Metal−Metal Multiply-Bonded Complexes of Technetium. 3.1 Preparation and Characterization of Phosphine Complexes of Technetium Possessing a Metal−Metal Bond Order of 3.5

Antiferromagnetic Coupling Supported by Metallophilic Interactions: Theoretical View

The antiferromagnetic coupling supported by metallophilic interactions has been studied in the framework of the broken symmetry approach (BS) and multiconfigurational calculations (CASSCF). A series of heterobimetallic complexes of the form [PtCo(X)₄(Y)]₂ (X = tba thiobenzoate, SAc thioacetate, and Y = H₂O, NO₂py, py), previously reported, have been used as model systems. Magnetic coupling constants were found in good agreement with the experimental reports, and it could be concluded that axial ligands with a pure σ-donor character have a marked effect on the J value strengthening the antiferromagnetic coupling, as shown for [PtCo(SAc)₄(H₂O)]₂ and [PtNi(SAc)₄(H₂O)]₂. The latter complex, included for comparative purposes, also made it possible to evidence that the interaction between magnetic orbitals and low-level excitation in the Pt···Pt region is also relevant favoring the stronger antiferromagnetic coupling found in this case. A careful analysis of the energetic components involved in Pt···Pt interaction suggests that the stabilization arises from a combination of favorable orbital contributions, which allows a weak covalent Pt···Pt σ(dz²...dz²) bond. Theoretical tools evidence that the weak σ-bond found between monomeric units is responsible for a spin polarization mechanism resulting in the observed antiferromagnetic interaction. Multiconfigurational calculations finally allowed us to establish that the spin polarization mechanism involves not only the d_z² orbitals in the M-Pt···Pt-M bond direction but also the empty 6p_z orbitals of Pt atoms. The inclusion of these orbitals favors a correlation-induced delocalization of magnetic orbitals and therefore a better balance among direct and kinetic exchange. The results shown in this work are relevant in the molecular design of systems supported by metallophilic interactions not only between platinum atoms but also could be extended to other cases with similar interactions.

Probing the presence of multiple metal-metal bonds in technetium chlorides by X-ray absorption spectroscopy: implications for synthetic chemistry

The cesium salts of [Tc(2)X(8)](3-) (X = Cl, Br), the reduction product of (n-Bu(4)N)[TcOCl(4)] with (n-Bu(4)N)BH(4) in THF, and the product obtained from reaction of Tc(2)(O(2)CCH(3))(4)Cl(2) with HCl(g) at 300 °C have been characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy. For the [Tc(2)X(8)](3-) anions, the Tc-Tc separations found by EXAFS spectroscopy (2.12(2) Å for both X = Cl and Br) are in excellent agreement with those found by single-crystal X-ray diffraction (SCXRD) measurements (2.117[4] Å for X = Cl and 2.1265(1) Å for X = Br). The Tc-Tc separation found by EXAFS in these anions is slightly shorter than those found in the [Tc(2)X(8)](2-) anions (2.16(2) Å for X = Cl and Br). Spectroscopic and SCXRD characterization of the reduction product of (n-Bu(4)N)[TcOCl(4)] with (n-Bu(4)N)BH(4) are consistent with the presence of dinuclear species that are related to the [Tc(2)Cl(8)](n-) (n = 2, 3) anions. From these results, a new preparation of (n-Bu(4)N)(2)[Tc(2)Cl(8)] was developed. Finally, EXAFS characterization of the product obtained from reaction of Tc(2)(O(2)CCH(3))(4)Cl(2) with HCl(g) at 300 °C indicates the presence of amorphous α-TcCl(3). The Tc-Tc separation (i.e., 2.46(2) Å) measured in this compound is consistent with the presence of Tc═Tc double bonds in the [Tc(3)](9+) core.

