ETA(7)-]ETA(3) HAPTICITY INTERCONVERSION IN CYCLOHEPTATRIENYL COMPLEXES OF MOLYBDENUM AND TUNGSTEN - CRYSTAL-STRUCTURES OF [MO(NCS)(CO)(2)-(C10H8N2)(ETA(3)-C7H7)] AND )(2)(PH(2)PCH(2)CH(2)PPH(2))-(ETA(3)-C7H6C6H4F-4)]

A series of reactions of the general type [MX(CO)(2)(eta(7)-C(7)H(6)R) ](n+) + 2L' --> [MX(CO)(2)L'(2)(eta(3)-C(7)H(6)R)](n+) involving eta(7 )-->eta(3) hapticity conversion at a molybdenum or tungsten co-ordinat ed cycloheptatrienyl ring have been examined with the objective of det ermining the effects of variation of X, M and R on the reactivity of [ MX(CO)(2)(eta(7)-C(7)H(6)R)](n+) towards adduct formation with ligands L' = NCMe, L'(2) = Ph(2)PCH(2)CH(2)PPh(2) (dppe) or 2,2'-bipyridyl (b ipy). The complexes [MoX(CO)(2)(eta(7)-C7H7)] exhibit a markedly X-dep endent reactivity. The derivatives with X = NCO or NCS form adducts [M oX(CO)(2)(dppe)(eta(3)-C7H7)]1 and 2, [MoX(CO)(2)(bipy)(eta(3)-C7H7)]3 and 4 and [MoX(CO)(2)(NCMe)(2)(eta(3)-C7H7)] {as an equilibrium mixtu re with [MoX(CO)(2)(eta(7)-C7H7)] in NCMe}. By contrast [MoX(CO)(2)(et a(7)-C7H7)] (X = C=Ph,C6F5 or SnPh(3)) do not react with dppe, bipy or NCMe at room temperature. Tungsten complexes [WX(CO)(2)(eta(7)-C7H7)] (n+) are activated towards adduct formation with L' = NCMe by comparis on with the molybdenum analogues. Thus [WX(CO)(2)(eta(7)-C7H7)](X = NC O or Br) react with NCMe to form [WX(CO)(2)(NCMe)(2)(eta(3)-C7H7)] (X = NCO 5 or Br 6). The composition of equilibrium mixtures of [Mo(CO)(2 )(NCMe)(eta(7)-C(7)H(6)R)](+) and [Mo(CO)(2)(NCMe)(3)(eta(3)-C(7)H(6)R )](+) in NCMe is dependent upon R and temperature; the % composition o f the eta(7)-C(7)H(6)R component is enhanced by electron-donating ring substituents (R = Me) and by an increase in temperature, The crystal structures of 4 and [MoCl(CO)(2)(dppe)(eta(3)-C7H6C6H4F-4)] have been determined. Both exhibit a pseudo-octahedral ligand arrangement at the Mo but, whilst 4 possesses a symmetrical structure with NCS located t rans to the eta(3)-C7H7 ring, the latter complex adopts an asymmetric ligand arrangement with one phosphorus of the dppe ligand positioned t rans to the cycloheptatrienyl ring.