Metal-metal and metal-carbon bond energy terms for the rhodium carbonyl clusters Rh4(CO)12 and Rh6(CO)16

Alternative ways of obtaining metal-metal and metal-carbon bond energy terms, E(M-M) and E(M-C), from published thermochemical data for the clusters Rh₄(CO)₁₂ and Rh₆(CO)₁₆ are outlined. One gives average bond energy terms for the two clusters of E(M-M) = 86±11 and E(M-C) = 178±8 kJ.mol⁻¹. The other, which makes allowance for the greater length, and presumably lower strength, of the metal-metal bonds of Rh₆(CO)₁₆, gives E(M-M) = 89±11 kJ.mol⁻¹ for this cluster, 91±11 kJ.mol⁻¹ for Rh₄(CO)₁₂, and E(M-C) = 175±8 kJ.mol⁻¹.     

Thermal decomposition of a series of tetranuclear carbonyl clusters Rh4(CO)12, Rh2Co2(CO)12, RhCo3(CO)12 and Co4(CO)12

Decarbonylation of the unsupported clusters Rh₄(CO)₁₂, Rh₂Co₂(CO)₁₂, RhCo₃(CO)₁₂ and Co₄(CO)₁₂ in a stream of hydrogen has been investigated by temperature programmed decomposition. Kinetic parameters for the thermal decomposition are presented, and the stabilities of the clusters are discussed. The profile for evolution of CO from Rh₄(CO)₁₂ indicates that a stable intermediate is formed. In all four cases methane is formed stepwise until most of the CO groups are evolved.     

Synthesis and electrochemistry of new Rh-centered and conjuncto rhodium carbonyl clusters. X-ray structure of [NEt(4)](3)[Rh(15)(CO)(27)], [NEt(4)](3)[Rh(15)(CO)(25)(MeCN)(2)] x 2MeCN, and [NEt(4)](3)[Rh(17)(CO)(37)

A reinvestigation of the redox chemistry of [Rh7(CO)16]3- resulted in the finding of new alternative syntheses for a series of previously reported Rh-centered carbonyl clusters, i.e., [H4-nRh14(CO)25]n- (n = 3 and 4) and [Rh17(CO)30]3-, as well as new species such as a different isomer of [Rh15(CO)27]3-, the carbonyl-substituted [Rh15(CO)25(MeCN)2]3-, and the conjuncto [Rh17(CO)37]3- clusters. All of the above clusters are suggested to derive from oxidation of [Rh7(CO)16]3- with H+, arising from dissociation either of [M(H2O)n]2+ aquo complexes or nonoxidizing acids. The nature of the previously reported species has been confirmed by IR, electrospray ionization mass spectrometry, and complete X-ray diffraction studies. Only the molecular structures of the new clusters are reported in some details. The ready conversion of [Rh7(CO)16]3- in [HRh14(CO)25]3- upon oxidation has been confirmed by electrochemical techniques. In addition, electrochemical studies point out that the close-packed [H3Rh13(CO)24]2- dianion undergoes a reversible monoelectronic reduction followed by an irreversible reduction. The irreversibility of the second reduction is probably a consequence of H2 elimination from a purported [H3Rh13(CO)24]4- species. Conversely, the body-centered-cubic [HRh14(CO)25]3- and [Rh15(CO)27]3- trianions display several well-defined redox changes with features of electrochemical reversibility, even at low scan rate. The major conclusion of this work is that mild experimental conditions and a tailored oxidizing reagent may enable more selective conversion of [Rh7(CO)16]3- into a higher-nuclearity rhodium carbonyl cluster. It is also shown that isonuclear Rh clusters may display isomeric metal frameworks [i.e., [Rh15(CO)27]3-], as well as almost identical metal frames stabilized by a different number of carbonyl groups [i.e., [Rh15(CO)27]3- and [Rh15(CO)30]3-]. Other isonuclear Rh clusters stabilized by a different number of CO ligands more expectedly exhibit completely different metal geometries [i.e., [Rh17(CO)30]3- and [Rh17(CO)37]3-]. The first pair of isonuclear and isoskeletal clusters is particularly astonishing in that [Rh15(CO)30]3- features six valence electrons more than [Rh15(CO)27]3-. Finally, the electrochemical studies seem to suggest that interstitial Rh atoms are less effective than Ni and Pt interstitial atoms in promoting redox properties and inducing molecular capacitor behavior in carbonyl clusters.     

