Photochemical reaction of (η 5 -C 5 R 5 )Re(CO) 3 (R ) H, Me) with pentachlorobenzene (λ ) 350 nm, hexane solution) yields trans-(η 5 -C 5 R 5 )Re(CO) 2 (2,3,4,5-C 6 HCl 4 )Cl (trans-1, R ) Me; 2, R ) H) formed by insertion of the fragment (η 5 -C 5 R 5 )Re(CO) 2 into the C-Cl bond of C 6 -HCl 5 . Complex trans-1 converts to cis-(η 5 -C 5 Me 5 )Re(CO) 2 (2,3,4,5-C 6 HCl 4 )Cl (cis-1) in MeCN. The stereochemistry of both isomers has been assigned from examination of ν(CO) IR intensities and is supported by an X-ray structure determination of both complexes. A pure sample of either trans-1 or cis-1 dissolved in organic solvents produces an equilibrium mixture. In nonpolar solvents trans-1 is favored, whereas cis-1 is more stable in polar solvents. A mixture of cis-1 and trans-1 in CH 2 Cl 2 solution reacts on silica gel to produce the new tetramethylfulvene complex (η 6 -C 5 Me 4 CH 2 )Re(CO) 2 (2,3,4,5-C 6 HCl 4 ), 3. Supporting Information Available: Tables of coordinates and displacement coefficients for the non-hydrogen atoms, all bond lengths and interbond angles, anisotropic displacement coefficients, and H atom coordinates and isotropic displacement coefficients for cis-1 and trans-1. This material is available free of charge via the Internet at http://pubs.acs.org. OM000032V

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