Synthetic Strategies To Obtain V-P-O Open Frameworks Containing Organic Species as Structural Directing Agents. Crystal Structure of the V(IV)-Fe(III) Bimetallic Phosphate [H(3)N(CH(2))(2)NH(3)](2)[H(3)N(CH(2))(2)NH(2)][Fe(III)(H(2)O)(2)(V(IV)O)(8)(OH)(4)(HPO(4))(4)(PO(4))(4)].4H(2)O

A general synthetic approach to rationalize the solution preparative chemistry of oxovanadium phosphates containing organic species as structural directing agents is presented. Careful attention is payed to the hydrolysis and condensation processes involving the ionic species in solution, and a simple restatement of the partial charge model (PCM) has been used in order to organize the experimental results. The structure of a new V(IV)-Fe(III) bimetallic oxovanadium phosphate, [H(3)N(CH(2))(2)NH(3)](2)[H(3)N(CH(2))(2)NH(2)] [Fe(III)(H(2)O)(2)(V(IV)O)(8)(OH)(4)(HPO(4))(4)(PO(4))(4)].4H(2)O, has been determined by X-ray single crystal diffraction methods. This compound crystallizes in the monoclinic system, space group P2(1)/n and the cell dimensions are as follows: a = 14.383(3) ?, b = 10.150(2) ?, c = 18.355(4) ?, and beta = 90.39(3) degrees (Z = 2). The existence of a complex intercrossing channel system, including a very large channel of 18.4 ? of diameter (in which both water molecules and ethylenediamine species are located), is the more interesting feature of this structure. Thermal decomposition, including the dehydration/rehydration process, has been studied by thermal analysis and variable temperature X-ray powder diffraction techniques. A complementary SEM study of the different intermediate decomposition products is presented.     

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.     

Further evidence for the tetraoxoiodate(V) anion,IO(4)(3-): hydrothermal syntheses and structures of Ba[(MoO(2))(6)(IO(4))(2)O(4)] x H(2)O and Ba(3)[(MoO(2))(2)(IO(6))(2)] x 2H(2)O

The hydrothermal reaction of MoO(3) with BaH(3)IO(6) at 180 degrees C for 3 days results in the formation of Ba[(MoO(2))(6)(IO(4))(2)O(4)] x H(2)O (1). Under similar conditions, the reaction of Ba(OH)(2) x 8H(2)O with MoO(3) and Ba(IO(4))(2) x 6H(2)O yields Ba(3)[(MoO(2))(2)(IO(6))(2)] x 2H(2)O (2). The structure of 1, determined by single-crystal X-ray diffraction, consists of corner- and edge-sharing distorted MoO(6) octahedra that create two-dimensional slabs. Contained within this molybdenum oxide framework are approximately C(2v) tetraoxoiodate(V) anions, IO(4)(3-), that are involved in bonding with five Mo(VI) centers. The two equatorial oxygen atoms of the IO(4)(3-) anion chelate a single Mo(VI) center, whereas the axial atoms are mu(3)-oxo groups and complete the octahedra of four MoO(6) units. The coordination of the tetraoxoiodate(V) anion to these five highly electropositive centers is probably responsible for stabilizing the substantial anionic charge of this anion. The Ba(2+) cations separate the layers from one another and form long ionic contacts with neighboring oxygen atoms and a water molecule. Compound 2 also contains distorted MoO(6) octahedra. However, these solely edge-share with octahedral hexaoxoiodate(VII), IO(6)(5-), anions to form zigzagging one-dimensional, (1)(infinity)[(MoO(2))(IO(6))](3-), chains that are polar. These chains are separated from one another by Ba(2+) cations that are coordinated by additional water molecules. Bond valence sums for the iodine atoms in 1 and 2 are 5.01 and 7.03, respectively. Crystallographic data: 1, monoclinic, space group C2/c, a = 13.584(1) A, b = 7.3977(7) A, c = 20.736(2) A, beta = 108.244(2) degrees, Z = 4; 2, orthorhombic, space group Fdd2, a = 13.356(7) A, b = 45.54(2) A, c = 4.867(3) A, Z = 8.     

