of dimers, cyclotetramers, and extended sheets (m = Mo, W, Tc, Ru, and Rh)

The electronic structures of D(4h)-M(2)(O(2)CH)(4) and the oxalate-bridged complexes D(2h)-[(HCO(2))(3)M(2)](2)(mu-O(2)CCO(2)) and D(4h)-[(HCO(2))(2)M(2)](4)(mu-O(2)CCO(2))(4) have been investigated by a symmetry analysis of their MM and oxalate-based frontier orbitals, as well as by electronic structure calculations on the model formate complexes (M = Mo and W {d(4)-d(4)}, Tc, Ru {d(6)-d(6)}, and Rh {d(7)-d(7)}). Significant changes in the ordering, interactions, and electronic occupation of the molecular orbitals (MOs) arise through both the progression from d(4) to d(7) metals and the change from second to third row transition metals. For M = Mo and W, the highest-occupied orbitals are delta based, while the lowest-unoccupied orbitals are oxalate pi based; for M = Tc, the highest-occupied orbitals are an energetically tight delta-based set of MOs, while the lowest-unoccupied orbitals are MM-based pi. For both Ru and Rh, the highest-occupied MOs are the MM pi* and delta*, respectively, while the lowest-unoccupied MOs, in both instances, are MM-based sigma. With the exception of M = Ru, all of the complexes are closed shell. From the progression M(2) --> [M(2)](2) --> [M(2)](4), we can envision the nature of bandlike structures for a 2-dimensional square grid of formula [M(2)(mu-O(2)CCO(2))](infinity). Only for Mo and W oxalates should good electronic communication between MM centers generate a band of significant width to lead to metallic conductivity upon oxidation.     

Expeditious access to the most easily ionized closed-shell molecule, W2(hpp)4

A new synthetic path, far superior to either of those previously available, to the W2(hpp)4 molecule (Hhpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) is reported. The reaction of W(CO)6 with Hhpp in o-dichlorobenzene at 200 degrees C produces W2(hpp)4Cl2 in a one-pot reaction in over 90% yield. This compound is stable and easily stored for further use, and it can be efficiently reduced in a one-step reaction to the title compound W2(hpp)4.     

Electronic Structure of [Pt(2)(&mgr;-O(2)CCH(3))(4)(H(2)O)(2)](2+) Using the Quasi-Relativistic Xalpha-SW Method: Analysis of Metal-Metal Bonding, Assignment of Electronic Spectra, and Comparison with Rh(2)(&mgr;-O(2)CCH(3))(4)(H(2)O)(2)

The electronic structure and metal-metal bonding in the classic d(7)d(7) tetra-bridged lantern dimer [Pt(2)(O(2)CCH(3))(4)(H(2)O)(2)](2+) has been investigated by performing quasi-relativistic Xalpha-SW molecular orbital calculations on the analogous formate-bridged complex. From the calculations, the highest occupied and lowest unoccupied metal-based levels are delta(Pt(2)) and sigma(Pt(2)), respectively, indicating a metal-metal single bond analogous to the isoelectronic Rh(II) complex. The energetic ordering of the main metal-metal bonding levels is, however, quite different from that found for the Rh(II) complex, and the upper metal-metal bonding and antibonding levels have significantly more ligand character. As found for the related complex [W(2)(O(2)CH)(4)], the inclusion of relativistic effects leads to a further strengthening of the metal-metal sigma bond as a result of the increased involvement of the higher-lying platinum 6s orbital. The low-temperature absorption spectrum of [Pt(2)(O(2)CCH(3))(4)(H(2)O)(2)](2+) is assigned on the basis of Xalpha-SW calculated transition energies and oscillator strengths. Unlike the analogous Rh(II) spectrum, the visible and near-UV absorption spectrum is dominated by charge transfer (CT) transitions. The weak, visible bands at 27 500 and 31 500 cm(-)(1) are assigned to Ow --> sigma(Pt(2)) and OAc --> sigma(Pt(2)) CT transitions, respectively, although the donor orbital in the latter transition has around 25% pi(Pt(2)) character. The intense near-UV band around 37 500 cm(-)(1) displays the typical lower energy shift as the axial substituents are changed from H(2)O to Cl and Br, indicative of significant charge transfer character. From the calculated oscillator strengths, a number of transitions, mostly OAc --> sigma(Pt-O) CT in nature, are predicted to contribute to this band, including the metal-based sigma(Pt(2)) --> sigma(Pt(2)) transition. The close similarity in the absorption spectra of the CH(3)COO(-), SO(4)(2)(-), and HPO(4)(2)(-) bridged Pt(III) complexes suggests that analogous spectral assignments should apply to [Pt(2)(SO(4))(4)(H(2)O)(2)](2)(-) and [Pt(2)(HPO(4))(4)(H(2)O)(2)](2)(-). Consequently, the anomalous MCD spectra reported recently for the intense near-UV band in the SO(4)(2)(-) and HPO(4)(2)(-) bridged Pt(III) complexes can be rationalized on the basis of contributions from either SO(4) --> sigma(Pt-O) or HPO(4) --> sigma(Pt-O) CT transitions. The electronic absorption spectrum of [Rh(2)(O(2)CCH(3))(4)(H(2)O)(2)] has been re-examined on the basis of Xalpha-SW calculated transition energies and oscillator strengths. The intense UV band at approximately 45 000 cm(-)(1) is predicted to arise from several excitations, both metal-centered and CT in origin. The lower energy shoulder at approximately 40 000 cm(-)(1) is largely attributed to the metal-based sigma(Rh(2)) --> sigma(Rh(2)) transition.     

