Carbene complexes. Part 11. Steric and conformational effects upon the transition-metal reactivity of electron-rich olefins and derived species; heteroatom-donor-olefin–metal (Cr 0 , Mo 0 , W 0 , or Rh I ) and 2-imidazoline-N-donor–metal complexes, and the crystal and molecular structure of tetracarbonylNN′N″N‴-tetramethylbi(imidazolidin-2-ylidene)-NN″-chromium( 0 )