Disordered crystal structures of six complexes of the type, Me2XCR1R2·CF2XME2·M(CO)4(M = Mo or Cr; X = As or P; R1= F or H; and R2= H, CF3, or CL)

Abstract

Crystal structures of four compounds of the type Me₂XCR¹R²·CF₂XMe₂·M(CO)₄(M = Mo or Cr; X = As or P; R¹= F or H; and R²= H or Cl) have been determined, and two previously reported crystal structures of this type (R²= H or CF₃) have been reinterpreted. Four compounds with R²= H are isostructural, space group Pbca, a ca. 18, b ca. 15, c ca. 13 Å, Z= 8. Two compounds with R²= Cl and CF₃ have structures which are quite similar to each other, the former having space group Pbca, a= 23·41, b= 12·59, c= 11·57 Å, Z= 8, and the latter C2/c, a= 25·06, b= 13·27, c= 11·56 Å, β= 102·8°, Z= 8. The structures were determined from diffractometer data by Patterson, electron-density, and full-matrix least-squares methods, the final R varying from 0·05–0·08 for 1200–1800 observed reflexions.

The crystals all contain disordered arrangements of molecules with normal geometries and dimensions; each crystal has two molecular configurations distributed at each lattice site, the occupancy of the two orientations varying from 1 : 1 to ca. 6 : 1. The metal atoms are co-ordinated in slightly distorted octahedral arrangements to two arsenic or phosphorus atoms and to four carbonyl groups, the X–M–X angles being 82–85°. The Mo–As, Mo–P, and Cr–As bond lengths are 2·58, 2·48, and 2·43 Å. The five-membered chelate rings have the usual puckered conformation, with one carbon atom displaced further (ca. 0·4 Å) from the X, M, X plane than the other (ca. –0·25 Å); the ligands thus have staggered conformations with X–C–C–X dihedral angles of ca. 50°. Intermolecular packing is governed by van der Waals inteactions, the most interesting feature being the disordered arrangements.