C-H BOND ACTIVATION OF SIMPLE ALKENES WITH (C5H5)CO FRAGMENTS - PREPARATION, MOLECULAR-STRUCTURE, AND DYNAMICAL BEHAVIOR OF TRI-NUCLEAR ANDTETRANUCLEAR CYCLOPENTADIENYLCOBALT CLUSTER COMPLEXES WITH MU-CYCLOALKYNE LIGANDS

Vicinal C-H bonds of the cycloalkenes CnH2n-2 (n = 5-8) were activated upon treatment under mild conditions with reactive sources of the (C5 H5)Co fragment. With [(C5H5)Co(C2H4)(2)] (2a), the trinuclear cluster complexes [H-2{(C5H5)Co}(3){mu(3)-C-2(CH2)(n-2)}] (5a: n = 5; 5c: n = 7; 5d: n = 8) were formed. Using the more reactive [(C5H5)(2)Co]/K, 5a -5d were obtained, along with small amounts of the tetranuclear [{(C5H 5)Co}(4){mu(4)-C-2(CH2)(n-2)}] (8a-c: n = 5-7). X-ray structure analys es of 5c, d and 8b, c were performed. A mu(3)-parallel to (or mu(3)-et a(1):eta(2):eta(1)) coordination of the cycloalkyne ligands was found in the trinuclear cluster complexes. The tetranuclear 8b, c had the cy cloalkynes quadruply bridging (mu(4)-eta(1):eta(2):eta(2):eta(1)) a bu tterfly arrangement of the metal atoms. The acetylenic carbon-carbon b onds are considerably lengthened upon coordination [1.392(4), 1.398(3) Angstrom in 5c, d; 1.464(8), 1.499(5) Angstrom in 8b, c]. Three dynam ic processes - alkyne ''walk'' on top of the Co-3 triangle, mu(2)-hydr ide migration, and mu(2)/mu(3)-hydride exchange - are operational in t he trinuclear 5a-d. Dynamic NMR data for 5a are consistent with a ''wi ndshield-wiper'' mechanism for the alkyne migration. Hydride exchange is associated with a higher energy barrier, and is probably an indepen dent dynamic process.