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.

Preparation and Structure of Two Ditungsten Compounds Synthesized with the 3,5-Dichlorophenylformamidinate Ligand.

The structure, preparation, and spectroscopy of a W(2)(II, II) paddlewheel complex with four formamidinate ligands is reported. Upon exposure to air and moisture in solution, the W(2)(II, II) core undergoes an oxidative addition reaction to form a W(2)(III, III) edge-sharing bioctahedral (ESBO) complex with two bridging hydroxides. The products, W(2)(&mgr;-DCPF)(4) (1) and W(2)(&mgr;-OH)(2)(&mgr;-DCPF)(2)(eta(2)-DCPF)(2) (2) where DCPF is [(3,5-Cl(2)C(6)H(3))NC(H)N(3,5-Cl(2)C(6)H(3))(-)], were characterized by UV-vis and (1)H NMR spectroscopy. Structural data are presented for W(2)(&mgr;-DCPF)(4) with four bridging formamidinate ligands and W(2)(&mgr;-OH)(2)(&mgr;-DCPF)(2)(eta(2)-DCPF)(2) with the formamidinate ligands in both chelating and bridging positions. Crystallographic data for W(2)(&mgr;-DCPF)(4) (1) and W(2)(&mgr;-OH)(2)(&mgr;-DCPF)(2)(eta(2)-DCPF)(2) (2) are as follows: (1), C(68)H(60)Cl(16)N(8)O(4)W(2), a = 12.6906(3) Å, b = 12.7818(4) Å, c = 13.0450(3) Å, alpha = 109.173(1) degrees, beta = 93.865(1) degrees, gamma = 103.746(1) degrees, triclinic, P&onemacr; (No. 2), and Z = 1 and (2), C(59)H(35)Cl(16)N(8)O(2)W(2), a = 17.1524(1) Å, b = 20.6568(3) Å, c = 19.3959(3) Å, beta = 102.791(1) degrees, monoclinic, C2/c (No. 15), and Z = 4.

Solution and solid-state variation of cupric phenanthroline complexes.

The 1:1 cupric-phenanthroline complexes, [Cu(5,6-Me2-phen)(MeCN)2(BF4)](BF4) (1), [Cu(o-phen)(MeCN)2(H2O)](BF4)2 (2), and [Cu(5-Cl-phen)(MeCN)2(BF4)](BF4) (3), have been prepared and characterized by X-ray crystallography. The structures of 1 and 3 are characterized by an equatorial plane about the copper center consisting of a phenanthroline ligand and two acetonitrile ligands. The copper units are connected by bridging counterions in the axial positions of the pseudo-octahedral metal centers to form one-dimensional solid-state linkages. The structure of 2 contains the same equatorial plane as 1 and 3, but an axial water ligand completes a square pyramidal geometry for each discrete metal unit. Although the solid-state structures vary for the three complexes, characterization through electronic spectroscopy and cyclic voltammetry reveals similar behavior for all three complexes in solution.

Photoinduced one-electron reduction of alkyl halides by dirhodium(II,II) tetraformamidinates and a related complex with visible light.

Various substituted dirhodium tetraformamidinate complexes, Rh(2)(R-form)(4) (R = p-CF(3), p-Cl, p-OCH(3), m-OCH(3); form = N,N'-diphenylformamidinate), and the new complex Rh(2)(tpgu)(4) (tpgu = 1,2,3-triphenylguanidinate) have been investigated as potential agents for the photoremediation of saturated halogenated aliphatic compounds, RX (R = alkyl group). The synthesis and characterization of the complexes is reported, and the crystal structure of Rh(2)(tpgu)(4) is presented. The lowest energy transition of the complexes is observed at approximately 870 nm and the complexes react with alkyl chlorides and alkyl bromides under low energy irradiation (lambda(irr) > or = 795 nm), but not when kept in the dark. The metal-containing product of the photochemical reaction with RX (X = Cl, Br) is the corresponding mixed-valent Rh(2)(II,III)X (X = Cl, Br) complex, and the crystal structure of Rh(2)(p-OCH(3)-form)(4)Cl generated photochemically from the reaction of the corresponding Rh(2)(II,II) complex in CHCl(3) is presented. In addition, the product resulting from the dimerization of the alkyl fragment, R(2), is also formed during the reaction of each dirhodium complex with RX. A comparison of the dependence of the relative reaction rates on the reduction potentials of the alkyl halides and their C-X bond dissociation energies are consistent with an outer-sphere mechanism. In addition, the relative reaction rates of the metal complexes with CCl(4) decrease with the oxidation potential of the dirhodium compounds. The mechanism of the observed reactivity is discussed and compared to related systems.

Diosmium(III) compounds supported by 2-anilinopyridinate and novel alkynyl derivatives.

The reaction between Os(2)(OAc)(4)Cl(2) and Hap (Hap is 2-anilinopyridine) under prolonged refluxing conditions resulted in an Os(III)(2) compound, Os(2)(ap)(4)Cl(2) (1), that can be crystallized as either the cis-(2,2) isomer from a CH(3)OH-CH(2)Cl(2) solution or the (3,1) isomer from a hexanes-CH(2)Cl(2) solution. Compound 1 undergoes facile reactions with LiC(2)Y to yield a series of Os(2)(ap)(4)(C(2)Y)(2) compounds with Y as Ph (2), ferrocenyl (3), SiMe(3) (4), and C(2)SiMe(3) (5). X-ray diffraction study of compound 2 reveals solvent-dependent isomerism similar to that of the parent compound 1. Compound 1 has Os-Os distances of 2.3937(8) and 2.3913(8) Angstroms for the cis-(2,2) and (3,1) isomers, respectively, and is paramagnetic (S = 1). Both the ethynyl derivatives 2-4 and butadiynyl derivative 5 are diamagnetic and have Os-Os distances of 2.456(1), 2.471(1), and 2.481(1) Angstroms for the cis-(2,2) and (3,1) isomers of 2 and (3,1) isomer of 4, respectively. Compounds 1-5 exhibit multiple one-electron redox couples in their cyclic voltammograms, including a reversible Os(2)(8+/7+) couple for 2. Resonance Raman spectra of both compounds 1 and 2 are reported.

Direct observation of the luminescence from the (3)deltadelta excited state of Re(2)Cl(2)(p-OCH(3)form)(4).

There are only a few reports on the measurement of the energy of the low-lying (3)deltadelta state of quadruply bonded bimetallic complexes, and the direct observation of the (1)deltadelta excited electronic state was only recently reported. In the quadruply bonded bimetallic complexes reported to date, luminescence arises from their (1)deltadelta excited state, and the (3)deltadelta state is nonemissive. Here we report the luminescence of Re(2)Cl(2)(p-OCH(3)form)(4) [p-OCH(3)form = (p-CH(3)OC(6)H(4))NCHN(p-CH(3)OC(6)H(4))(-)] observed upon 400-460 nm excitation with maxima at 820 nm (CH(2)Cl(2), tau = 1.4 micros) and 825 nm (CH(3)CN, tau = 1.3 micros) at 298 K. From the large Stokes shift, the vibronic progression at 77 K, the quenching by O(2), the long lifetime, and the calculated energy of the (3)deltadelta state, the luminescence of Re(2)Cl(2)(p-OCH(3)form)(4) and the corresponding transient absorption signal are assigned as arising from the (3)deltadelta ((3)A(2u)) excited state of the complex.