Complex Chemistry of Reactive Organic Compounds. 46.^†1Conformation of Metal Carbene Complexes: Synthesis and Structure of (η⁵-C₅H₅)Cr(CO)(NO)[C(C₆H₅)₂and Its Relationship to the Isoelectronic Compound (η⁵-C₅H₅)Mn(CO)₂[C(C₆H₅)₂]

Reaction of CpCr(CO)(NO)(thf) (Cp = η⁵-C₅H₅, thf = tetrahydrofuran) with diphenyldiazomethane gives the new carbene complex CpCr(CO)(NO)[C(C₆H₅)₂] (2). The complex crystallizes in the monoclinic space group P2₁/n (a = 11.090 (2) Å, b = 10.579 (3) Å, c = 13.804 (3) Å, β = 92.43 (2)°). The structure shows the carbene plane to be nearly coplanar with the plane described by the N-Cr-C(carbene) fragment and nearly perpendicular to the C(carbonyl)-Cr-C(carbene) plane. In this orientation, the carbene ligand chooses to align itself with the most basic of the two potential π-donor orbitals of the CpCr(CO)(NO) fragment, e.g., the one not stabilized by the nitrosyl ligand. The resulting Cr-C(carbene) bond length of 1.912 Å is the shortest Cr-carbene distance yet reported. The structure of the isoelectronic compound CpMn(CO)₂[C(C₆H₅)₂] (4) is reported for comparison. The manganese complex crystallizes in the space group P2₁/c (a = 12.882 (2) Å, b = 7.380 (2) Å, c = 17.243 (2) Å, β = 106.95 (1)°). This latter structure sharply contrasts with that of the chromium derivative 2 in that the carbene plane is coincident with the mirror plane of the CpMn(CO)₂fragment. Analysis of the room-temperature ¹H and ¹³C NMR spectra of the two complexes shows the chromium-carbene conformation is locked whereas manganese-carbene rotation is fast.