The hydration of phenylacetylene in the presence of the complex mer,trans-(PNP)RuCl 2 (PPh 3 ) in THF at 60°C leads to the cleavage of the C-C triple bond with formation of the carbonyl complex fac,cis-(PNP)RuCl 2 (CO) and toluene [PNP ) CH 3 CH 2 CH 2 N(CH 2 CH 2 PPh 2 ) 2 ]. A study under different experimental conditions, the use of model and isotope labeling experiments, and the detection of several intermediates, taken altogether, show that the C-C bond cleavage reaction comprises a number of steps, among which the most relevant to the mechanism are 1-alkyne to vinylidene tautomerism, conversion of a vinylidene ligand to hydroxycarbene by intramolecular attack of water, deprotonation of hydroxycarbene to σ-acyl, deinsertion of CO from the acyl ligand, and hydrocarbon elimination by protonation of the metal-alkyl moiety. The following intermediate species have been isolated and characterized: the vinylidene fac,cis-(PNP)RuCl 2 {CdC(H)Ph}, the (aquo)(σ-alkynyl) complex fac-(PNP)RuCl-(CtCPh)(OH 2 ), and the (benzyl)carbonyl mer-(PNP)RuCl(η 1 -CH 2 Ph)(CO). Other intermediates such as the σ-acyl mer-(PNP)RuCl(η 1 -COCH 2 Ph)(CO) have been intercepted by addition of appropriate reagents, while the independent synthesis of the aminocarbene complex fac,cis-(PNP)RuCl 2 {C(NC 5 H 10 )(CH 2 Ph)} and its reaction with water have provided evidence for the intermediacy of a hydroxycarbene species in the C-C bond cleavage reaction.

The Ru(II) complex mer,trans-(PNP)RuCl2(PPh3) (1) reacts with either phenylacetylene or p-tolylacetylene to give fac,cis-(PNP)RuCl2{CCH(R)} (R = Ph (2), p-tolyl (3)) in refluxing tetrahydrofuran (THF) and mer,trans-(PNP)RuCl2{CCH(R)} (R = Ph (4), p-tolyl (5)) in a refluxing THF/EtOH mixture (1:2 v:v) [PNP = CH3CH2CH2N(CH2CH2PPh2)2]. The fac,cis vinylidene complexes 2 and 3 react with an excess of LiC⋮CPh converting to the σ-alkynyl−η3-enynyl complexes anti,mer-(PNP)Ru(C⋮CPh){η3-PhC3CH(Ph)} (8) and anti,mer-(PNP)Ru(C⋮CPh){η3-PhC3CH(p-tolyl)} (9), respectively. Conversely, treatment of the mer,trans vinylidene isomers with an excess of LiC⋮CPh, followed by addition of a primary alcohol, exclusively gives the σ-alkynyl−η-dienynyl complexes mer-(PNP)Ru(C⋮CPh){η-PhCC(C⋮CPh)CHCH(R)} (R = Ph, 12; R = p-tolyl, 13). Single-crystal X-ray analyses have been carried out on 8 and 13. In both compounds the coordination geometry around the Ru atom approximates an octahedron with three positions taken by a mer PNP ligand and one position taken by a phenylethynyl group. The coordination sphere around the metal center is completed by an η3-1,4-diphenylbut-3-en-1-ynyl ligand in 8 and by a 1-p-tolyl-3-(phenylethynyl)-4-phenylbuta-1(E),3(Z)-dien-4-yl ligand in 13. The latter ligand essentially uses the C1 carbon atom to bind the metal, although a weak bonding interaction may be envisaged also with the alkynyl substituent in the 3-position. A single-crystal X-ray analysis has also been carried out on the octahedral fac,cis-(PNP)RuCl2(CO) complex obtained by treatment of the vinylidenes 2 and 3 with molecular oxygen. The influence of the bonding mode of the PNP ligand on the different reactivity shown by the fac and mer vinylidene complexes toward LiC⋮CPh is discussed in light of a multiform experimental study (X-ray structure determinations, multinuclear NMR spectroscopy, deuterium- and p-tolyl-labeling experiments, independent reactions with isolated compounds).

After displacement of the η 2 -H 2 ligand from [(triphos)Re(CO) 2 (η 2 -H 2 )]BF 4 (2), ethyne and various 1-alkynes, HC≡CR, are tautomerized at the Re(I) center to vinylidene ligands (R ) H, Ph, p-tolyl, COOEt, n-C 6 H 13 , SiMe 3 ). A kinetic π-alkyne adduct is intercepted at low temperature during the reaction between 2 and ethyne. The primary vinylidene complex [(triphos)Re(CO) 2 (CdCH 2 )]BF 4 (7-BF 4 ) can also be obtained by reaction of the (trimethylsilyl)vinylidene complex [(triphos)Re(CO) 2 {CdC(H)SiMe 3 }]BF 4 with stoichiometric water. Unprecedented examples of addition of either water or alcohols to Re-vinylidene moieties to give hydroxycarbene or alkoxycarbene complexes are presented. In particular, an excess of water transforms 7-BF 4 into the secondary hydroxycarbene complex [(triphos)Re(CO) 2 {C(OH)-CH 3 }]BF 4 (11), which can be isolated in the solid state. Upon thermolysis in refluxing tetrahydrofuran, 11 selectively converts to the tricarbonyl complex [(triphos)Re(CO) 3 ]BF 4 and methane. Deprotonation of 11 by mild bases gives the acetyl derivative (triphos)Re-(CO) 2 (COCH 3 ), which regenerates the hydroxycarbene precursor by protonation with strong acids. The ethoxycarbene complexes [(triphos)Re(CO) 2 {C(OEt)CH 2 R}]BPh 4 (R ) H, COOEt) are prepared by nucleophilic addition of ethanol across the CdC double bond of the corresponding vinylidene derivatives. Neutral σ-alkynyl complexes of the general formula (triphos)Re(CO) 2 (C≡CR) (R ) Ph, p-tolyl, COOEt, n-C 6 H 13 , H) are obtained by reaction of the vinylidene derivatives with strong bases. The reaction of the σ-alkynyl complexes with various methylating agents affords disubstituted vinylidene derivatives, herein exemplified by [(triphos)Re(CO) 2 {CdC(Me)Ph}](OSO 2 CF 3 ). The structural identities in the solid state of the phenylvinylidene complex [(triphos)Re(CO) 2 {CdC(H)Ph}]BF 4 and of the ethoxycarbene complex [(triphos)Re(CO) 2 {C(OEt)CH 3 }]BPh 4 have been determined by X-ray diffraction analyses. In both complexes the metal center is octahedrally coordinated by a fac triphos ligand, by two mutually cis terminal carbonyls, and by the organyl ligand.

A new class of precursors for water-soluble unsaturated carbenes was synthesized by reaction of [Cp*RuCl(PMe 3 ) 2 ] with a terminal alkyne and a propargyl alcohol, each bearing a polyhydroxylated lateral chain derived from D-xylose. In the reaction with the terminal glycoynitol, the presence of a pair of noninterconverting π-alkyne intermediates of [Cp*Ru-(PMe 3 ) 2 ] + species was observed for the first time and the kinetics of isomerization to the same vinylidene species measured by NMR experiments.