Structural, spectroscopic, and multiconfigurational quantum chemical investigations of the electron-rich metal-metal triple-bonded Tc(2)X(4)(PMe(3))(4) (X = Cl, Br) complexes

The compounds Tc(2)Cl(4)(PMe(3))(4) and Tc(2)Br(4)(PMe(3))(4) were formed from the reaction between (n-Bu(4)N)(2)Tc(2)X(8) (X = Cl, Br) and trimethylphosphine. The Tc(II) dinuclear species were characterized by single-crystal XRD, UV-visible spectroscopy, and cyclic voltammetry techniques, and the results are compared to those obtained from density functional theory and multiconfigurational (CASSCF/CASPT2) quantum chemical studies. The compound Tc(2)Cl(4)(PMe(3))(4) crystallizes in the monoclinic space group C2/c [a = 17.9995(9) A, b = 9.1821(5) A, c = 17.0090(9) A, beta = 115.4530(10) degrees ] and is isostructural to M(2)Cl(4)(PMe(3))(4) (M = Re, Mo, W) and to Tc(2)Br(4)(PMe(3))(4). The metal-metal distance (2.1318(2) A) is similar to the one found in Tc(2)Br(4)(PMe(3))(4) (2.1316(5) A). The calculated molecular structures of the ground states are in excellent agreement with the structures determined experimentally. Calculations of effective bond orders for Tc(2)X(8)(2-) and Tc(2)X(4)(PMe(3))(4) (X = Cl, Br) indicate stronger pi bonds in the Tc(2)(4+) core than in Tc(2)(6+) core. The electronic spectra were recorded in benzene and show a series of low intensity bands in the range 10 000-26 000 cm(-1). Assignment of the bands as well as computing their excitation energies and intensities were performed at both TD-DFT and CASSCF/CASPT2 levels of theory. Calculations predict that the lowest energy band corresponds to the delta* --> sigma* transition, the difference between calculated and experimental values being 228 cm(-1) for X = Cl and 866 cm(-1) for X = Br. The next bands are attributed to delta* --> pi*, delta --> sigma*, and delta --> pi* transitions. The cyclic voltammograms exhibit two reversible waves and indicate that Tc(2)Br(4)(PMe(3))(4) exhibits more positive oxidation potentials than Tc(2)Cl(4)(PMe(3))(4.) This phenomenon is discussed and ascribed to stronger metal (d) to halide (d) back bonding in the bromo complex. Further analysis indicates that Tc(II) dinuclear species containing pi-acidic phosphines are more difficult to oxidize, and a correlation between oxidation potential and phosphine acidity was established.

Mixed Chloride/Phosphine Complexes of the Dirhenium Core. 5. Reduction or Disproportionation at the Re(2)(6+) Unit in Reactions with Tertiary Phosphines in Different Solvents

For reactions of the octachlorodirhenium anion, [Re(2)Cl(8)](2)(-), with tertiary phosphines PEt(3) and PPr(n)(3), the influence of solvent on the reaction pathway has been studied. It has been shown that in 1-propanol at room temperature the reaction leads to a one-electron reduction of the Re(2)(6+) core. The novel dirhenium(II,III) paramagnetic products of stoichiometries [Bu(n)(4)N][Re(2)Cl(6)(PEt(3))(2)] (1a) and Re(2)Cl(5)(PPr(n)(3))(3) (2a) have been isolated and characterized. In benzene a disproportionation process occurs resulting in mononuclear rhenium(IV) complexes ReCl(4)(PR(3))(2) (PR(3) = PEt(3) (1b) and PPr(n)(3) (2b)) in addition to 1a and 2a, respectively. The solid state structures of compounds 1a, 1b, 2a.0.25C(6)H(14), and 2b have been investigated by X-ray crystallography. The crystallographic parameters are as follows: for 1a, tetragonal space group P4(2)/m with a = 11.821(2) Å, c = 14.770(3) Å, and Z = 2; for 1b, monoclinic space group P2(1)/n with a = 8.0555(4) Å, b = 12.536(2) Å, c = 10.2041(6) Å, beta = 99.167(9) degrees, and Z = 2; for 2a.0.25C(6)H(14), monoclinic space group P2(1)/n with a = 11.7679(9) Å, b = 18.574(5) Å, c = 19.688(4) Å, beta = 93.37(2) degrees, and Z = 4; for 2b, monoclinic space group P2(1)/c with a = 8.219(2) Å, b = 11.210(1) Å, c = 14.331(3) Å, beta = 93.171(9) degrees, and Z = 2. It has also been found that the disproportionation process in benzene goes by way of an intermediate complex which is an initial product of interaction between the [Re(2)Cl(8)](2)(-) and the phosphine. In the case of the triethylphosphine ligand we formulate this complex as [Bu(n)(4)N][Re(2)Cl(7)(PEt(3))(3)] (1c).