Generation of one light-induced metastable nitrosyl linkage isomer in [Pt(NH3)4Cl(NO)]Cl2 in the red spectral range

One metastable linkage nitrosyl isomer can be generated in [Pt(NH(3))(4)Cl(NO)]Cl(2) by irradiation with light in the red spectral range. The potential energy barrier for the thermal relaxation of the metastable state to the ground state has an amount of E(A) = (0.27 +/- 0.03) eV. The decay follows the Arrhenius law and E(A) is independent of temperature. At room temperature the metastable state has a lifetime of tau = 3.8 x 10(-5) s after generation by pulsed laser illumination. Below T = 100 K about 30% linkage NO isomers can be generated in a powder sample by irradiation with lambda = 658 nm. DFT calculations demonstrate the rotation of the NO ligand from Pt-N-O to Pt-O-N as a unique linkage isomer. Consequently, only one new nu(NO) stretching vibration is detected with a shift from 1673 cm(-1) to 1793 cm(-1) by 120 cm(-1), to higher frequencies in good agreement with the DFT calculations. In the metastable state new electronic absorption bands are observed in the blue-green and near infrared spectral range. The metastable state can be optically accessed via a (5d + pi(NO)) -->pi*(NO) transition. [Pt(NH(3))(4)Cl(NO)]Cl(2) is diamagnetic with a Pt(5d(8)) configuration and thus represents the first {MNO}(8) complex with experimental evidence for a light-induced nitrosyl linkage isomer.     

Molecular and electronic structures of bis-(o-diiminobenzosemiquinonato)metal(II) complexes (Ni, Pd, Pt), their monocations and -anions, and of dimeric dications containing weak metal-metal bonds

Two series of square planar, diamagnetic, neutral complexes of nickel(II), palladium(II), and platinum(II) containing two N,N-coordinated o-diiminobenzosemiquinonate(1-) pi radical ligands have been synthesized and characterized by UV-vis and (1)H NMR spectroscopy: [M(II)((2)L(ISQ))(2)], M = Ni (1), Pd (2), Pt (3), and [M(II)((3)L(ISQ))(2)] M = Ni (4), Pd (5), Pt (6). H(2)[(2)L(PDI)] represents 3,5-di-tert-butyl-o-phenylenediamine and H(2)[(3)L(PDI)] is N-phenyl-o-phenylenediamine; (L(ISQ))(1-) is the o-diiminobenzosemiquinonate pi radical anion, and (L(IBQ))(0) is the o-diiminobenzoquinone form of these ligands. The structures of complexes 1, 4, 5, and 6 have been (re)determined by X-ray crystallography at 100 K. Cyclic voltammetry established that the complete electron-transfer series consisting of a dianion, monoanion, neutral complex, a mono- and a dication exists: [M(L)(2)](z)z = -2, -1, 0, 1+, 2+. Each species has been electrochemically generated in solution and their X-band EPR and UV-vis spectra have been recorded. The oxidations and reductions are invariably ligand centered. Two o-diiminobenzoquinones(0) and two fully reduced o-diiminocatecholate(2-) ligands are present in the dication and dianion, respectively, whereas the monocations and monoanions are delocalized mixed valent class III species [M(II)(L(ISQ))(L(IBQ))](+) and [M(II)(L(ISQ))(L(PDI))](-), respectively. One-electron oxidations of 1 and trans-6 yield the diamagnetic dications [cis-[Ni(II)((2)L(ISQ))((2)L(IBQ))](2)]Cl(2) (7) and [trans-[Pt(II)((3)L(ISQ))((3)L(IBQ))](2)](CF(3)SO(3))(2) (8), respectively, which have been characterized by X-ray crystallography; both complexes possess a weak M.M bond and the ligands adopt an eclipsed configuration due to weak bonding interactions via pi stacking.     

Pt(II) mono-carbonyl complexes of a cyclometallating 2-(2'-thienyl)-(pyridinato-C,3N') ligand: nature and dynamics of the lowest excited state of the chloro- and thiolato-complexes

The synthesis of a cyclometallated Pt(II) thiolate carbonyl complex Pt(thpy)(CO)(mts), (thpy = 2-(2'-thienyl)-pyridinate, mts = methylthiosalicylate) is reported. A combination of emission and time-resolved infrared (TRIR) techniques revealed for both Pt(thpy)(CO)(mts) and its chloride analogue Pt(thpy)(CO)Cl the predominant intra 2-(2'-thienyl)-pyridinate 3pi pi* character of the lowest electronic excited state. The unusually short lifetime (780 ps) of the intraligand 3pi pi* lowest excited state of Pt(thpy)(CO)(mts) indicates that this electronic state is influenced by another close-lying excited state, probably charge-transfer in origin.     

Proof of large positive zero-field splitting in a Ru2 5+ paddlewheel

We present the synthesis, as well as the structural and magnetic characterization, of [Ru2(D(3,5-Cl2Ph)F)4Cl(0.5H2O)].C6H14 (D(3,5-Cl2Ph)F = N,N'-di(3,5-dichlorophenyl)formamidinate), a Ru2(5+) compound having a 4B(2u) ground state derived from a sigma2pi4delta2pi2delta electron configuration. The persistence of this configuration from 27 to 300 K is shown by the invariance of the Ru-Ru distance. Orientation-dependent magnetic susceptibility (chiT) and magnetization (M(H)) data are in accord with a spin quartet ground state with large magnetocrystalline anisotropy associated with a large axial zero-field splitting (D) parameter. Theoretical fits to chiT and M(H) plots yielded D/kB = +114 K, implying an S = +/-1/2 Kramers doublet ground state at low temperature. Single-crystal and powder EPR data are consistent with this result, as the only observed transition is between the M(s) = +/1/2 Zeeman levels. The g values are g(perpendicular) = 2.182, g(parallel) = 1.970, and D = 79.8 cm(-1). The totality of the results demands D > 0.