A series of reactions of the general type [Mo(CO),~(q5-C,H,)] + + L' -[Mo(CO),(L') L(q3-C,Hg)] +, involving q5 --+ q3 hapticity conversion at a molybdenum co-ordinated cycloheptadienyl ring, have been examined. The specific examples investigated comprise reactions of [ Mo(CO),L,(q5-C7H,)] + [L, = 2,2'-bipyridyl (bipy) 1, Ph,PCH,PPh, (dppm) 6, Ph,PCH,CH,PPh, (dppe) 7; L = CNBu' 17 or PPh, 181 with NCMe or CNBu'. Complex 1 reacts reversibly with NCMe to give [Mo(CO),-(NCMe)(bipy)(q3-C7Hg)]+ 2 and, in NCMe, 7 exists as an equilibrium mixture with [Mo(CO),(NCMe)(dppe)(q3-C7Hg)]+ 8. The complexes 6, 17 and 18 do not react with NCMe at ambient temperature. Treatment of 1, 6 and 7 with CNBu' affords [Mo(CO),(CNBut)L,(q3-C7Hg)]+ (L, = bipy 4, dppm 9 or dppe 11) although 4 is better prepared by reaction of [Mo(CO),-(CNBu') (NCMe),(q3-C7Hg)] + 5 with bipy. Complex 4 is resistant to carbonyl elimination but 9 and 11 undergo facile loss of CO to give [Mo(CO)(CNBu')L,(q5-C,Hg)]+ ( 4 = dppm 10 or dppe 12) so establishing examples of associative ligand-substitution processes which proceed with ' q5 -+ q3 -q5 hapticity interconversion at the C,H, dienyl ligand. The reaction between 17 andCNBu' affords [Mo(CO)(CNBu'),(q5-C7H,)]+ 19 with no observable q3-C,H, intermediate but 19 reacts with further CNBu' to yield [Mo(CO) (CNBut),(q3-C,Hg)] + 20. The crystal structures of complexes 1 and 4 have been determined. Complex 1 adopts an asymmetric ligand arrangement similar to that previously described for 6. Complex 4 has a pseudo-octahedral molecular geometry in which CNBu' is located trans to the q3-bonded C,Hg ligand.It is well established that the reactivity of complexes of dienyl ligands is enhanced by the potential for q5 -q3 hapticity interconversion/'ring slip' of the dienyl ligand. However, notwithstanding extensive investigations on complexes of cyclopentadienyl, indenyl,' and acyclic pentadienyl ligands,, very few studies have systematically addressed the design of complexes which undergo facile qq interconversion or