Reaction of bis(phosphine)(hydrotris(3,5-dimethylpyrazolyl)borato)rhodium(I) with phenylacetylene,p-nitrobenzaldehyde,and triphenyltin hydride: structures of [Rh(Tp)(PPh(3))(2)], [Rh(Tp)(H)(C(2)Ph)(P(4-C(6)H(4)F)(3))], [Rh(Tp)(H)(COC(6)H(4)-4-NO(2))(PPh(3))], and [Rh(Tp)(H)(SnPh(3))(PPh(3))

The complexes [Rh(Tp)(PPh(3))(2)] (1a) and [Rh(Tp)(P(4-C(6)H(4)F)(3))(2)] (1b) combine with PhC(2)H, 4-NO(2)-C(6)H(4)CHO and Ph(3)SnH to give [Rh(Tp)(H)(C(2)Ph)(PR(3))] (R = Ph, 2a; R = 4-C(6)H(4)F, 2b), [Rh(Tp)(H)(COC(6)H(4)-4-NO(2))(PR(3))] (R = Ph, 3a), and [Rh(Tp)(H)(SnPh(3))(PR(3))] (R = Ph, 4a; R = 4-C(6)H(4)F, 4b) in moderate to good yield. Complexes 1a, 2b, 3a, and 4a have been structurally characterized. In 1a the Tp ligand is bidentate, in 2b, 3a, and 4a it is tridentate. Crystal data for 1a: space group P2(1)/c; a = 11.9664(19), b = 21.355(3), c = 20.685(3) A; beta = 112.576(7) degrees; V = 4880.8(12) A(3); Z = 4; R = 0.0441. Data for 2b: space group P(-)1; a = 10.130(3), b = 12.869(4), c = 17.038(5) A; alpha = 78.641(6), beta = 76.040(5), gamma = 81.210(6) degrees; V = 2100.3(11) A(3); Z = 2; R = 0.0493. Data for 3a: space group P(-)1; a = 10.0073(11), b = 10.5116(12), c = 19.874(2) A; alpha = 83.728(2), beta = 88.759(2), gamma = 65.756(2) degrees; V =1894.2(4) A(3); Z = 2; R = 0.0253. Data for 4a: space group P2(1)/c; a = 15.545(2), b = 18.110(2), c = 17.810(2) A; beta = 95.094(3) degrees; V = 4994.1(10) A(3); Z = 4; R = 0.0256. NMR data ((1)H, (31)P, (103)Rh, (119)Sn) are also reported.     

Synthesis and Characterization by (19)F and (125)Te NMR and Raman Spectroscopy of cis-ReO(2)(OTeF(5))(3) and cis-ReO(2)(OTeF(5))(4)(-), and X-ray Crystal Structure of [N(CH(3))(4)(+)][cis-ReO(2)(OTeF(5))(4)(-)

The pentafluorooxotellurate compound ReO(2)(OTeF(5))(3) has been synthesized from the reaction of ReO(2)F(3) with B(OTeF(5))(3) and structurally characterized in solution by (19)F and (125)Te NMR spectroscopy and in the solid state by Raman spectroscopy. The NMR and vibrational spectroscopic findings are consistent with a trigonal bipyramidal arrangement in which the oxygen atoms and an OTeF(5) group occupy the equatorial plane. The (19)F and (125)Te NMR spectra show that the axial and equatorial OTeF(5) groups of ReO(2)(OTeF(5))(3) are fluxional and are consistent with intramolecular exchange by means of a pseudorotation. The Lewis acid behavior of ReO(2)(OTeF(5))(3) is demonstrated by reaction with OTeF(5)(-). The resulting cis-ReO(2)(OTeF(5))(4)(-) anion was characterized as the tetramethylammonium salt in solution by (19)F and (125)Te NMR spectroscopy and in the solid state by Raman spectroscopy and X-ray crystallography. The compound crystallizes in the triclinic system, space group P&onemacr;, with a = 13.175(7) ?, b = 13.811(5) ?, c = 15.38(1) ?, alpha = 72.36(5)(o), beta = 68.17(5)(o), gamma = 84.05(4)(o), V = 2476(2) ?(3), D(calc) = 3.345 g cm(-)(3), Z = 4, R = 0.0547. The coordination sphere about Re(VII) in cis-ReO(2)(OTeF(5))(4)(-) is a pseudooctahedron in which the Re-O double bond oxygens are cis to one another.     

