THEORETICAL-STUDIES OF BRIDGING-LIGAND EFFECTS IN QUADRUPLY BONDED DICHROMIUM(II) COMPOUNDS .3. THE 1ST COMPLETE GEOMETRY OPTIMIZATIONS OF TRANSITION-METAL DIMER COMPLEXES
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Generalized valence bond (GVB) calculations are reported for a series of dichromium(II) complexes. Analytical gradient techniques were used to completely optimize the geometries of Cr2(02CH)4, Cr2((NH)OCH)4, and Cr2((NH)2CH)4 as well as partially optimize the geometries of Cr2(02CH)4(H20)2 and Cr2((NH)2CH)4(H20)2 at the GVB level. The geometry of the bridging ligand strongly affects the calculated Cr-Cr bond length; thus, complete geometry optimizations are necessary for dichromium complexes with different bridging ligands. Even with complete optimization, the Cr-Cr bond is calculated to be too long at the GVB level, but the calculated trends in Cr-Cr bond lengths are more accurate. The calculated changes in Cr-Cr bond length are 0.08 and 0.11 for the bridging ligand changes formato to amidato and amidato to amidinato, respectively. Upon axial hydration, the Cr-Cr bond is calculated to lengthen by 0.11 for Cr2(02CH)4 and 0.18 for Cr2((NH)2CH)4. Again, calculations underestimate the effect of axial ligands on Cr-Cr bond length, but the trends in Cr-Cr bond lengths for the axially ligated complexes as well as the relative response to axial ligands are consistent with experimental results. Our calculated trends in Cr-Cr bond length are consistent with a Cr-Cr bond length of 2.05-2.10 for dichromium tetracarboxylates without axial ligands. 1989, American Chemical Society. All rights reserved.
