Manipulating magnetism: Ru25⁺ paddlewheels devoid of axial interactions.

Variable-temperature magnetic and structural data of two pairs of diruthenium isomers, one pair having an axial ligand and the formula Ru2(DArF)4Cl (where DArF is the anion of a diarylformamidine isomer and Ar = p-anisyl or m-anisyl) and the other one being essentially identical but devoid of axial ligands and having the formula [Ru2(DArF)4]BF4, show that the axial ligand has a significant effect on the electronic structure of the diruthenium unit. Variable temperature crystallographic and magnetic data as well as density functional theory calculations unequivocally demonstrate the occurrence of π interactions between the p orbitals of the chlorine ligand and the π* orbitals in the Ru2(5+) units. The magnetic and structural data are consistent with the existence of combined ligand σ/metal σ and ligand pπ/metal-dπ interactions. Electron paramagnetic resonance data show unambiguously that the unpaired electrons are in metal-based molecular orbitals.

A strong metal-to-metal interaction in an edge-sharing bioctahedral compound that leads to a very short tungsten-tungsten double bond.

An ionic edge-sharing bioctahedral (ESBO) species has been prepared having a tetramethylated bicyclic guanidinate with two fused six-membered rings characterized by a fairly flat N-C(N)-N skeleton and abbreviated as TMhpp. The anion has two W(IV) atoms bridged by two oxo groups; the metal atoms are also spanned by two bridging guanidinate ligands, and each has two monodentate chlorine atoms. The complex formulated as (H2TMhpp)2[W(µ-O)(µ-TMhpp)Cl2]2 has the shortest W-W distance (2.3318(8) Å) of any species with a σ(2)π(2) electronic configuration. The anion and cations are connected by hydrogen bonds. To unambiguously ascertain the existence of the double-bonded W2(µ-O)2 entity, density functional theory calculations and natural bond orbital analyses were done on an analogous but hypothetical species with a W2(µ-OH)2 core having trivalent tungsten atoms and a possible σ(2)π(2)δ(2) electronic configuration. The calculations decidedly support the presence of tungsten-oxo instead of tungsten-hydroxo groups and thus the existence of the double-bonded W2(µ-O)2 core. The strong bonding interaction between metal atoms is a clear indication that under certain circumstances the two octahedra in ESBO species do not behave as the sum of two mononuclear compounds.

Solubilizing the most easily ionized molecules and generating powerful reducing agents.

Two very soluble compounds having W2(bicyclic guanidinate)4 paddlewheel structures show record low ionization energies (onsets at 3.4 to 3.5 eV) and very negative oxidation potentials in THF (-1.84 to -1.90 V vs Ag/AgCl). DFT computations show the correlation from the gas-phase ionization energies to the solution redox potentials and chemical behavior. These compounds are thermally stable and easy to synthesize in high yields and good purity. They are very reactive and potentially useful stoichiometric reducing agents in nonpolar, nonprotonated solvents.

Direct evidence from electron paramagnetic resonance for additional configurations in uncommon paddlewheel Re2(7+) units surrounded by an unsymmetrical bicyclic guanidinate.

Three rare compounds have been synthesized and structurally characterized; these species have paddlewheel structures and Re(2)(7+) cores surrounded by four bicyclic guanidinates and two axial ligands along the Re-Re axis. Each possesses a formal bond order of 3.5 and a σ(2)π(4)δ(1) electronic configuration that entails the presence of one unpaired electron for each compound. The guanidinate ligands characterized by having CH(2) entities and a central C(N)(3) unit that joins two cyclic units--one having two fused 6-membered rings (hpp) and the other having a 5- and a 6-membered ring fused together (tbn)--allowed the isolation of [Re(2)(tbn)(4)Cl(2)]PF(6), 1, [Re(2)(tbn)(4)Cl(2)]Cl, 2, and [Re(2)(hpp)(4)(O(3)SCF(3))(2)](O(3)SCF(3)), 3. Because of the larger bite angle of the tbn relative to the hpp ligand, the Re-Re bond distances in 1 and 2 (2.2691(14) and 2.2589(14) Å, respectively) are much longer than that in 3 (2.1804(8) Å). Importantly, electron paramagnetic resonance (EPR) studies at both X-band (~9.4 GHz) and W-band (112 GHz) in the solid and in frozen solution show unusually low g-values (~1.75) and the absence of zero-field splitting, providing direct evidence for the presence of one metal-based unpaired electron for both 1 and 3. These spectroscopic data suggest that the unsymmetrical 5-/6-membered ligand leads to the formation of isomers, as shown by significantly broader EPR signals for 1 than for 3, even though both compounds possess what appears to be similar ideal crystallographic axial symmetry on the X-ray time scale.