Synthesis and structures of new heteronuclear cluster complexes [PPh4][Fe4Rh3Se2(CO)16] and [PPh4]2[Fe3Rh4Te2(CO)15]

The reaction of the K₂[Fe₃Q(CO)₉] clusters (Q = Se or Te) with Rh₂(CO)₄Cl₂ under mild conditions is accompanied by complicated fragmentation of cores of the starting clusters to form large heteronuclear cluster anions. The [PPh₄][Fe₄Rh₃Se₂(CO)₁₆] and [PPh₄]₂[Fe₃Rh₄Te₂(CO)₁₅] compounds were isolated by treatment of the reaction products with tetraphenylphosphonium bromide. The structures of the products were established by X-ray diffraction. In both compounds, the core of the heteronuclear cluster consists of two octahedra fused via a common Rh₃ face. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 775–778, May, 2006.     

High nuclearity PtRh carbonyl clusters. Synthesis and X-ray characterization of the [Pt2Rh9(CO)22]3− trianion

The new mixed PtRh cluster trianion [Pt₂Rh₉(CO)₂₂]^3? has been isolated as a minor product of the pyrolysis of [PtRh₅(CO)₁₅]^?, and has been characterized by X-ray diffraction. The metal skeleton, which has ideal D_3h symmetry, consists of three face-to-face condensed octahedra, as previously found in the isoelectronic species [Rh₁₁CO)₂₃]^3?, with the Pt atoms on the three-fold axis, in the positions of maximum MM connectivity.     

Cationic carbonyl complexes of rhodium(I) and rhodium(III): syntheses,vibrational spectra,NMR studies,and molecular structures of tetrakis(carbonyl)rhodium(I) heptachlorodialuminate and -gallate, [Rh(CO)4][Al2Cl7] and [Rh(CO)4][Ga2Cl7

Dimeric rhodium(I) bis(carbonyl) chloride, [Rh(CO)(2)(mu-Cl)](2), is found to be a useful and convenient starting material for the syntheses of new cationic carbonyl complexes of both rhodium(I) and rhodium(III). Its reaction with the Lewis acids AlCl(3) or GaCl(3) produces in a CO atmosphere at room temperature the salts [Rh(CO)(4)][M(2)Cl(7)] (M = Al, Ga), which are characterized by Raman spectroscopy and single-crystal X-ray diffraction. Crystal data for [Rh(CO)(4)][Al(2)Cl(7)]: triclinic, space group Ponemacr; (No. 2); a = 9.705(3), b = 9.800(2), c = 10.268(2) A; alpha = 76.52(2), beta = 76.05(2), gamma = 66.15(2) degrees; V = 856.7(5) A(3); Z = 2; T = 293 K; R(1) [I > 2sigma(I)] = 0.0524, wR(2) = 0.1586. Crystal data for [Rh(CO)(4)][Ga(2)Cl(7)]: triclinic, space group Ponemacr; (No. 2); a = 9.649(1), b = 9.624(1), c = 10.133(1) A; alpha = 77.38(1), beta = 76.13(1), gamma = 65.61(1) degrees; V = 824.4(2) A(3); Z = 2; T = 143 K; R(1) [I > 2sigma(I)] = 0.0358, wR(2) = 0.0792. Structural parameters for the square planar cation [Rh(CO)(4)](+) are compared to those of isoelectronic [Pd(CO)(4)](2+) and of [Pt(CO)(4)](2+). Dissolution of [Rh(CO)(2)Cl](2) in HSO(3)F in a CO atmosphere allows formation of [Rh(CO)(4)](+)((solv)). Oxidation of [Rh(CO)(2)Cl](2) by S(2)O(6)F(2) in HSO(3)F results in the formation of ClOSO(2)F and two seemingly oligomeric Rh(III) carbonyl fluorosulfato intermediates, which are easily reduced by CO addition to [Rh(CO)(4)](+)((solv)). Controlled oxidation of this solution with S(2)O(6)F(2) produces fac-Rh(CO)(3)(SO(3)F)(3) in about 95% yield. This Rh(III) complex can be reduced by CO at 25 degrees C in anhydrous HF to give [Rh(CO)(4)](+)((solv)); addition of SbF(5) at -40 degrees C to the resulting solution allows isolation of [Rh(CO)(4)][Sb(2)F(11)], which is found to have a highly symmetrical (D(4)(h)()) [Sb(2)F(11)](-) anion. Oxidation of [Rh(CO)(2)Cl](2) in anhydrous HF by F(2), followed in a second step by carbonylation in the presence of SbF(5), is found to be a simple, straightforward route to pure [Rh(CO)(5)Cl][Sb(2)F(11)](2), which has previously been structurally characterized by us. All new complexes are characterized by vibrational and NMR spectroscopy. Assignment of the vibrational spectra and interpretation of the structural data are supported by DFT calculations.     

