The reactions of RuCl2(PPh(3))(3) with a number of diazoalkanes were surveyed, and alkylidene transfer to give RuCl2(=CHR)(PPh(3))2 (R = Me (1), Et (2)) and RuCl2(=CH-p-C(6)H(4)X)(PPh(3))(2) (X = H (3), NMe(2) (4), OMe (5), Me (6), F (7), Cl (8), NO2 (9)) was observed for alkyl diazoalkanes RCHN(2) and various para-substituted aryl diazoalkanes p-C(6)H(4)XCHN(2). Kinetic studies on the living ring-opening metathesis polymerization (ROMP) of norbornene using complexes 3-9 as catalysts have shown that initiation is in all cases faster than propagation (k(i)/k(p) = 9 for 3) and that the electronic effect of X on the metathesis activity of 3-9 is relatively small. Phosphine exchange in 3-9 with tricyclohexylphosphine leads to RuCl2(=CH-p-C(6)H(4)X)(PCy(3))(2) 10-16, which are efficient catalysts for ROMP of cyclooctene (PDI = 1.51-1.63) and 1,5-cyclooctadiene (PDI = 1.56-1.67). The crystal structure of RuCl2(=CH-p-C6H4Cl)(PCy(3))(2) (15) indicated a distorted square-pyramidal geometry, in which the two phosphines are trans to each other, and the alkylidene unit lies in the Cl-Ru-Cl plane. The benzylidenes RuCl2(=CHPh)(PR(3))(2) (R = Cy (cyclohexyl) (10), Cp (cyclopentyl) (17), i-Pr (18)) are quantitatively available via one-pot synthesis with RuCl2(PPh(3))(3), PhCHN(2), and PR(3) as reaction components. 10 is an efficient catalyst for metathesis of acyclic olefins : On reaction with excess ethylene, the methylidene complex RuCl2(=CH2)(PCy(3))(2) (19) is formed quantitatively and various alkylidene compounds RuCl2(=CHR)(PCy(3))(3) (R = Me (20), Et (21), n-Bu (22)) are isolated as the kinetic products from the reaction of 10 with an excess of the corresponding terminal or disubstituted olefins. Metathesis of conjugated and cumulated olefins with 10 results in the formation of vinylalkylidene and vinylidene complexes, as shown by the synthesis of RuCl2(=CHCH=CH2)(PCy(3))(2) (23) and RuCl2(=C=CH2)(PCy(3))(2) (24) from 1,3-butadiene or 1,2-propadiene, respectively. Also, functional groups such as -OAc, -Cl, and -OH can be introduced into the alkylidene moiety via cross metathesis with the appropriate alkene.