Reactions of tricarbonyl-n-indenylmethylmolybdenum with phosphorus ligands yield, in the first instance, products of the same type as those obtained from the analogous a-cyclopentadienyl complex. The rates of reaction, both in tetrahydrofuran and in n-hexane, are however appreciably faster than those for the a-cyclopentadienyl complex.Possible explanations for this increase in rate are discussed.In all cases, the initial product undergoes a further reaction which is believed to be an isomerization similar to those previously described for complexes of the type ( T -C~H~) M O ( C O )~( L ) I .

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NMR studies reveal that complexes Ru(CO)(2)(H)(2)L(2) (L = PMe(3), PMe(2)Ph, and AsMe(2)Ph) can have three geometries, ccc, cct-L, and cct-CO, with equilibrium ratios that are highly dependent on the electronic properties of L; the cct-L form is favored, because the sigma-only hydride donor is located trans to CO rather than L. When L = PMe(3), the ccc form is only visible when p-H(2) is used to amplify its spectral features. In contrast, when L = AsMe(2)Ph, the ccc and cct-L forms are present in similar quantities and, hence, must have similar free energies; for this complex, however, the cct-CO isomer is also detectable. These complexes undergo a number of dynamic processes. For L(2) = dppe, an interchange of the hydride positions within the ccc form is shown to be accompanied by synchronized CO exchange and interchange of the two phosphorus atoms. This process is believed to involve the formation of a trigonal bipyramidal transition state containing an eta(2)-H(2) ligand; in view of the fact that k(HH)/k(DD) is 1.04 and the synchronized rotation when L(2) = dppe, this transition state must contain little H-H bonding character. Pathways leading to isomer interconversion are suggested to involve related structures containing eta(2)-H(2) ligands. The inverse kinetic isotope effect, k(HH)/k(DD) = 0.5, observed for the reductive elimination of dihydrogen from Ru(CO)(2)(H)(2)dppe suggests that substantial H-H bond formation occurs before the H(2) is actually released from the complex. Evidence for a substantial steric influence on the entropy of activation explains why Ru(CO)(2)(H)(2)dppe undergoes the most rapid hydride exchange. Our studies also indicate that the species [Ru(CO)(2)L(2)], involved in the addition of H(2) to form Ru(CO)(2)(H)(2)L(2), must have singlet electron configurations.

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Enhancements of NMR signals by parahydrogen induced polarisation aids detection of a minor isomer of complexes [RuL~(CO)~(H)~] (L = AsMe2Ph, PMeZPh, PMe3) containing inequivalent hydride ligands: some enhancement is also observed for the major isomer in which the hydrides are equivalent, and this is increased by use of [RuL~(WO)(~~CO)(H)~].We have recently shown that the enhanced absorption and emission signals observed in NMR spectra of nuclei arising from para-enriched-hydrogen (p-H2) can be used to detect and characterise materials present in concentrations too low for detection by normal methods. 1,2 In this communication, we describe how the complexes [RuL2(CO),(H),] (L = AsMe2Ph, PMeZPh, PMe3) behave with p-H2. We show that, using this

The photochemical reaction of Ru(CO)(3)(dppe) and Fe(CO)(3)(dppe)(dppe = Ph(2)PCH(2)CH(2)PPh(2)) with parahydrogen has been studied by in situ-photochemistry resulting in NMR spectra of Ru(CO)(2)(dppe)(H)(2) that show significant enhancement of the hydride resonances while normal signals are seen in Fe(CO)(2)(dppe)(H)(2). This effect is associated with a singlet electronic state for the key intermediate Ru(CO)(2)(dppe) while Fe(CO)(2)(dppe) is a triplet. DFT calculations reveal electronic ground states consistent with this picture. The fluxionality of Ru(CO)(2)(dppe)(H)(2) and Fe(CO)(2)(dppe)(H)(2) has been examined by NMR spectroscopy and rationalised by theoretical methods which show that two pathways for ligand exchange exist. In the first, the phosphorus and carbonyl centres interchange positions while the two hydride ligands are unaffected. A second pathway involving interchange of all three ligand sets was found at slightly higher energy. The H-H distances in the transition states are consistent with metal-bonded dihydrogen ligands. However, no local minimum (intermediate) was found along the rearrangement pathways.