Tuning the electrochemistry of Re2(6+) species with divergent bicyclic guanidinate ligands and by modification of axial π interactions.

Four Re(2)(6+) paddlewheel compounds with equatorial bicyclic guanidinate ligands and two monodentate anions in axial positions show a large change in the metal-metal distance that depends on the bite angle of the ligands and whether there are pi interactions between the dimetal unit and the axial ligands. These processes are accompanied by significant changes in the redox behavior. The two pairs of compounds that have been synthesized are Re(2)(tbn)(4)Cl(2), 1, and Re(2)(tbn)(4)(SO(3)CF(3))(2), 2, as well as Re(2)(tbo)(4)Cl(2), 3, and Re(2)(tbo)(4)(SO(3)CF(3))(2), 4, where tbn is the anion of a bicyclic guanidinate with six- and five-membered rings (1,5,7-triazabicyclo[4.3.0]non-6-ene) and tbo is an analogous species with two five-membered rings (the anion of 1,4,6-triazabicyclo[3.3.0]oct-4-ene). For both 1 and 2 as well as for 3 and 4, the metal-metal distances are shorter for the triflate species than for the chloride analogues because of the π interactions of the Cl with the π bonds of the triply bonded Re(2)(6+) cores compounded by a small but symmetry allowed interaction between the antisymmetric combination of the filled σp orbitals of the chlorine atom and the empty σ* orbital of the metal atoms. In addition there is a significant increase in the Re-Re distance from that in the six/five tbn-membered ring to the five/five-membered tbo species. Electrochemical measurements show two redox processes for each set of compounds corresponding to the uncommon Re(2)(6+) → Re(2)(7+) and Re(2)(7+) → Re(2)(8+) processes, which are strongly affected by the bite angle of the guanidinate ligand as well as the ability of the axial ligands to interact with the π orbitals of the dirhenium unit. For 1 and 3, the first redox couples are at 0.146 and 0.487 V, respectively, while for 2 and 4 these are at 0.430 and 0.698 V, respectively.

Nature of bonding in complexes containing "supershort" metal-metal bonds. raman and theoretical study of M2(dmp)4 [M = Cr (natural abundance Cr, 50Cr, and 54Cr) and Mo; dmp = 2,6-dimethoxyphenyl].

We report an investigation of complexes of the type M(2)(dmp)(4) (M = Mo, Cr; dmp = 2,6-dimethoxyphenyl) using resonance Raman (RR) spectroscopy, Cr isotopic substitution, and density functional theory (DFT) calculations. Assignment of the Mo-Mo stretching vibration in the Mo(2) species is straightforward, as evidenced by a single resonance-enhanced band at 424 cm(-1), consistent with an essentially unmixed metal-metal stretch, and overtones of this vibration. On the other hand, the Cr(2) congener has no obvious metal-metal stretching mode near 650-700 cm(-1), where empirical predictions based on the Cr-Cr distance as well as DFT calculations suggest that this vibration should appear if unmixed. Instead, three bands are observed at 345, 363, and 387 cm(-1) that (a) have relative RR intensities that are sensitive to the Raman excitation frequency, (b) exhibit overtones and combinations in the RR spectra, and (c) shift in frequency upon isotopic substitution ((50)Cr and (54)Cr). DFT calculations are used to model the vibrational data for the Mo(2) and Cr(2) systems. Both the DFT results and empirical predictions are in good agreement with experimental observations in the Mo(2) complex, but both, while mutually consistent, differ radically from experiment in the Cr(2) complex. Our experimental and theoretical results, especially the Cr isotope shifts, clearly demonstrate that the potential energy of the Cr-Cr stretching coordinate is distributed among several normal modes having both Cr-Cr and Cr-ligand character. The general significance of these results in interpreting spectroscopic observations in terms of the nature of metal-metal multiple bonding is discussed.

