As observed by variable temperature and pressure (13)C NMR, the intramolecular scrambling of carbonyl ligands in Rh(4)(CO)(12) and IrRh(3)(CO)(12) is due to a merry-go-round process (3 &mgr;(2)-CO <--> 3 eta(1)-CO) about any triangular face of the metal tetrahedron. Both cluster compounds have a negative activation volume on going from the bridged ground-state structure of C(3)(v)() symmetry to an unbridged transition state, suggesting that bridged M-M distances are longer than unbridged M-M distances. Site exchange is faster in Rh(4)(CO)(12) than in IrRh(3)(CO)(12) where the apical position is occupied by an iridium atom. Density functional calculations on the bridged and unbridged forms of both cluster compounds have been made at two levels of approximation (LDA and GGA) including relativistic effects for Ir and Rh. LDA reproduces best the experimental distances and shows that opening bridges shortens the M-M bonds. The difference in volume of the bridged and unbridged forms of Rh(4)(CO)(12), as calculated from Connoly surfaces, agrees fairly well with the experimental activation volume. Calculations at the GGA level give the correct trends in energies. ELF maps and overlap population analysis indicate that iridium is more electropositive than rhodium, as suggested by the experimental results.

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NMR spectra of [RuC1Oi-C1)(tj3:7)3-C10H16)]2 (1) in noncoordinating solvents indicate that an equilibrium between two diastereoisomeric forms is rapidly established in solution (k298 = (3 ± 2) x 10"1 s"1; AG*298 = 76 ± 2 kJ mol"1) and provide evidence that the local C2 symmetry of the [Ru(ij3:n3-CI0H16)] fragments is retained. Monomeric solvates of formula [RuC12(jj3:t/3-C10H16)S] form on dissolution of 1 in coordinating solvents and have been isolated for S = pyridine (2), DMSO (S-txiund) (3), MeCN (4), and DMF (O-bound) (5). A trigonal-bipyramidal ruthenium coordination, with the octadienediyl ligand locked in a local C2 symmetry configuration and occupying two of the equatorial sites, is proposed for all the solvates. In solution, 2 and 3 feature solvent coordination uniquely in the third equatorial site, and the chiral octadienediyl configuration results in diastereotopic methyl groups coordinated to sulfur in 3. Variable-temperature NMR measurements in acetone-¿6 show that these diastereotopic methyl groups are rendered equivalent as a result of intermolecular DMSO exchange between the enantiomers (AG*308 = 66.5 ± 0.5 kj mol"1). The formation of 4 and 5 from 1 proceeds via initial formation of equatorial solvates, which rapidly isomerize to an equilibrium mixture containing both axially and equatorially solvated isomers. Equilibrium constants and reaction enthalpies for the isomerizations have been determined. The kinetic activation parameters for the interconversion of the isomers of 4 have also been evaluated. A mechanism involving solvent exchange is proposed. Equilibria of the type [RuCl2(7/3:n3-C10H16)S] ¡= [RuCl2(n3:n3-C,oH16)S'] or [RuC12(t73:t¡3-C]0H16)S] 1 are set up on dissolution of 2-5 in certain coordinating or noncoordinating solvents, respectively. Evaluation of the equilibrium constants suggests that the affinities for coordination to the monomers of 1 fall in the sequence pyridine > DMSO > MeCN > DMF.(2,7-Dimethyloctadienediyl)ruthenium(IV) Complexes

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Methyltrioxorhenium, CH 3 ReO 3 (1; MTO), hydrolyzes rapidly in basic aqueous solutions and much more slowly in acidic media. At low concentrations (c MTO < 8.0 × 10 -3 M) the formation of CH 4 gas and perrhenate was detected. The hydrolysis was followed by spectrophotometry, and the rate constants were determined as a function of temperature and pressure according to the rate lawThe rate constant, activation enthalpy, entropy, and volume at 298.2 K are as follows:) 15.9 ( 1, ∆S 1 q /J mol -1 K -1 ) -135 ( 3, and ∆V 1 q /cm 3 mol -1 ) -2.4 ( 0.4. Attempts to perform the reverse reaction, the synthesis of MTO from [ReO 4 ] -and CH 4 (200 MPa), failed: the decomposition of MTO seems to be irreversible. At higher MTO concentrations a second reaction, a faster reversible polymerization-precipitation, takes place to yield a gold solid of net formula (C 0.92 H 3.3 ReO 3.0 ) n . The rate of polymerizationprecipitation was studied as a function of temperature in D 2 O by 1 H NMR spectroscopy; it follows first-order reversible kinetics. The rate constant, activation enthalpy, and entropy of polymerization-precipitation at 298.2 K are as follows: k 2 /s -1 ) (2.1 ( 0.4) × 10 -6 , ∆H 2 q / kJ mol -1 ) 111.9 ( 5, ∆S 2 q /J mol -1 K -1 ) 25 ( 16 (the parameters for the dissolution of the polymer are estimated data). The rate of the polymerization-precipitation is independent of the concentration of H + , and the reaction does not occur in the presence of [ClO 4 ] -, [NO 3 ] -, and other oxidants.