SIGNIFICANCE OF CARBENOID CHARACTER DURING THE PROTONATION OF 4-MEMBERED AND 5-MEMBERED DIRUTHENACYCLES FORMED FROM THE REACTION OF RU-2(ME(2)PCH(2)PME(2))(2)(CO)(5) WITH DIMETHYL ACETYLENEDICARBOXYLATE

Protonation of the diruthenacyclobutene -2(dmpm)2(CO)(4)[mu-eta(1):eta (1)-C-2(CO(2)Me)(2)] (dmpm = bis(dimethylphosphino)methane) resulted i n stereoselective formation of the chelated (Z)alkenyl complex {Ru-2(d mpm)(2)(CO)(4)[C-2(H)(CO(2)Me)(2)]}[BF4], as deternnined by single-cry stal X-ray crystallography (P2(1)2(1)2 (No. 18) space group, a = 15.61 (2) Angstrom, b = 22.67(2) Angstrom, c = 9.34(1) Angstrom, V = 3308(5) Angstrom(3), Z = 4). Protonation of the diruthenacyclopentenone Ru-2( dmpm)(2)(CO)(4) [mu-C(O)C-2(CO(2)Me)(2)] occurs reversibly at the carb onyl oxygen of the bridging group, giving the corresponding diruthenac yclopentenol. The X-ray crystal structure of the solvated molecule [P2 (1) (No. 4) space group, a = 16.010(7) Angstrom, b = 11.833(6) Angstro m, c = 19.450(8) Angstrom, beta = 103.44(3)degrees, V = 3584(5) Angstr om(3), Z = 4) and its unusual blue color suggest a highly delocalized pi-system between the two metals. Reaction of the metal hydride [HRu2( dmpm)(2)-(CO)(5)][BF4] with dimethyl acetylenedicarboxylate (DMAD) res ults in formation of a new C-H bond and a sigma-bound (E)-alkenyl liga nd. This same (E)-alkenyl product can also be formed by addition of CO to and alkene isomerization of the chelated (Z)-alkenyl ex{Ru-2(dmpm) (2)(CO)(4)[C-2(H)(CO(2)Me)(2)]}[BF4].