Carbene complexes. Part 11. Steric and conformational effects upon the transition-metal reactivity of electron-rich olefins and derived species; heteroatom-donor-olefin–metal (Cr0, Mo0, W0, or RhI) and 2-imidazoline-N-donor–metal complexes, and the crystal and molecular structure of tetracarbonyl[NN′N″N‴-tetramethylbi(imidazolidin-2-ylidene)-NN″]-chromium(0)