Large changes in electronic structures of Ru2(6+) species caused by the variations of the bite angle of guanidinate ligands: tuning magnetic behavior.

Syntheses and characterization of two Ru(2)(6+) paddlewheel compounds having very different magnetic behavior are reported. The compounds Ru(2)(tbn)(4)Cl(2), 1, and Ru(2)(tbo)(4)Cl(2), 2 (where tbn = the anion of 1,5,7-triazabicyclo[4.3.0]non-6-ene and tbo = the anion of 1,4,6-triazabicyclo[3.3.0]oct-4-ene), have four equatorial bicyclic guanidinate ligands and two chloride ions in axial positions. They show large disparity in Ru-Ru distances of about 0.11 A (2.389(3) and 2.499(3) A at 30 K for 1 and 2, respectively) that is attributed to the divergence in the bite angle of the ligand. Variable temperature structural data show no significant changes in the Ru-Ru distances between 30 and 213 K suggesting that the electronic structure remains unchanged in this temperature range for both compounds. Magnetic studies of 1 indicate there are two unpaired electrons at room temperature but the compound behaves as essentially diamagnetic at approximately 2 K. Compound 2 is non-magnetic across all temperatures in the range of 2 to 300 K. Density functional theory calculations suggest a pi(4)pi*(4)delta(2) electronic configuration for 2, while the magnetic behavior and structural data for 1 are consistent with a sigma(2)pi(4)delta(2)pi*(2) electronic configuration. This shows the importance of the ligand bite angle in determining the electronic configuration of the diruthenium unit and a way to tune magnetic behavior.

Proof by EPR spectroscopy that the unpaired electron in an Os(2)(7+) species is in a delta* metal-based molecular orbital.

Variable temperature structural and EPR studies are reported on the paddlewheel compound [Os(2)(hpp)(4)Cl(2)]PF(6), 1, (hpp = the anion of the bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) that contains a rare M(2)(7+) species, with the goal of determining whether the unpaired electron resides in a metal- or ligand-based molecular orbital. Crystallographic studies show that the Os-Os distance in 1 remains essentially unchanged from 213 to 30 K, which is consistent with no changes in electronic structure in this range of temperature. It is noteworthy that the metal-metal distance in 1 is about 0.05 A shorter than that in the precursor Os(2)(hpp)(4)Cl(2), which is consistent with the loss of an electron in a delta* orbital. EPR spectra of 1 were measured in dilute frozen solution, powder, and single crystals. The spectra were observable only below about 50 K, with an exceptionally large line width, approximately 3,750 gauss, for a powdered sample, due to dipolar interactions and to short relaxation times. There is a very small average g value of approximately 0.750 and a cylindrical symmetry about the Os-Os bond. These data are consistent with the unpaired electron orbital having a large L value, such as that of a delta* orbital. The combination of X-ray structural data, the short relaxation time, and the magnetic data provide strong evidence that the unpaired electron in this nine-electron Os(2)(7+) species is localized in a metal-based orbital with this electron residing predominantly in a delta* orbital rather than in a pi* orbital and, thus, having an electronic configuration of sigma(2)pi(4)delta(2)delta*.

Influence of bonding mode of the linkers in the electronic communication of molecular pairs having dimolybdenum units linked by pseudohalides.

