Square-planar carbyne complexes of the form Ru(tCR)(PCy 3 ) 2 X (X ) F, Cl, Br, I, O 3 SCF 3 ) are prepared by net dehydrohalogenation of the Grubbs catalysts Ru(dCHR)(PCy 3 ) 2 Cl 2 followed by substitution of the chloride ligand (when X * Cl). The dehydrohalogenation can be effected in one step (R ) n-Bu, Ph, p-C 6 H 4 Me) by Ge(CH[SiMe 3 ] 2 ) 2 or in two steps via treatment with excess aryloxide such as NaO-p-C 6 H 4 -t-Bu followed by SnCl 2 . The latter route gives greater yields but is more restricted in scope. Addition of HCl (1 equiv) to Ru(tCR)(PCy 3 ) 2 X (X ) Cl, Br, I) affords Ru(dCHR)(PCy 3 ) 2 ClX; those with mixed halide ligand sets undergo rapid halide exchange in solution. Upon treatment with the appropriate oxidant, each Ru(tC-p-C 6 H 4 Me)(PCy 3 ) 2 X complex undergoes two-electron oxidation. Oxidation of Ru(tC-p-C 6 H 4 Me)(PCy 3 ) 2 X (X ) F, Cl, Br, I) by XeF 2 , C 2 Cl 6 , Br 2 , and I 2 , respectively, yields either six-coordinate bis-phosphine complexes Ru(tC-p-C 6 H 4 Me)(PCy 3 ) 2 X 3 (X ) F, Cl) or square-pyramidal mono-phosphine complexes Ru(tC-p-C 6 H 4 Me)(PCy 3 )X 3 (X ) Br, I) depending on the size of the halide ligands. Cationic square-pyramidal complexes of the form [Ru(tC-p-C 6 H 4 Me)(PCy 3 ) 2 X 2 ] + (X ) Cl, I) can be prepared from Ru(tC-p-C 6 H 4 Me)(PCy 3 ) 2 Cl 3 by chloride abstraction using [Ph 3 C]BF 4 and from Ru(tC-p-C 6 H 4 -Me)(PCy 3 )X 3 by addition of PCy 3 . Hydride addition to Ru(tC-p-C 6 H 4 Me)(PCy 3 ) 2 Cl 3 yields the carbene complex Ru(dCHR)(PCy 3 ) 2 Cl 2 , whereas fluoride addition affords the carbyne complex Ru(tC-p-C 6 H 4 -Me)(PCy 3 ) 2 Cl 2 F, results with important implications for metathesis of vinyl fluorides. X-ray structures of Ru(tC-p-C 6 H 4 Me)(PCy 3 ) 2 X 2 F (X ) F, Cl), [Ru(tC-p-C 6 H 4 Me)(PCy 3 ) 2 Cl 2 ]BF 4 , and Ru(tC-p-C 6 H 4 -Me)(PCy 3 )I 3 reveal short RutC bonds in the 1.670(5)-1.714(3) Å range; when two PCy 3 ligands are present, they are mutually trans. The benzylidyne ligands occupy the apical sites in the two squarepyramidal complexes. Of the five-and six-coordinate complexes, only the two fluoride-containing complexes Ru(tC-p-C 6 H 4 Me)(PCy 3 ) 2 X 2 F (X ) F, Cl) display reactivity toward alkynes, serving as alkyne dimerization catalysts.

The activation parameters (delta G++298 = 11.5 (+/- 1.0) kcal mol-1, delta H++ = 16.3 (+/- 3.0) kcal mol-1, delta S++ = 16 (+/- 11) cal mol-1 K-1) have been determined for the rac to meso isomerization of a phosphametallocene, bis(3,4-dimethyl-2-phenylphospholyl)titanium dichloride, 2, which has been structurally characterized.

The dialkylgermylene [Ge(CH[SiMe3]2)2] (3) converts the carbene complexes [Ru(CHR)(PCy3)2Cl2] (R = p-C6H4Me [2], n-C4H9 [6]) into the corresponding square-planar carbyne complexes [Ru(CR)(PCy3)2Cl] (R = p-C6H4Me [1], n-C4H9 [7]) cleanly within minutes at 25 °C in benzene; addition of HCl to a benzene solution of 1 results in quantitative re-formation of 2. Replacement of chloride in 1 affords other halide and pseudohalide complexes, [Ru(C-p-C6H4Me)(PCy3)2X] (X = p-OC6H4-t-Bu, Br, F, O3SCF3), cleanly.

The first example of a low-valent phosphatitanocene has been prepared and structurally characterized, and a chiral Ti(II) phosphametallocene was prepared and was found to have a significantly higher barrier to isomerization than the Ti(IV) analogue. Descriptions of the molecular orbitals of the two complexes offer an explanation of the difference.