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.     

Magnetic Susceptibility Trends in Oxo-Bridged, Dinuclear Chromium(III) Complexes. Crystal Structure of [(tmpa)Cr(&mgr;-O)(&mgr;-CO(3))Cr(tmpa)](ClO(4))(2).2H(2)O

The synthesis and physical characterization of oxo-bridged [Cr(2)(tmpa)(2)(&mgr;-O)(X)](n)()(+) complexes (tmpa = tris(2-pyridylmethyl)amine) containing a variety of complementary ligands (X = CO(3)(2)(-), PhPO(4)(2)(-), HS(-)) are described, with the objective of understanding factors underlying variations in the antiferromagnetic coupling constant J. We also present the crystal structure of [(tmpa)Cr(&mgr;-O)(&mgr;-CO(3))Cr(tmpa)](ClO(4))(2).2H(2)O, for comparison with previous findings on [(tmpa)Cr(&mgr;-O)(&mgr;-CH(3)CO(2))Cr(tmpa)](ClO(4))(3). The carbonate-bridged complex crystallizes in the monoclinic space group P2(1)/c with a = 11.286(10) ?, b = 18.12(2) ?, c = 20.592(12) ?, beta = 95.99(5) degrees, and V = 4190 ?(3) and Z = 4. Asymmetric tmpa ligation pertains, with apical N atoms situated trans to bridging oxo and acido O atoms. Key structural parameters include Cr-O(b) bond lengths of 1.818(6) and 1.838(6) ?, Cr-OCO(2) distances of 1.924(7) and 1.934(7) ?, and a bridging bond angle of 128.3(3) degrees. Several attempts to prepare oxo, amido-bridged dimers were unsuccessful, but the nearlinear [Cr(tmpa)(N(CN)(2))](2)O(ClO(4))(2).3H(2)O complex was isolated from the reaction of dicyanamide ion with [Cr(tmpa)(OH)](2)(4+). In contrast to the behavior of analogous diiron(III) complexes, antiferromagnetic coupling constants of [Cr(2)(tmpa)(2)(&mgr;-O)(X)](n)()(+) dinuclear species are highly responsive to the X group. Considering the complexes with X = CO(3)(2)(-), PhPO(4)(2)(-), HS(-), SO(4)(2)(-), and RCO(2)(-) (10 R substituents), we find a reasonably linear, empirical relationship between J and oxo bridge basicity, as measured by pK(a) (Cr(OH)Cr) values in aqueous solution. While there is no theoretical basis for such a correlation between solid-state and solution-phase properties, this relationship demonstrates that CrOCr pi-bonding contributes significantly to antiferromagnetic exchange. Thus, J tends to become less negative with increasing &mgr;-O(2)(-) basicity, showing that greater availability of a bridging oxo group lone pair toward the proton, with decreasing CrOCr pi-interaction, reduces the singlet-triplet gap.     

Mechanism of (&mgr;-H)(&mgr;-alkenyl)Re(2)(CO)(8) Formation in 350 nm Flash Irradiations of Re(2)(CO)(10)

The mechanism of (&mgr;-H)(&mgr;-alkenyl)Re(2)(CO)(8) formation upon UV irradiations of Re(2)(CO)(10) in presence of olefin (styrene, trans-stilbene, 4-methyl-1-cyclohexane, and ethylene) was investigated by laser flash photolyses. Such photoproducts result from reactions of the olefin with eq-Re(2)(CO)(9). No reactions of Re(CO)(5) leading to hydride alkenyl products were observed. Dependences of the reaction rate on olefin concentration and solvent revealed an additional intermediate formed after the addition of the olefin to eq-Re(2)(CO)(9) and before the appearance of the &mgr;-hydrido-&mgr;-alkenyl products.     

