SYNTHESIS AND CHARACTERIZATION OF NEW DINUCLEAR COMPLEXES (CO)(5)MNRE(CO)(3)(L) (L=2,2'-BIPYRIMIDINE, 2,3-BIS(2-PYRIDYL)PYRAZINE) AND TRINUCLEAR COMPOUNDS (CO)(5)MNRE(CO)(3)(L)R (R=RE(BR)(CO)(3), W(CO)(4)) - EVIDENCE FOR ASYMMETRIC DISTORTION OF THE BRIDGING 2,2'-BIPYRIMIDINE LIGAND (L) IN (CO)(5)MNRE(CO)(3)(L)RE(BR)(CO)(3) FROM THE CRYSTAL-STRUCTURE AND H-1-NMR AND RESONANCE RAMAN-SPECTRA

This article describes the synthesis and spectroscopic (H-1-NMR, UV/vi s, resonance Raman) properties of the new complexes (CO)(5)MnRe(CO)(3) (L), (CO)(5)MnRe(CO)(3)(L)Re(Br)(CO)(3) (L = 2,2'-bipyrimidine (BPYM), 2,3-bis(2-pyridyl)pyrazine (DPP)), and (CO)(5)MnRe(CO)(3)(BPYM)W(CO)( 4) and the single-crystal X-ray structure of (CO)(5)MnRe(CO)(3)(BPYM)R e(Br)(CO)(3). The crystals are triclinic, space group P $($) over bar$ $ 1, with cell dimensions a = 11.819(3) Angstrom, b = 14.957(9) Angstr om, c = 15.792(7) Angstrom, alpha = 64.09(4)degrees, beta = 88.88(7)de grees, gamma = 88.22(8)degrees, and Z = 4. Least-squares refinement on F, for 4620 observed reflections, converged to R = 0.127. The two dif ferent metal fragments (CO)(5)MnRe(CO)(3) and Re(Br)(CO)(3) are coordi nated to BPYM via Re with Mn(CO)(5) and Br- in a mutually trans positi on perpendicular to the BPYM plane. An important aspect of this struct ure is the asymmetric bonding within the bridging BPYM ligand. From th e C-N bonds connected to the inter-ring C-C bond, those involving N at oms coordinated to the Re-0 atom of the (CO)(5)MnRe(CO)(3) moiety are longer than those involving the N atoms coordinated to the Re-1 atom o f the Re(Br)(CO)(3) fragment. This asymmetry of BPYM is also evident f rom the H-1-NMR spectrum and is responsible for the occurrence of two absorption bands in the visible region belonging to separate transitio ns from the two metal fragments to BPYM. It even manifests itself in t he resonance Raman spectra by showing resonance enhancement of intensi ty for BPYM vibrations with slightly different frequencies upon excita tion into either of these MLCT bands. This asymmetry is not observed f or the corresponding DPP-bridged complex and also not for (CO)(5)MnRe( CO)(3)(BPYM)W(CO)(4). The resonance Raman spectra of the latter compou nd point to a reversal of the order of the a(u) and b(2u) pi orbitals of BPYM as compared with the free ligand. The distortion of the BPYM ligand appears to be responsible for a very weak electronic pi interac tion between the two metal fragments in (CO)(5)MnRe(CO)(3)(BPYM)Re(Br) (CO)(3) which may thus be viewed as a mixed-valence complex.