The reaction of [Cp*RuCl]4 (1; Cp* = η5-C5Me5) with the carbene ligand 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) affords a coordinatively unsaturated Cp*Ru(IMes)Cl (3) complex in 86% isolated yield. Solution calorimetric results in this system provide information concerning the electron donor properties of the carbene ligand, which are very similar to those of PCy3. Structural information from single-crystal X-ray studies for complex 3 allows the determination of steric parameters associated with this ligand. The thermochemical information is used to predict the magnitude of the enthalpic driving force behind substitution reactions involving RuCl2(C(H)Ph)(PCy3)2 (1) and the carbene ligand, IMes, affording the RuCl2(C(H)Ph)(PCy3)(IMes) (6) complex in high yield. A similar mixed carbene/phosphine ruthenium complex, RuCl2(C(H)Ph)(PPh3)(IMes), can also be isolated from RuCl2(C(H)Ph)(PPh3)2 and the IMes ligand. A single-crystal X-ray diffraction study has been performed on 6. The thermal stability of these mixed phosphine/carbene ruthenium carbene complexes has been studied at 60 °C in toluene. Their catalytic activity has been evaluated for the ring closing metathesis of diethyldiallylmalonate.

Clear experimental evidence from X-ray photoelectron spectroscopy and (31)P NMR spectroscopy has been obtained for the first time to confirm that the combination of Ag(+) cation with [L-Au](+) results in the formation of different complexes in solution. Re-evaluation of literature-reported gold-catalyzed reactions revealed a significant difference in the reactivities with and without silver. In extreme cases (more than "rare"), the conventional [L-Au](+) catalysts could not promote the reaction without the presence of silver. This investigation has therefore revealed a long-overlooked "silver effect" in gold catalysis and should lead to revision of the actual mechanism.

The coordination chemistry of N-pynolyl phosphines (P-NC&) is described. These ligands are prepared in excellent yield from pyrrole, a phosphorus halide, and base, and this synthesis has been applied to the series PPh,(pyrrolyl)3-, (x = 0-2) and the chelate (pyrrolyl)zP(CHz)zP(pyrrolyl)~. These ligands readily form coordination complexes, and the complexes tran~-RhCl(CO)[PPh,(pynoly1)3-,]~ (x = 0-2) and Mo(C0)4[(pyrrolyl)2P(CH2)2-P(pyrrolyl)z] are described. The carbonyl stretching frequencies of these complexes are shifted to significantly higher energy relative to "traditional" phosphine ligands, indicating that N-pyrrolyl phosphines are poor a-donors, exceeding phosphites and approaching fluoroalkylphosphines with respect to this property. For example, vco for trans-RhCl(C0)-[P(pyrrolyl)3]~ exceeds that of the PPh3 analogue by 59 cm-I. That these ligands are n-acceptors is suggested by the single crystal X-ray structure of tran~-RhCl(CO)[P(pyrrolyl)3]:! which shows shortened Rh-P distances and a lengthened Rh-C distance, consistent with enhanced Rh to P back-bonding. The X-ray structure of trans-RhCl(C0)-[P(pyrrolidinyl)3]2 is also reported; this complex possesses longer Rh-P distances which more closely resemble those found for other complexes of this type. The exceptional macceptor character of these ligands is convincingly demonstrated by their substitution chemistry with electron rich [PPN] [Rh(CO)4]. P(pyrrolyl)3 is found to displace CO in a stepwise manner to give the entire series [PPN][Rh(CO)4-,{P(pyrr01yl)3}~] (x = 1-4). Similar results are obtained with (pyrrolyl)2P(CH~)~P(pyrrolyl)~, and the anions [PPNI[Rh(CO),{ (pyrrolyl)2P(CH~)2P(pyrrolyl)~}~] (x = 2, y = 1; x = 0, y = 2) are reported. An X-ray structure analysis of [ PPN][Rh(CO){P(pyrrolyl)3}3] shows thatthe Rh-P bonds in this tetrahedral anion are shorter than those found in the Rh(1) complex, consistent with significantly greater n back-bonding in this more electron rich system. The infrared spectra of these anions again show a substantial shift in YCO to higher frequency relative to other phosphine ligands. The structural results further indicate that PPhx(pyrrolyl)3-, (x = 0-2), PPh3, and P(pyrrolidiny1)s possess nearly identical steric properties (cone angles). The wide range of electronic properties (n-acceptorla-donor) exhibited by this isosteric series, together with their ready availability, suggests that they, and N-pyrrolyl phosphines in particular, may find utility in physical inorganic and organometallic chemistry. IntroductionPhosphorus(II1) ligands of the type PZ3, where Z is a substituent such as hydrocarbyl, alkoxy, halo, etc., are ubiquitous in coordination chemistry, organometallic chemistry, and homogeneous catalysis.' The wide variety of Z groups available allows the systematic alteration of ligand steric and electronic (o-donor, n-acceptor) properties. This allows a great deal of leverage in fine tuning the reactivity of metal complexes of these