An Approach to Heterometallic Complexes with Selenolate and Tellurolate Ligands: Crystal Structures of cis-[Mn(CO)(4)(SePh)(2)](-), [(CO)(3)Mn(&mgr;-SeMe)(3)Mn(CO)(3)](-), (CO)(4)Mn(&mgr;-TePh)(2)Co(CO)(&mgr;-SePh)(3)Mn(CO)(3), and (CO)(3)Mn(&mgr;-SePh)(3)Fe(CO)(3)

Oxidative addition of diorganyl diselenides to the coordinatively unsaturated, low-valent transition-metal-carbonyl fragment [Mn(CO)(5)](-) produced cis-[Mn(CO)(4)(SeR)(2)](-). The complex cis-[PPN][Mn(CO)(4)(SePh)(2)] crystallized in triclinic space group P&onemacr; with a = 10.892(8) ?, b = 10.992(7) ?, c = 27.021(4) ?, alpha = 101.93(4) degrees, beta = 89.79(5) degrees, gamma = 116.94(5) degrees, V = 2807(3) ?(3), and Z = 2; final R = 0.085 and R(w) = 0.094. Thermolytic transformation of cis-[Mn(CO)(4)(SeMe)(2)](-) to [(CO)(3)Mn(&mgr;-SeMe)(3)Mn(CO)(3)](-) was accomplished in high yield in THF at room temperature. Crystal data for [Na-18-crown-6-ether][(CO)(3)Mn(&mgr;-SeMe)(3)Mn(CO)(3)]: trigonal space group R&thremacr;, a = 13.533(3) ?, c = 32.292(8) ?, V = 5122(2) ?(3), Z = 6, R = 0.042, R(w) = 0.041. Oxidation of Co(2+) to Co(3+) by diphenyl diselenide in the presence of chelating metallo ligands cis-[Mn(CO)(4)(SePh)(2)](-) and cis-[Mn(CO)(4)(TePh)(2)](-), followed by a bezenselenolate ligand rearranging to bridge two metals and a labile carbonyl shift from Mn to Co, led directly to [(CO)(4)Mn(&mgr;-TePh)(2)Co(CO)(&mgr;-SePh)(3)Mn(CO)(3)]. Crystal data: triclinic space group P&onemacr;, a = 11.712(3) ?, b = 12.197(3) ?, c = 15.754(3) ?, alpha = 83.56(2) degrees, beta = 76.13(2) degrees, gamma = 72.69(2) degrees, V = 2083.8(7) ?(3), Z = 2, R = 0.040, R(w) = 0.040. Addition of fac-[Fe(CO)(3)(SePh)(3)](-) to fac-[Mn(CO)(3)(CH(3)CN)(3)](+) resulted in formation of (CO)(3)Mn(&mgr;-SePh)(3)Fe(CO)(3). This neutral heterometallic complex crystallized in monoclinic space group P2(1)/n with a = 8.707(2) ?, b = 17.413(4) ?, c = 17.541(4) ?, beta = 99.72(2) degrees, V = 2621(1) ?(3), and Z = 4; final R = 0.033 and R(w) = 0.030.     

Synthesis, NMR Study, and Reactivity of Isomeric Early-Late Heterobimetallic Dihydrides. X-ray Crystal Structure of (PPh(3))HRu(&mgr;-H)(&mgr;-PMe(2)C(5)Me(4))(2)(&mgr;-Cl)ZrCl