The impact of weak C-H...Rh interactions on the structure and reactivity of trans-[Rh(CO)2(phosphine)2]+: an experimental and theoretical examination

The crystal structure of the new cationic Rh(I) complex trans-[Rh(CO)(2)(L)(2)]BF(4) (L=alpha(2)-(diisopropylphosphino)isodurene) was found to exhibit a nonlinear OC-Rh-CO fragment and weak intramolecular C-H...Rh interactions. These interactions, which have also been shown to occur in solution, have been examined by density functional theory calculations and found to be inextricably linked to the presence of the distorted OC-Rh-CO fragment. This linkage has also been demonstrated by comparison with a highly similar Rh(I) complex, in which these C-H...Rh interactions are absent. Furthermore, the presence of these weak interactions has been shown to have a significant effect on the reactivity of the metal center.     

The heats of formation of some protonated olefinic carbonyl compounds: [C5H9O]+ and [C4H7O2]+ ions

The proton transfer equilibrium reactions involving 3-penten-2-one, 3-methyl-3-buten-2-one, crotonic acid and methacrylic acid were carried out in an ion cyclotron resonance (ICR) spectrometer. The semiempirical method MNDO, used to estimate the heats of formation for 14 protonated [C₅H₉O]⁺ and [C₄H₇O₂]⁺ ions and the energetic aspect of the fragmentations of metastable [C₆H₁₂O]^+. and [C₆H₁₂O₂]^+. ions, leads to the conclusion that the ions corresponding to protonation at the carbonyl oxygen are the most stable. Thus the experimentally determined heats of formation of protonated olefinic carbonyl compounds can be attributed to the following structures: [CH₃COHCHCHCH₃]⁺ (δH_f = 490 KJ mol^?1), [CH₃COHC(CH₃)CH₂]⁺ (δH_f = 502 KJ mol^?1), [HOCOHCHCHCH₃]⁺ (δH_f = 330 KJ mol^?1) and [HOCOHC(CH₃)CH₂]⁺ (δH_f = 336 KJ mol^?1).     

Vibrational frequencies of the homoleptic cobalt carbonyls: Co4(CO)12 and Co6(CO)16

The homoleptic cobalt carbonyls Co4(CO)12 and Co6(CO)16 are characterized by their equilibrium geometries, thermochemistry, and vibrational frequencies using density functional theory (DFT) methods with the B3LYP, BLYP, and BP86 functionals. The B3LYP predicted CoCo distances are 2.51 and 2.47 A for the C3v and Td structures, respectively, of Co4(CO)12. The global minimum for Co4(CO)12 has C3v symmetry with three bridging and nine terminal carbonyls. The 2.51 and 2.52 A CoCo distances suggest the single bond required for an 18-electron configuration for the metal atoms. This structure is close to an experimentally realized structure. A more symmetrical Co4(CO)12 structure with Td symmetry, analogous to that observed in the valence isoelectronic Ir4(CO)12 molecule, lies approximately 28 kcal/mol higher in energy and exhibits a small imaginary vibrational frequency ( approximately 40i). It has a slightly shorter CoCo distance of 2.47 A. Both Co4(CO)12 structures satisfy the 18-electron rule. The Co6(CO)16 structure has Td symmetry and satisfies the Wade-Mingos rules for an octahedral cluster. The nu(CO) carbonyl frequencies for both Co4(CO)12 and Co6(CO)16 computed with the BP86 functional are closer to the experimental values than those computed with the B3LYP and BLYP functionals. The structure of Co6(CO)16 is not known experimentally, but the BP86 functional predicts 2.56 A (CoCo), 1.77 and 2.02 A (CoC), and 1.66 and 1.20 A (CO) for the bond distances.     

Post-protein-binding reactivity and modifications of the fac-[Re(CO)3]+ core

The reactivity of the [Re(CO)(3)(H(2)O)(2)](+) complex coordinated to the His15 residue of HEW lysozyme is described. In the fully metalated protein (Lys-1), the Re ion retains its reactivity only toward selected ligands, while others induce a ligand-mediated demetalation of the enzyme. It is further shown that some of the complexes that may be "engineered" on the lysozyme do not react with the free protein even if present in solution in excess. The formation of stable metal adducts starting from Lys-1 was confirmed by X-ray crystallography.     

Why [CpW(CO)3]+ reduces H2 to dihydride

The [CpW(CO)3]+ complex, with three pi acceptor ligands and a positive charge, is shown to have an unexpected reducing ability towards H2 because of a low lying triplet state energy.     