Depending on conditions the reactions of [Mo(2)(cis-DAniF)(2)(NCCH(3))(4)](BF(4))(2) (DAniF = N,N'-di-p-anisylformamidinate) with solutions containing thiocyanate anions lead to two compounds: a quadruple-bonded dinuclear species 1, (Bu(n)(4)N)(2)[Mo(2)(cis-DAniF)(2)(NCS)(4)], and a molecular pair 3, [Mo(2)(cis-DAniF)(2)](2)(mu(1,3)-NCS)(4). The latter has a cuboidal structure having two (cis-DAniF)(2)Mo(2)(2+) units, [Mo(2)], with four thiocyanate groups bridging two [Mo(2)] units in an end-to-end fashion in which the N and S atoms serve as the bridging units. On the contrary, the structure of the cyanate isomer, [Mo(2)(cis-DAniF)(2)](2)(mu(1,1)-NCO)(4) (2), shows an end-on binding of the cyanate linkers. Various physical measurements of 2 and its oxidized species 2.PF(6) indicate that there is strong electronic communication between the two dimetal cores. For 1, two reversible oxidation processes were observed in the cyclic voltammogram corresponding to the successive oxidation to 1(+) and an uncommon 1(2+) species that has a triple-bonded Mo(2)(6+) core. DFT calculations indicate that the antibonding character between the Mo-Mo delta orbitals and thiocyanate p orbitals plays a very important role in elevating the HOMO delta orbital energy that allows formation of the dication. A selenium isomer of 3 was also studied. In both the thiocyanate and selenocyanate bridged dimers of dimers, in which the pseudohalide bridges bind the two dimetal units in an end-to-end fashion, long separations between the dimetal units are observed, and these generate very weak electronic interactions.

Evidence of disruption of conjugation involving delta bonds in intramolecular electronic coupling.

A dimer of dimers containing two quadruply bonded [Mo(2)(DAniF)(3)](+) units (DAniF = N,N'-di(p-anisyl)formamidinate) linked by the S-donor linker, dimethyldithiooxamidate was synthesized, structurally characterized, and electronic communication was probed. The core of [Mo(2)(DAniF)(3)](2)(C(2)S(2)N(2)Me(2)), 1, formed by the Mo(2)NSC(2)SNMo(2) atoms shows two fused but non planar six-membered rings, which differs from that of the beta form of dimethyloxamidate analogue that has a heteronaphthalene-type structure (Cotton, F. A.; Liu, C. Y.; Murillo, C. A.; Villagran, D.; Wang, X. J. Am. Chem. Soc. 2004, 126, 14822). For these two analogous compounds electronic coupling between the two [Mo(2)] units, as determined by electrochemical measurements, diminishes considerably upon replacement of O-donor by S-donor atoms (DeltaE(1/2) = 531 mV and 440 mV, respectively). This suggests that the non planar conformation of the linker in 1 hampers a pathway leading to pi conjugation. Density functional theory (DFT) calculations show that the highest occupied molecular orbitals HOMO-HOMO-1 energy gap of 0.12 eV for 1 is much smaller than that of 0.61 eV for the O-donor analogue, which is consistent with the electrochemical data.

Perfluorinated polycyclic aromatic hydrocarbons: anthracene, phenanthrene, pyrene, tetracene, chrysene, and triphenylene.

The properties of perfluoroanthracene (1-C(14)F(10)), perfluorophenanthrene (2-C(14)F(10)), perfluoropyrene (C(16)F(10)), perfluorotetracene (1-C(18)F(12)), perfluorochrysene (2-C(18)F(12)), and perfluorotriphenylene (3-C(18)F(12)) and their radical anions have been studied using density functional theory (DFT). Three measures of neutral-anion energy separations reported in this work are the adiabatic electron affinity (EA(ad)), the vertical electron affinity (EA(vert)), and the vertical detachment energy (VDE). The vibrational frequencies of these perfluoro PAHs and their radical anions are also examined. The predicted adiabatic electron affinities (DZP++ B3LYP) are: 1.84 eV, 1-C(14)F(10); 1.41 eV, 2-C(14)F(10); 1.72 eV, C(16)F(10); 2.39 eV, 1-C(18)F(12); 1.83 eV (C(i) symmetry) and 1.88 eV (C(2) symmetry), 2-C(18)F(12); and 1.69 eV, 3-C(18)F(12). The perfluorotetracene is clearly the most effective electron captor. Perfluorophenanthrene, perfluoropyrene, perfluorochrysene, and perfluorotriphenylene, as well as their radical anions deviate from planarity. For example, the nonplanar perfluorochrysene structures are predicted to lie 7-13 kcal/mol below the pertinent C(2h) stationary points.