Synthesis,crystal structure,and solid state NMR spectroscopy of NH(4)[(V(2)O(3))(2)(4,4'-bpy)(2)(H(2)PO(4))(PO(4))(2)].0.5H(2)O,a mixed-valence vanadium(IV,V) phosphate with a pillared layer structure

A mixed-valence vanadium phosphate, NH(4)[(V(2)O(3))(2)(4,4'-bpy)(2)(H(2)PO(4))(PO(4))(2)].0.5H(2)O, has been synthesized under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction. It crystallizes in the monoclinic space group C2/c (No. 15) with a = 12.6354(8) A, b = 9.9786(6) A, c = 23.369(1) A, beta = 92.713(1) degrees, and Z = 4 with R(1) = 0.0389. The structure consists of dimers of edge-sharing vanadium(IV,V) octahedra that are connected by corner-sharing phosphate tetrahedra to form layers in the ab-plane, which are further linked through 4,4'-bipyridine pillars to generate a 3-D framework. Magnetic susceptibility confirms the valence of the vanadium atoms. The (31)P MAS NMR spectrum shows a resonance centered at 80 ppm with a shoulder at ca. 83 ppm in an intensity ratio close to 1:2, which correspond to two distinct P sites. The observed large downfield (31)P NMR shifts can be ascribed to magnetic exchange coupling involving phosphorus atoms. The unpaired electron spin density at the phosphorus nucleus was determined from variable-temperature (31)P NMR spectra. The (1)H MAS NMR spectrum was fitted to six components in accordance with the structure as determined from X-ray diffraction.     

Mixed-metal uranium(VI) iodates: hydrothermal syntheses,structures, and reactivity of Rb[UO(2)(CrO(4))(IO(3))(H(2)O)], A(2)[UO(2)(CrO(4))(IO(3))(2)] (A = K,Rb, Cs), and K(2)[UO(2)(MoO(4))(IO(3))(2)

The reactions of the molecular transition metal iodates A[CrO(3)(IO(3))] (A = K, Rb, Cs) with UO(3) under mild hydrothermal conditions provide access to four new, one-dimensional, uranyl chromatoiodates, Rb[UO(2)(CrO(4))(IO(3))(H(2)O)] (1) and A(2)[UO(2)(CrO(4))(IO(3))(2)] (A = K (2), Rb (3), Cs (4)). Under basic conditions, MoO(3), UO(3), and KIO(4) can be reacted to form K(2)[UO(2)(MoO(4))(IO(3))(2)] (5), which is isostructural with 2 and 3. The structure of 1 consists of one-dimensional[UO(2)(CrO(4))(IO(3))(H(2)O)](-) ribbons that contain uranyl moieties bound by bridging chromate and iodate anions as well as a terminal water molecule to create [UO(7)] pentagonal bipyramidal environments around the U(VI) centers. These ribbons are separated from one another by Rb(+) cations. When the iodate content is increased in the hydrothermal reactions, the terminal water molecule is replaced by a monodentate iodate anion to yield 2-4. These ribbons can be further modified by replacing tetrahedral chromate anions with MoO(4)(2)(-) anions to yield isostructural, one-dimensional [UO(2)(MoO(4))(IO(3))(2)](2)(-) ribbons. Crystallographic data: 1, triclinic, space group P(-)1, a = 7.3133(5) A, b = 8.0561(6) A, c = 8.4870(6) A, alpha = 88.740(1) degrees, beta = 87.075(1) degrees, gamma = 71.672(1) degrees, Z = 2; 2, monoclinic, space group P2(1)/c, a = 11.1337(5) A, b = 7.2884(4) A, c = 15.5661(7) A, beta = 107.977(1) degrees, Z = 4; 3, monoclinic, space group P2(1)/c, a = 11.3463(6) A, b = 7.3263(4) A, c = 15.9332(8) A, beta = 108.173(1) degrees, Z = 4; 4, monoclinic, space group P2(1)/n, a = 7.3929(5) A, b = 8.1346(6) A, c = 22.126(2) A, beta = 90.647(1) degrees, Z = 4; 5, monoclinic, space group P2(1)/c, a = 11.3717(6) A, b = 7.2903(4) A, c = 15.7122(8) A, beta = 108.167(1) degrees, Z = 4.     

