An explanation is given for the enhancement of Raman intensity of the (symmetrical) stretching mode of the carbonyl(s) cis with respect to L when complexes of the type (0 )4_ . [ ( )3] (M = Cr, Mo, W; x = 0, 1; L = 1,4-diazabutadiene, pyridine-2-carbaldehyde imine, 2,2'-bipyridine, 1,10-phenanthroline) are excited within the lowest MLCT band. The influence of the central metal atom, of the ligand L, and of the co-ligand is discussed in detail. Irradiation within the lowest MLCT band also causes photosubstitution of a cis carbonyl ligand with quantum yields varying from 10"1 2345 to 10"2. A relationship appears to exist between these photosubstitution quantum yields and the resonance enhancement of the carbonyl stretching mode.

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The resonance Raman (r.R.) spectra of a series of [Re(CO),L(X)] complexes [L = aryl or alkyl NN'-disubstitutedethane-l,2-di-imine or 2-(N-monosubstituted-iminomethy1)pyridine; X = CI or Br], obtained by excitation within the lowest-energy absorption band are reported. The influence of the ligand L and of the halogen atom X is discussed. The excitation profiles of [Re(CO),(p-MeC6H4N=CHCH=NC6H4Me-p)C1] are compared with those of the free ligand, NN'-di(p-tolyl)ethane-l,2-di-imine. These spectra revealed that upon excitation within both metal-to-ligand charge-transfer and intraligand transitions also resonance enhancement occurs of a carbonyl stretching mode. The low-temperature r.R. spectra of [Re(CO),(NC,H4CH=NC6H,Me-p){Mn(CO),}] are reported and used for the characterization of the electronic absorption spectrum and for the explanation of the photochemistry of this complex. IN recent articles1-' we reported the resonance Raman (r.R.) spectra of a series of [M(C0)4-ZL(L')Z] complexes (M = Cr, Mo, or W; L = di-imine; L' = phosphine or isocyanide; x = 0 or l), obtained by excitation withintheir lowest-energy metal-to-ligand charge-transfer (m.1.c.t.) transitions. Special attention was paid to the r.R. effect of v(COcis) and v(CEN), the stretching modes of the carbonyl and isocyanide ligands cis with respect to the di-imine ligand L. This effect was explained by an interaction between virtual molecular orbitals having predominantly L x* and CN-R x* or CO x* character, respectively .In this article we extend these studies to a series of fac-[Re(CO),L(X)] complexes. When X = C1 or Br, the metal has a d6 configuration, and a direct relationship with the results of the Group 6B analogues is therefore expected. When X = Mn(CO),, the rhenium metal is formally zerovalent, having a d7 configuration. This has a pronounced influence on the properties of the complexes, as shown by several authors.8-11 Some controversy exists about the assignment of the electronic absorption spectral0P1l and in order to elucidate the character of the electronic transitions, we studied the r.R. effect for one representative complex.First, we report the properties of the lowest-energy m.1.c.t. transitions for a series of rhenium d6 complexes, as derived from the r.R. results, in comparison with those of the zerovalent Group 6B complexes reported earlier. In the second part we present the excitation profiles of a representative complex, and finally, we report the r.R. spectra of a binuclear rhenium-manganese complex upon excitation within the visible region of the spectrum. In this latter complex, the di-imine ligand is c,c bonded to the rhenium metal atom via the nitrogen lone pairs, while the Mn-(CO), moiety occupies a position cis with respect to the di-imine ligand, resulting in the formation of a single Re-Mn C T bond.This article consists of three parts.The structure of a representative complex is shown below. trans 0 cis X =CL,Br, or Mn(CO), EXPERIMENTALThe complexes were prepared following standard procedures. [11][12][13] Electronic absorp...

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