Exceptionally strong electronic coupling between [Mo2] units linked by substituted dianionic quinones.

Ligands derived from N-CH(3) substituted benzoquinonemonoimines are exceedingly good facilitators of electronic communication between two quadruply bonded dimolybdenum units and provide record values for comproportionation constants.

Tetra-kis(1,3,4,6,7,8-hexa-hydro-2H-pyrimido[1,2-a]pyrimidin-9-ido-κN,N)niobium(V) hexa-fluorido-phosphate.

The title complex, [Nb(C(7)H(12)N(3))(4)]PF(6), features chelating hpp anions (hpp is 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrim-idine) that define a distorted dodeca-hedral coordination geometry based on an N(8) donor set. The Nb atom is situated on a site of symmetry , and the PF(6) (-) anion has crystallographic fourfold symmetry.

Photoelectron spectroscopy and DFT calculations of easily ionized quadruply bonded Mo2(4+) compounds and their bicyclic guanidinate precursors.

A series of five bicyclic guanidinate compounds containing various combinations of five- and six-membered rings and substituted alkyl groups have been shown by photoelectron spectroscopy to be easily ionized, with the one having two six-membered rings and four ethyl groups being the most easily ionized. The corresponding anions are capable of forming paddlewheel compounds having quadruply bonded Mo2(4+) units which are also easy to ionize. The most easily ionized compound is the ethyl-substituted Mo2(TEhpp)4 complex which has a broad first ionization band centered around 4.27 +/- 0.03 eV and an ionization onset at the very low energy of 3.93 +/- 0.03 eV. Even the compound with ligands containing two five-membered rings, which favors a long Mo-Mo separation because of the large ligand bite, has an ionization energy (4.78 eV) that is less than those of well-known organometallic reducing agents such as (eta5-C9Me7)2Co and (eta5-C5Me5)2Cr.

Remarkable electron accepting properties of the simplest benzenoid cyanocarbons: hexacyanobenzene, octacyanonaphthalene and decacyanoanthracene.

The optimised structures, electron affinities, and vibrational frequencies of the simplest benzenoid cyanocarbons, namely hexacyanobenzene C6(CN)6, octacyanonaphthalene C10(CN)8, and decacyanoanthracene C14(CN)10, have been studied using carefully calibrated density functional methods (Chem. Rev., 2002, 102, 231-282); the predicted adiabatic electron affinities are 3.53 eV for C6(CN)6, 4.35 eV for C10(CN)8 and 5.02 eV for C14(CN)10, which are significantly larger than those of the analogous benzenoid fluorocarbons as well as tetracyanoethane and tetracyanoquinodimethane.

The radical anions and the electron affinities of perfluorinated benzene, naphthalene and anthracene.

Although benzene and naphthalene do not have electron affinities in the conventional sense, perfluorobenzene and perfluoronaphthalene have nonzero electron affinities. Theoretical methods extensively calibrated with experiment for the prediction of electron affinities (EAs) predict the EAs of perfluorobenzene, perfluoronaphthalene and perfluoroanthracene as 0.69, 1.02 and 1.84 eV, respectively. A rough estimate of 2.39 eV is made for the electron affinity of perfluorotetracene. Thus the perfluoro polycyclic aromatic hydrocarbons (PAHs) are predicted to be effective electron acceptors.

A mixed-valence compound with one unpaired electron delocalized over four molybdenum atoms in a cyclic tetranuclear ion.

The first oxidation of a species derived from a compound having two linked, quadruply-bonded Mo2 units has been performed and [[cis-Mo2(DAniF)2]2(mu-Cl)4]PF6, 2, has been isolated and characterized in many ways; it has one unpaired electron and a fully delocalized structure where the Mo-Mo distances increase from 2.1191(4) A in the reduced species to 2.1453(3) A in 2 and the Kc of 1.3 x 10(9) is three orders of magnitude larger than that of the Creutz-Taube ion.

Mononuclear Molybdenum(IV) Complexes with Two Multiply Bonded Chalcogen Ligands in Trans Configuration and Chelating Biphosphine Ligands.