Hydroarylation reactions of olefins are catalyzed by the octahedral Ru(II) complex TpRu-(CO)(NCMe)(Ph) (1) (Tp ) hydridotris(pyrazolyl)borate). Experimental studies and density functional theory calculations support a reaction pathway that involves initial acetonitrile/ olefin ligand exchange and subsequent olefin insertion into the ruthenium-phenyl bond. Metal-mediated C-H activation of arene to form a Ru-aryl bond with release of alkyl arene completes the proposed catalytic cycle. The cyclopentadienyl complex CpRu(PPh 3 ) 2 (Ph) produces ethylbenzene and styrene from a benzene/ethylene solution at 90 °C; however, the transformation is not catalytic. A benzene solution of (PCP)Ru(CO)(Ph) (PCP ) 2,6-(CH 2 P t -Bu 2 ) 2 C 6 H 3 ) and ethylene at 90 °C produces styrene in 12% yield without observation of ethylbenzene. Computational studies (DFT) suggest that the C-H activation step does not proceed through the formation of a Ru(IV) oxidative addition intermediate but rather occurs by a concerted pathway.

Combined solution calorimetric and quantum mechanics studies of reactions involving saturated and unsaturated N-heterocyclic carbene (NHC) ligands show that the difference in their relative bond dissociation energies is very small (1 kcal·mol-1). Structural and computational studies reveal small metric parameter differences. These observations in conjunction with relative reactivity profiles of NHC-modified ruthenium-based olefin metathesis catalysts suggest that very small changes in the donor properties of the NHC ligands can translate into significant differences in catalytic properties.

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The homologous series of ansa-monocyclopentadienyl amido complexes [(C 5 H 4 )SiMe 2 (Nt-Bu)]M(NMe 2 ) 2 (M ) Ti (2), Zr (3), Hf (4)) and [(C 5 Me 4 )SiMe 2 (N-t-Bu)]Zr(NMe 2 ) 2 ( 8) are prepared in 70-85% isolated yields by heating the neat 1:1 reaction mixture of (C 5 R 4 H)-SiMe 2 (N(H)-t-Bu), where R ) H, Me, and M(NMe 2 ) 4 , at 110-120 °C for 24-48 h under a N 2 purge. Whereas the reactions of 2 with 2 equiv of [NEt 3 H]Cl or HCl produce a mixture of [(C 5 H 4 )SiMe 2 (N-t-Bu)]TiCl 2 (5) and [(C 5 H 4 )SiMe 2 Cl]TiCl 2 (NMe 2 )(NMe 2 H) (6), the reaction of 2 with SiMe 3 Cl in toluene provides a suitable route for the quantitative conversion of 2 to 5. Alternatively, the reactions of 3 and 8 with 2 equiv of [NEt 3 H]Cl or HCl afford solely the dimethylamine adduct [(C 5 H 4 )SiMe 2 (N-t-Bu)]ZrCl 2 (NMe 2 H) (7) and the dichloride derivative [(C 5 Me 4 )SiMe 2 (N-t-Bu)]ZrCl 2 (1), respectively. Compounds 1-8 have been characterized in solution by NMR measurements, and the molecular structures of 2, 7, and 8 have been determined by X-ray crystallography. The M-N(appended amido) bond distances and Cp(centroid)-M-N bond angles in 2, 7, and 8 are 1.972(4), 2.055(5), 2.108(4) Å and 105.5, 101.1, 100.2°, respectively.

Titanium and zirconium complexes carrying two 2,6-bis(pyrrolyl)pyridine ligands have been synthesized and characterized. The neutral complexes Ti(PDP) and Zr(PDP) (PDP = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine) show intense ligand-to-metal charge-transfer bands in the visible region and undergo multiple reversible redox events under highly reducing conditions. Zr(PDP) exhibits photoluminescent behavior and its excited state can be quenched by mild reductants to generate a powerful electron transfer reagent with a ground state potential of -2.16 V vs Fc. This reactivity was utilized to facilitate dehalogenation reactions, the reduction of electron-poor olefins, and the reductive coupling of benzyl bromide via photoredox catalysis. In these reactions, the earth-abundant metal complex Zr(PDP) acts as a substitute for the precious metal photosensitizer [Ru(bpy)].