Structural and dynamic systematics in the isoelectronic series [(C 5H 5)ML 2] 2, where M = Cr, Mn, and Fe and L = CO and NO

Richard M. Kirchner, Tobin J Marks, J. S. Kristoff, James A. Ibers

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Abstract

This work reports a detailed comparison of solution and solid-state structural properties for the isoelectronic series [(C 5H 5)Cr(NO)] 2, [(C 5H 5)Mn(CO)(NO)] 2, and [(C 5H 5)Fe(CO)] 2. The structure of [(C 5H 5)Mn-(CO)(NO) 2 has been determined from three-dimensional X-ray data collected by counter methods. The opaque compound crystallizes in space group C 2H 5-P2 1/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 F 2 > 3σ(F 2). 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 nitrosyl ligands disordered. Some important bond lengths (in A) are Mn-Mn, 2.571 (1); Mn-B (av), 1.906 (5); Mn-T (av), 1.723 (4); and Mn-C (av C 5H 5), 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 stereo-chemistry of the system. Activation parameters and thermodynamic data are reported for all three systems.

Original languageEnglish
Pages (from-to)6602-6613
Number of pages12
JournalJournal of the American Chemical Society
Volume95
Issue number20
Publication statusPublished - 1973

ASJC Scopus subject areas

  • Chemistry(all)

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