A series of molybdenum(IV) complexes of the type trans-Mo(Q)(Q')(P&arcraise;P)(2) has been prepared where Q and Q' are chalcogen ligands (O, S, Se, Te) and P&arcraise;P is either cis-1,2-bis(diphenylphosphino)ethylene (dppee) or 1,2-bis(diphenylphosphino)ethane (dppe). X-ray crystallographic studies were carried out to investigate how the Mo-Q distance is influenced by the mutually competitive d(pi)-p(pi) interactions between the p(x)() and p(y)() orbitals of the chalcogen ligands and the d(xz)() and d(yz)() orbitals of the molybdenum center. trans-Mo(O)(2)(dppee)(2) (1) has been prepared by the hydrolysis and deprotonation reaction of [Mo(O)(Cl)(dppee)(2)]Cl with NaOH in methanol. The compounds where Q represents the heavier chalcogens (S, Se, Te) have been prepared by the reaction between trans-Mo(N(2))(2)(P&arcraise;P)(2) and a chalcogen source: trans-Mo(S)(2)(dppee)(2) (2), BzS(3)Bz (dibenzyl trisulfide); trans-Mo(Se)(2)(dppee)(2) (3), elemental Se; trans-Mo(Te)(2)(dppee)(2) (4), TePEt(3) (Et = ethyl); trans-Mo(S)(2)(dppe)(2) (5), BzS(3)Bz; trans-Mo(Se)(2)(dppe)(2) (6), Se. trans-Mo(O)(S)(dppee)(2) (7) was obtained from the reaction of MoCl(3)(THF)(3) (THF = tetrahydrofuran), dppee, and NaHS in a mixture of THF and methanol. The attempted preparation of 7 by the reaction of SO(2) and trans-Mo(N(2))(2)(dppee)(2) yielded Mo(SO(2))(2)(dppee)(2) (8). 1 crystallizes in the monoclinic space group P2(1)/c (No. 14, Z = 2) with a = 11.1340(15) Å, b = 18.435(2) Å, c = 12.515(2) Å, and beta = 110.999(9) degrees; 2 crystallizes in the triclinic space group P&onemacr; (No. 2, Z = 1) with a = 10.102(4) Å, b = 10.722(4) Å, c = 12.195(3) Å, alpha = 100.95(3) degrees, beta = 95.04(4) degrees, and gamma = 117.81(2) degrees; 3 crystallizes in the monoclinic space group P2(1)/c (No. 14, Z = 2) with a = 11.186(5) Å, b = 18.005(8) Å, c = 12.761(9) Å, and beta = 110.35(4) degrees; 4 crystallizes in the triclinic space group P&onemacr; (No. 2, Z = 2) with a = 12.681(4) Å, b = 19.280(5) Å, c = 10.454(3) Å, alpha = 104.60(2) degrees, beta = 111.61(2) degrees, and gamma = 75.12(2) degrees; 5 crystallizes in the monoclinic space group C2/c (No. 15, Z = 8) with a = 49.515(7) Å, b = 10.9286(12) Å, c = 18.203(3) Å, and beta = 98.306(12) degrees; 6 crystallizes in the monoclinic space group C2/c (No. 15, Z = 8) with a = 49.566(9) Å, b = 10.9765(15) Å, c = 18.282(3) Å, and beta = 98.541(13) degrees; 7 crystallizes in the triclinic space group P&onemacr; (No. 2, Z = 1) with a = 10.040(1) Å, b = 10.563(1) Å, c = 12.162(2) Å, alpha = 75.30(1) degrees, beta = 85.93(1) degrees, and gamma = 63.21(1) degrees; 8 crystallizes in the monoclinic space group C2/c (No. 15, Z = 4) with a = 21.534(6) Å, b = 12.4271(13) Å, c = 19.550(5) Å, and beta = 118.480(14) degrees. The UV/vis and the (31)P{(1)H} NMR data for compounds 1-7 are also reported and discussed.

Theoretical Study of Electronic Structures and Spectra of Chalcogenido Complexes of Molybdenum, trans-Mo(Q)(2)(PH(3))(4) (Q = O, S, Se, Te).

