This work reports a detailed comparison of solution and solid-state structural properties for the isoelectronic series [(C5H5)Cr(NO)2]2, [(C5H5)Mn(CO)(NO)]2, and [(C5H5)Fe(CO)2]2. The structure of [(C5H5)Mn- (CO)(NO)]2 has been determined from three-dimensional X-ray data collected by counter methods. The opaque compound crystallizes in space group C2h5-P21/c of the monoclinic system with two dimeric molecules in a cell of dimensions a = 7.002 (1) Å, b = 12.491 (3) Å, c = 8.023 (2) Å, and β = 108.07 (1)°. The observed and calculated densities are 1.75 (1) and 1.772 g cm-3, respectively. Full-matrix least-squares refinement gave a final value of the conventional R factor (on F) of 0.040 for the 1197 reflections having F2> 3α (F2). The structure consists of discrete dimeric ligand-bridged molecules with trans cyclopentadienyl rings. The dimers have an imposed crystallographic center of symmetry which makes the bridging (B) as well as the terminal (T) carbonyl and nit rosy 1 ligands disordered. Some important bond lengths (in Å) are Mn-Mn, 2.571 (1); Mn-B (av), 1.906 (5); Mn-T (av), 1.723 (4); and Mn-C (av C5H5), 2.13 (1). Comparisons with similar bridged dimeric molecules such as the iron carbonyl analog are made. In solution, all three compounds exist as a mixture of cis and trans ligand-bridged dimers, the ratio [cis]: [trans] following the order Fe > Mn > Cr. The activation energy for cis-trans interconversion and bridge-terminal ligand interchange decreases in the order Cr > Mn > Fe. For both the iron and the manganese compounds, bridge-terminal ligand interchange is more rapid for the trans than for the cis isomers; for manganese, both processes are more rapid than cis-trans interconversion, and this result is interpreted in terms of the stereochemistry of the system. Activation parameters and thermodynamic data are reported for all three systems.
ASJC Scopus subject areas
- Colloid and Surface Chemistry