Transition metal .sigma.- acyls. II. Crystallographic determination of the molecular structure of (pentahaptocyclopentadienyl)-1,3',4'-trihapto{2-methyl-2-(6'6'-dimethylbicyclo[3.2.0]hept-3'-en-7'-on-2'-yl)propionyl}nickel(II), including the location and refinement of all hydrogen atoms