Clusters as Ligands. 4. Synthesis, Structure, and Characterization of the Tungsten(II)-Tungsten(III) Cluster Carboxylate {[Na][W(2){OOCCCo(3)(CO)(9)}(2)(OOCCF(3))(4)(THF)(2)]}(2)

The reaction of W(2)(OOCCF(3))(4) with (CO)(9)Co(3)CCOOH and Na[OOCCF(3)] in a nonpolar solvent mixture leads to the formation of the cluster of clusters {[Na][W(2){OOCCCo(3)(CO)(9)}(2)(OOCCF(3))(4)(THF)(2)]}(2), 1, in 40% yield. The structure of 1.3C(6)H(5)CH(3) in the solid state corresponds to a dimer of W(2) dinuclear complexes (monoclinic P2(1)/c, a = 15.234(6) ?, b = 23.326(11) ?, c = 20.658(7) ?, beta = 102.46(3) degrees; V = 7,168(5) ?(3); Z = 4; R(F)() = 8.39%). Each W(2) unit is bridged by two cis cluster carboxylates, and the remaining four equatorial sites are occupied by monodentate [OOCCF(3)](-) ligands. The axial positions contain coordinated THF. The W(2) carboxylate is opened up (W-W distance of 2.449(2) ?) so that the free ends of the [OOCCF(3)](-) ligands on both W(2) carboxylate units can cooperate in chelating two Na(+) ions thereby forming a dimer of W(2) complexes. A distinctive EPR spectrum with g = 2.08 is consistent with each W(2) carboxylate being a mixed-valent W(II)-W(III) species. The reaction of W(2)(OOCCF(3))(4) with (CO)(9)Co(3)CCOOH in THF in the absence of Na[OOCCF(3)] leads to the expected diamagnetic W(II)-W(II) cluster carboxylate W(2){OOCCCo(3)(CO)(9)}(3)(OOCCF(3))(THF)(2), 3.     

Nanoscale Hemispheres in Novel Mixed-Valent Uranyl Chromate(V,VI), (C(3)NH(10))(10)[(UO(2))(13)(Cr(12)(5+)O(42))(Cr(6+)O(4))(6)(H(2)O)(6)](H(2)O)(6)

The structure of a novel mixed-valent chromium uranyl compound, (C(3)NH(10))(10)[(UO(2))(13)(Cr(12)(5+)O(42))(Cr(6+)O(4))(6)(H(2)O)(6)](H(2)O)(6) (1), obtained by the combination of a hydrothermal method and evaporation from aqueous solutions with isopropylammonium, contains uranyl chromate hemispheres with lateral dimensions of 18.9 × 18.5 ?(2) and a height of about 8 ?. The hemispheres are centered by a UO(8) hexagonal bipyramid surrounded by six dimers of Cr(5+)O(5) square pyramids, UO(7) pentagonal bipyramids, and Cr(6+)O(4) tetrahedra. The hemispheres are linked into two-dimensional layers so that two adjacent hemispheres are oriented in opposite directions relative to the plane of the layer. From a topological point of view, the hemispheres have the formula U(21)Cr(23) and can be considered as derivatives of nanospherical cluster U(26)Cr(36) composed of three-, four-, and five-membered rings.     

Structure Determination of a Complex Tubular Uranyl Phenylphosphonate, (UO(2))(3)(HO(3)PC(6)H(5))(2)(O(3)PC(6)H(5))(2).H(2)O, from Conventional X-ray Powder Diffraction Data

The three-dimensional structure of a complex tubular uranyl phosphonate, (UO(2))(3)(HO(3)PC(6)H(5))(2)(O(3)PC(6)H(5))(2).H(2)O, was determined ab initio from laboratory X-ray powder diffraction data and refined by the Rietveld method. The crystals belong to the space group P2(1)2(1)2(1), with a = 17.1966(2) ?, b = 7.2125(2) ?, c = 27.8282(4) ?, and Z = 4. The structure consists of three independent uranium atoms, among which two are seven-coordinated and the third is eight-coordinated. These metal atoms are connected by four different phosphonate groups to form a one-dimensional channel structure along the b axis. The phenyl groups are arranged on the outer periphery of the channels, and their stacking forces keep the channels intact in the lattice. The determination of this structure which contains 50 non-hydrogen atoms in the asymmetric unit, from conventional X-ray powder data, represents significant progress in the application of powder techniques to structure solution of complex inorganic compounds, including organometallic compounds.     

