CRYSTAL STRUCTURE OF SOME DICARBONYL-ALLYL DERIVATIVES OF SUBSTITUTED CARBONYLS OF MOLYBDENUM .I. CRYSTAL STRUCTURE OF (ISOTHIOCYANATO)DICARBONYL-2,2'-BIPYRIDINE-PI-ALLYLMOLYBDENUM NCS(CO)2(C10H8N2)(PI-C3H5)MO

The crystal structure and molecular configuration of the complex (isothiocyanato)dicarbonyl-2,2′-bipyridine-π-allylmolybdenum, NCS(CO)2(C10H8N2)(π-C3H5)Mo, have been determined from three-dimensional data collected photographically at 20°. The unit cell is monoclinic, spacegroup P21/c, with four formula units in a cell of dimensions a = 8.290 ± 0.003 Å, b = 10.160 ± 0.001 Å, c = 21.631 ± 0.007 Å, β = 113° 58′ ± 2′. The structure has been refined to R = 0.086 by the application of an isotropic/anisotropic block-diagonal-approximation least-squares refinement using 2606 independent non-zero reflections. The complex has essentially an octahedral arrangement of ligands. If it is viewed such that the 2,2′-bipyridine and dicarbonyl ligands lie in equatorial positions, the allyl and isothiocyanato ligands lie in the axial positions, one on each side of the equatorial plane. The equatorial ligands are each planar and dip away from the plane of the allylic carbon atoms. The inter-planar angles between these ligands are: 2,2′-bipyridine/dicarbonyl 15.1°; π-allyl/dicarbonyl 8.3°; 2,2′-bipyridine/π-allyl 22.7°. The central allylic carbon atom is nearer the molybdenum, at 2.196 ± 0.009 Å, than the two terminal atoms at 2.291 ± 0.011 Å and 2.351 ± 0.012 Å. The allylic inter-carbon angle is 115.7 ± 1.1° and the mean CC bond length is 1.440 ± 0.023 Å. The mean MoC (carbonyl) and CO (carbonyl) bond lengths are 1.941 ± 0.007 Å and 1.178 ± 0.014 Å respectively. The thiocyanato ligand is nitrogen-coordinated at a distance of 2.117 ± 0.007 Å with bond lengths NC = 1.163 ± 0.010 Å and CS = 1.643 ± 0.008 Å. The aims of the investigation of the MCBAS structure were threefold, namely: 1. To investigate the orientation of the allyl ligand and to show that it was a π-allyl, and not a σ-allyl, ligand. 2. To determine whether the thiocyanato ligand was sulphur- or nitrogen-coordinated to the molybdenum. 3. To investigate the stereochemical position of the thiocyanato ligand in relation to the other ligands. In their infra-red work, Hull and Stiddard8 observed a strong spectral band at 2080 cm-1 which they assigned to v(CN) and a weak band at 825 cm-1, assigned to v(CS). On the basis of the latter mode they inferred that the thiocyanato ligand was N-bonded to the molybdenum. © 1969.