Electronic structures of the title complexes have been studied using quantum chemical computations by different methods. It is shown that the results of Xalpha calculations agree well with expectations from classical ligand-field theory, but both are far from being in agreement with the results given by ab initio calculations. The HOMO in the ab initio Hartree-Fock molecular orbital diagrams of all these complexes is a chalcogen p(pi) lone pair orbital rather than the metal nonbonding d(xy)() orbital previously proposed. Electronic transition energies were calculated by CASSCF and CI methods. The results suggest that in the cases when Q = S, Se, and Te the lowest energy transitions should be those from the p(pi) lone pair orbitals to the metal-chalcogen pi orbitals. The calculated and observed electronic spectra of the oxo complex are in good agreement and very different from the spectra of the other complexes, and the lowest absorptions were accordingly assigned to transitions of different origins.

Synthesis, Structure, and Reactivity of [Zr(6)Cl(18)H(5)](2)(-), the First Paramagnetic Species of Its Class.

Reaction of [Zr(6)Cl(18)H(5)](3)(-) (1) with 1 equiv of TiCl(4) yields a new cluster anion, [Zr(6)Cl(18)H(5)](2)(-) (2), which can be converted back into [Zr(6)Cl(18)H(5)](3)(-) (1) upon addition of 1 equiv of Na/Hg. Cluster 2 is paramagnetic and unstable in the presence of donor molecules. It undergoes a disproportionation reaction to form 1, some Zr(IV) compounds, and H(2). It also reacts with TiCl(4) to form [Zr(2)Cl(9)](-) (4) and a tetranuclear mixed-metal species, [Zr(2)Ti(2)Cl(16)](2)(-) (3). The oxidation reaction of 1 with TiCl(4) is unique. Oxidation of 1 with H(+) in CH(2)Cl(2) solution results in the formation of [ZrCl(6)](2)(-) (5) and H(2), while in py solution the oxidation product is [ZrCl(5)(py)](-) (6). There is no reaction between 1 and TiI(4), ZrCl(4), [TiCl(6)](2)(-), [ZrCl(6)](2)(-), or CrCl(3). Compounds [Ph(4)P](2)[Zr(6)Cl(18)H(5)] (2a), [Ph(4)P](2)[Zr(2)Ti(2)Cl(16)] (3a), [Ph(4)P](2)[Zr(2)Cl(9)] (4a), [Ph(4)P](2)[ZrCl(6)].4MeCN (5a.4MeCN), and [Ph(4)P][ZrCl(5)(py)] (6a) were characterized by X-ray crystallography. Compound 2a crystallized in the trigonal space group R&thremacr; with cell dimensions (20 degrees C) of a = 28.546(3) Å, b = 28.546(3) Å, c = 27.679(2) Å, V = 19533(3) Å(3), and Z = 12. Compound 3a crystallized in the triclinic space group P&onemacr; with cell dimensions (-60 degrees C) of a = 11.375(3) Å, b = 13.357(3) Å, c = 11.336(3) Å, alpha = 106.07(1) degrees, beta = 114.77(1) degrees, gamma = 88.50(1) degrees, V = 1494.8(7) Å(3), and Z = 1. Compound 4a crystallized in the triclinic space group P&onemacr; with cell dimensions (-60 degrees C) of a = 12.380(5) Å, b = 12.883(5) Å, c = 11.000(4) Å, alpha = 110.39(7) degrees, beta = 98.29(7) degrees, gamma = 73.12(4) degrees, V = 1572(1) Å(3), and Z = 2. Compound 5a.4MeCN crystallized in the monoclinic space group P2(1)/c with cell dimensions (-60 degrees C) of a = 9.595(1) Å, b = 19.566(3) Å, c = 15.049(1) Å, beta = 98.50(1) degrees, V = 2794.2(6) Å(3), and Z = 2. Compound 6a crystallized in the monoclinic space group P2(1)/c with cell dimensions (20 degrees C) of a = 10.3390(7) Å, b = 16.491(2) Å, c = 17.654(2) Å, beta = 91.542(6) degrees, V = 3026.4(5) Å(3), and Z = 4.