The new isomeric ruthenium/zirconium dihydrides of the formula (PPh(3))HRuH(&mgr;-PMe(2)Cp)(2)ClZrCl (1, 2) (Cp = C(5)Me(4)) have been characterized by elemental analysis and NMR ((1)H, (31)P and (1)H relaxation data). Complex 1, stabilized by Cl and H bridges, has been isolated from the room temperature reaction between RuH(2)(H(2))(PPh(3))(3) and (PMe(2)Cp)(2)ZrCl(2). The X-ray crystallographic study of 1 revealed a bimetallic complex. The six-coordinate Ru atom and the five-coordinate Zr atom are held together by two bifunctional phosphinocyclopentadienyl ligands and by H and Cl bridges. Crystal data for 1: monoclinic space group P2(1)/c, a = 13.901(2) ?, b = 18.205(6) ?, c = 16.633(3) ?, beta = 92.43(1) degrees, V = 4206 ?(3), Z = 4, d(calc) = 1.472 g cm(-)(3), R(F) = 0.056, R(w)(F) = 0.058. Complex 2 with two H bridges and terminal Cl ligands at Ru and Zr has been obtained by an irreversible isomerization of 1 in the presence of HNEt(3)BPh(4). This transformation has been proposed to occur through slow protonation of one of the phosphorus ligands with the five-coordinate Ru center formed by undergoing rapid pseudorotation. Complexes 1 and 2 do not react with H(2), N(2), or 3,3-dimethyl-but-1-ene. Treatment of 1 with 1 equiv of NaHBEt(3) in C(6)D(6) gives a mixture of new trihydrides (PPh(3))HRu(&mgr;-Cl)(&mgr;-H)(&mgr;-PMe(2)Cp)(2)ZrH (3) and (PPh(3))HRu(&mgr;-H)(2)(&mgr;-PMe(2)Cp)(2)ZrCl (4). Complex 3 transforms to 4 upon standing in solution for a period of several days. Under the same conditions, complex 2 leads smoothly to trihydride 4. Both trihydrides are new and have been characterized by (1)H, (31)P NMR, and (1)H NMR relaxation data. Complexes 1 and 4 are fluxional in solution at room temperature, showing hydride exchange between the terminal and bridging positions. The variable-temperature (1)H NMR spectra allowed determinations of the DeltaG() values of 16.4 (313 K, THF-d(8)) and 13.5 kcal/mol (295 K, toluene-d(8)) for the exchange in complexes 1 and 4, respectively. Possible exchange mechanisms have been discussed. Complex 2 is rigid on the NMR time scale.     

Isolation of a Bismuth Chloride-Iron Carbonyl Adduct: Synthesis and Structural Characterization of [PhCH(2)NMe(3)](2)[Bi(2)Cl(4)(&mgr;-Cl)(2){&mgr;-Fe(CO)(4)}

The reaction of bismuth(III) chloride with [PhCH(2)NMe(3)](2)[Fe(CO)(4)] at a ratio of 2:1 in acetonitrile yields the iron carbonyl-bismuth chloride adduct [PhCH(2)NMe(3)](2)[Bi(2)Cl(4)(&mgr;-Cl)(2){&mgr;-Fe(CO)(4)}] cleanly in high yield. The complex consists of two BiCl(3) groups bridged by an [Fe(CO)(4)](2)(-) unit. Two chloride ligands are shared between the Bi atoms, producing square-pyramidal coordination at bismuth and octahedral coordination at the iron center. The production of this complex represents the synthesis of a stable adduct of a highly nucleophilic metal carbonyl anion with a strongly Lewis acidic main group halide. The compound C(24)H(32)N(2)O(4)Bi(2)Cl(6)Fe crystallizes in the orthorhombic space group Pba2 (No. 32) with cell parameters a = 14.624(3) ?, b = 17.010(3) ?, c = 7.1990(10) ?, V = 1790.8(5) ?(3), and Z = 2.     

Synthetic, Structural, Spectroscopic, and Theoretical Studies of Structural Isomers of the Cluster Pt(3)(&mgr;-PPh(2))(3)Ph(PPh(3))(2). A Unique Example of Core Isomerism in Phosphine Phosphido-Rich Clusters