Hydrothermal Synthesis, Structure, and Solid State (19)F NMR Study of ULM-17: (H(3)O)(2)[V(4)(HPO(4))(PO(4))(3)O(6)F](2)[NC(7)H(14)](6)

(H(3)O)(2)[V(4)(HPO(4))(PO(4))(3)O(6)F](2)[NC(7)H(14)](6) (labeled ULM-17) has been hydrothermally synthesized (150 degrees, 24 h, autogeneous pressure). It is monoclinic (space group P2(1)/c (No. 14)) with a = 21.4747(6) ?, b = 17.7223(5) ?, c = 20.1616(6) ?, beta = 94.329(1) degrees, and Z = 4. The structure consists in the hexagonal close packing of discrete hydronium cations, protonated quinuclidine and molecular anions [V(4)(HPO(4))(PO(4))(3)O(6)F](4)(-) (1) The structure presents two kinds of octameric anions built up from the tetrahedral arrangement of V(V)O(5)F octahedra sharing edges and vertices, capped by phosphorus tetrahedra. The stability of the solid is ensured via strong hydrogen bonds between the oxygens of the polyanions and the hydrogens of both hydronium and quinuclidinium cations. The particuliar location of fluorine at the center of the molecular anion 4-fold coordinated by V(V) was studied by solid state NMR.     

Hydrothermal synthesis in the system Ni(OH)(2)-NiSO(4): nuclear and magnetic structures and magnetic properties of Ni(3)(OH)(2)(SO(4))(2)(H(2)O)(2)

We present the synthesis, characterization by DT-TGA and IR, single crystal X-ray nuclear structure at 300 K, nuclear and magnetic structure from neutron powder diffraction on a deuterated sample at 1.4 K, and magnetic properties as a function of temperature and magnetic field of Ni(3)(OH)(2)(SO(4))(2)(H(2)O)(2). The structure is formed of chains, parallel to the c-axis, of edge-sharing Ni(1)O(6) octahedra, connected by the corners of Ni(2)O(6) octahedra to form corrugated sheets along the bc-plane. The sheets are connected to one another by the sulfate groups to form the 3D network. The magnetic properties measured by ac and dc magnetization, isothermal magnetization at 2 K, and heat capacity are characterized by a transition from a paramagnet (C = 3.954 emu K/mol and theta = -31 K) to a canted antiferromagnet at T(N) = 29 K with an estimated canting angle of 0.2-0.3 degrees. Deduced from powder neutron diffraction data, the magnetic structure is modeled by alternate pairs of Ni(1) within a chain having their moments pointing along [010] and [010], respectively. The moments of Ni(2) atoms are oppositely oriented with respect to their adjacent pairs. The resulting structure is that of a compensated arrangement of moments within one layer, comprising one ferromagnetic and three antiferromagnetic superexchange pathways between the nickel atoms.     

Synthesis, Crystal Structure, and EPR Studies of the Five-Coordinate [CuCl(3)(H(2)O)(2)](-) Complex in (dabcoH(2))(2)Cl(3)[CuCl(3)(H(2)O)(2)].H(2)O

The compound (dabcoH(2))(2)Cl(3)[CuCl(3)(H(2)O)(2)].H(2)O, where dabco = 1,4-diazabicyclo[2.2.2]octane, has been synthesized, its structure has been determined by single-crystal structural analysis, and its properties have been investigated by powder and single-crystal EPR spectroscopy. The compound crystallizes in space group Pnma at room temperature with unit cell dimensions of a = 15.227(1) ?, b = 7.467(1) ?, and c = 20.166(2) ? with Z = 4. The structure was solved by the Patterson method and refined by full-matrix least-squares to R = 4.3% for 1681 observed reflections (I > 2sigma(I)). The [CuCl(3)(H(2)O)(2)](-) anion exists with a slightly distorted trigonal bipyramidal geometry in which the three Cl atoms lie in equatorial positions and the two water molecules are in axial positions. The distortion appears to be driven by the presence of N-H.Cl hydrogen bonds. The EPR spectra are also consistent with the presence of only a small distortion from trigonal bipyramidal geometry since g(1) approximately g(2) > g(3) approximately 2.0. Analysis of the geometry indicates the distortion consists primarily of a "negative" C(2)(v)() type. Analysis of the thermal parameters supports the supposition that the observed geometry corresponds to disorder over two "positive" C(2)(v)() distortions.     