Energy deposition in [Fe(CO)5]+˙ upon collision with a metal surface

Collision of the title ion upon a stainless steel surface at near-normal incidence leads to deposition of internal energy in a well-defined narrow distribution. The energy deposited increases with laboratory collision energy and exceeds 7 eV (average) for 100 eV collisions. The translational-to-vibrational energy transfer efficiency is 15% (assuming an infinitely massive target) at 25 eV collision energy. Comparison is made with the internal energy distributions associated with gas-phase collisional activation using both low and high ion kinetic energies. The narrowness of the distribution of internal energies, the easy access to ions excited to different extents, and the high internal energies accessible, make the ion/surface collision process superior to gas-phase collisional activation for this system.     

Pentafluorophenyl carbonyl compounds in the Reformatsky-type reactions promoted with Fe(CO)5-based metal complex systems

Iron pentacarbonyl is an effective promoter for additions of halogenated acid esters and nitriles to pentafluorophenyl carbonyl compounds 1, 4, and 5 by the Reformatsky-type reaction and reductive coupling of compound 1. The electron-withdrawing character of the pentafluorophenyl group has a significant effect on the reaction pathway and the type of the reaction products. The reactions involving metal complex systems derived from Fe(CO)₅ have a number of advantages such as a simple procedure for carrying out, the lack of necessity to use anhydrous solvents and an inert atmosphere. The schemes of the reactions have been proposed and the conditions for preparative syntheses of most products have been optimized.     

Crystal structure of a platinum carbonyl phosphine cluster Pt4(CO)5[P(C2H5)3]4 and the IR bands of CO groups with different coordination to the metal atoms

Conclusions The crystal structure of Pt₄(CO)₅(PEt₃)₄ was determined and the IR spectra of this compound in the V_CO region characteristic for such clusters were studied. Four models for the interaction of CO with metals upon adsorption were proposed on the basis of the geometrical parameters obtained for the coordination of CO in this and other clusters. Calculation of the frequencies and shape of the vibrations of all four models showed that we must independently evaluate the force constants of the interaction of the CO groups with the remaining part of the complex in order to develop reliable spectral indicators for the specific type of CO coordination.Translated from Zhurnal Strukturnoi Khimii, Vol. 26, No. 2, pp. 84–93, March–April, 1985.     

The nu(CO) vibrational spectra of planar transition metal carbonyl clusters

The nu(CO) vibrational spectra of planar transition cluster carbonyls containing M(CO)(4) groups are studied. It is possible to anticipate qualitatively, both for the infrared and Raman, the band intensity changes associated with increasing metallic nature of the cluster. These enable a unification of the band patterns shown by the species reported. As for (idealized) spherical clusters, the spherical harmonic model (SHM), suitably modified, becomes of more general applicability as cluster size increases, although for smaller species the tensor harmonic model (THM) makes a contribution.     

Formation of a six-coordinate fac-[Re(CO)3]+ complex by the N-C bond cleavage of a potentially tetradentate ligand

The rhenium(I) compound fac-[Re(CO)₃(daa)].Hpab.H₂O (Hpab = N,N’-(1,2-phenylene)bis(2′-aminobenzamide); Hdaa = 2- amino-N-(2-aminophenyl)benzamide) was synthesized from the reaction of [Re(CO)₅Br] with two equivalent of Hpab in toluene. The monoanionic tridentate ligand daa was formed by the rhenium-mediated cleavage of an amido N-C bond of the potentially tetradentate ligand Hpab. The compound was characterized by IR spectroscopy and X-ray crystallography, and daa is coordinated as a diaminoamide via three nitrogen-donor atoms.     

Kinetics and mechanism of the acid-catalysed decarboxylation ofcis-[Co(cyclen)CO3]+

Summary The acid catalysed decarboxylation ofcis-[Co(cyclen)CO₃] has been studied over a range of nitric acid concentrations, at 25, 35.4 and 45°. The rate expression takes the form: k_obs=k₀+k₁ [H⁺], where k_obs is the observed first order rate constant at constant hydrogen ion concentration. The ko term which represents the spontaneous or water reaction is kinetically unimportant at the acidities used in the study. The activation parameters for the acid-catalysed decarboxylation are H=100.4 kj mol^–1 and S ₂₉₈=+51 JK^–1mol^–1. The acid catalysed reaction is subject to a deuterium solvent isotope effect consistent with a mechanism involving a rapid preequilibrium protonation of the complex followed by a slow ratedetermining ring opening of the carbonate ring.