Two isomers of the phosphido-bridged platinum cluster Pt(3)(&mgr;-PPh(2))(3)Ph(PPh(3))(2) (2 and 3) have been isolated, and their structures have been solved by single-crystal X-ray diffraction. Compound 2 crystallizes in the orthorhombic space group Cmc2(1) with a = 22.192(10) ?, b = 17.650(9) ?, c = 18.182(8) ?, and Z = 4. Compound 3 crystallizes with 2 molecules of dichloromethane in the monoclinic space group C2/c with a = 21.390(10) ?, b = 18.471(9) ?, c = 19.021(11) ?, beta = 105.27(5) degrees, and Z = 4. The two isomers differ essentially in their metal-metal distances and Pt-(&mgr;-PPh(2))-Pt angles. Thus 2, having an imposed C(s) symmetry, contains a bent chain of metal atoms with two short Pt-Pt distances of 2.758(3) ? and a long separation of 3.586(2) ?. In 3, which has an imposed C(2) symmetry, the metal atoms form an isosceles triangle with two Pt-Pt distances of 2.956(3) ? and one of 3.074(4) ?. These isomers can be smoothly interconverted by changing the crystallization solvents. Solution and solid-state (31)P NMR studies have been performed in order to assign the resonances of the different P nuclei and relate their chemical shifts with their structural environments. Raman spectroscopy was used to assign the nu(Pt-Pt) modes of the two structural isomers. Theoretical studies based on extended Hückel calculations and using the fragment molecular orbital approach show that the isomer with the three medium Pt-Pt distances is slightly more stable, in agreement with earlier theoretical predictions. Cluster core isomerism remains a rare phenomenon, and the present example emphasizes the role and the importance of flexible phosphido bridges in stabilizing clusters as well as the unprecedented features which can be observed in phosphine phosphido-rich metal clusters.     

Modeling the Formation of Molybdenum Oxides from Alkoxides: Crystal Structures of [Mo(4)O(4)Cl(4)(&mgr;(2)-OEt)(4)(HOEt)(2)(&mgr;(3)-O)(2)] and [PPN](+)[Et(3)NH](+)[Cl(2)(O)Mo(&mgr;(2)-O)(2)Mo(O)Cl(2)](2)(-)

The reactions of the binuclear oxomolybdenum(V) complex [Cl(2)(O)Mo(&mgr;-OEt)(2)(&mgr;-HOEt)Mo(O)Cl(2)] (1) with Me(3)Si(allyl) and SbF(3) produce the compounds [Mo(6)O(6)Cl(6)(&mgr;(3)-O)(2)(&mgr;(2)-OEt)(6)(&mgr;(2)-Cl)(2)] (2) and [Mo(8)O(8)Cl(6)(&mgr;(3)-O)(4)(OH)(2)(&mgr;(2)-OH)(4)(&mgr;(2)-OEt)(4)(HOEt)(4)] (3), respectively. Treatment of 1 with the Lewis base PMe(3) affords the tetrameric complex [Mo(4)O(4)Cl(4)(&mgr;(2)-OEt)(4)(HOEt)(2)(&mgr;(3)-O)(2)] (4), which represents another link in the chain of clusters produced by the reactions of 1 and simulating the build-up of polymeric molybdenum oxides by sol-gel methods. The crystal structure of 4 has been determined [C(12)H(32)Cl(4)Mo(4)O(12), triclinic, P&onemacr;, a = 7.376(2) ?, b = 8.807(3) ?, c = 11.467(4) ?, alpha = 109.61(1) degrees, beta = 92.12(3) degrees, gamma = 103.75(2) degrees, Z = 1]. By contrast, reaction of 1 with the nitrogen base NEt(3), followed by treatment with [PPN]Cl.2H(2)O ([PPN](+) = [Ph(3)P=N=PPh(3)](+)), gives the complex [PPN](+)[Et(3)NH](+)[Cl(2)(O)Mo(&mgr;(2)-O)(2)Mo(O)Cl(2)](2)(-) (6) in 90% yield. Its crystal structure [C(36)H(30)Cl(4)MoNOP(2), triclinic, Pna2(1), a = 21.470(6) ?, b = 16.765(2) ?, c = 9.6155(14) ?, alpha = 90 degrees, beta = 90 degrees, gamma = 90 degrees, Z = 16] includes the anion [Cl(2)(O)Mo(&mgr;(2)-O)(2)Mo(O)Cl(2)](2)(-), which is a charged derivative of the species forming the gels in sol-gel processes starting from chloromolybdenum ethoxides. Furthermore, compound 1 is found to be catalytically active in esterification and dehydration reactions of alcohols.