Deoxygenation of Polynuclear Metal-Oxo Anions: Synthesis, Structure, and Reactivity of the Condensed Polyoxoanion [(C(4)H(9))(4)N](4)(NbW(5)O(18))(2)O

The deoxygenation of the mixed-metal polyoxoanion [(C(4)H(9))(4)N](3)NbW(5)O(19) with benzoyl chloride in dichloromethane forms quantitatively the condensed polyoxanion [(C(4)H(9))(4)N](4)(NbW(5)O(18))(2)O, in which two polyoxoanion fragments are linked together by a Nb-O-Nb oxo bridge. The product is characterized by a strong IR band at 692 cm(-)(1) assigned to a Nb-O-Nb stretch and a broad single (93)Nb NMR resonance at 975 ppm. Partial hydrolysis of [(C(4)H(9))(4)N](4)(NbW(5)O(18))(2)O to NbW(5)O(19)O(3)(-) in wet acetonitrile was observed by IR and (17)O NMR spectroscopy. The reaction of [(C(4)H(9))(4)N](4)(NbW(5)O(18))(2)O with a variety of alcohols and phenol forms alkoxide-derivatized polyoxoanions [(C(4)H(9))(4)N](2)Nb(OR)W(5)O(18) (R = methyl, ethyl, isopropyl, cholesteryl, phenyl). The similarity of the IR spectra of these deriviatives suggests that functionalization occurs at the terminal NbO oxygen. A crystallographic study of [(C(4)H(9))(4)N](4)(NbW(5)O(18))(2)O revealed a crystallographically imposed linear Nb-O-Nb oxo bridge (Nb-O(bridge) = 1.887(3) ?) and a structure in which the terminal tungsten-oxo bonds on the adjoining polyoxoanion fragments are eclipsed. Crystal data: orthorhombic, Cmca; Z = 4, a = 15.817(2) ?, b = 17.870(2) ?, c = 35.058(2) ?; V = 9928.0(10) ?(3); R = 5.52%.     

Electronic Structure of Rh(2)(&mgr;-CO)(CO)(2)(H(2)PCH(2)PH(2))(2). An Example of a Non-A-Frame Structure

Calculations based on density functional theory (DFT) and Hartree-Fock configuration interaction (HF-CI) methodology have been carried out to investigate the rhodium-rhodium coupling in Rh(2)(CO)(2)(dppm)(2), 1 (dppm = Ph(2)PCH(2)PPh(2)) and in Rh(2)(&mgr;-CO)(CO)(2)(dppm)(2), 2. DFT geometries, obtained with the Dgauss program, are in good agreement with those determined from X-ray, but HF geometries, calculated using the same basis sets, yield bond distances systematically too long. Calculations indicate that the rhodium atoms in 1 are linked by a single bond. The insertion of a semibridging carbonyl between the two metal atoms leads to a shortening of the rhodium-rhodium distance and also to a noticeable weakening of the metal-metal interaction. Both effects, and also the stabilization of the HOMO of 2, are related to an observed change from square planar to tetrahedral of the ligand environment of the Rh atom proximal to the inserted CO. Both MO analysis and bond characterization from the topology of the charge density confirm the existence of a bonding interaction between the semibridging carbonyl and the distal rhodium atom. The electronic structures of the dicationic complex [Rh(2)(CO)(3)(dppm)(2)](2+) and of the A-frame-like, isoelectronic system Rh(2)Br(2)(&mgr;-CO) (dppm)(2) are also discussed. The electron deformation density is derived from 2 by means of several methodological approaches, namely, HF, HF-CI, DFT, and DFT + gradient corrections. The HF deformation density obtained in the plane containing the metals and the three CO ligands is discussed, as well as the "correlation density" obtained from the difference maps DFT - HF and CI - HF.