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.

The complexes (bpym)PtCl(2) (1) and the new (micro-bpym)[PtCl(2)](2) (2), bpym = 2,2'-bipyrimidine, were synthesized and, in the case of 1, crystallized in solvent-free form for X-ray diffraction. The molecules 1 exhibit two different kinds of stacking motifs in the crystal with an interstack CH--N interaction. Complexes 1 and 2 were found to be sufficiently soluble for cyclic voltammetry, spectroscopy (absorption and emission), and spectroelectrochemical studies (UV-vis, EPR). As a result of single or double coordination of the strongly sigma-accepting [PtCl(2)] fragment to bpym, the paramagnetic anions 1(*)(-)() and 2(*)(-)() and the dianions 1(2-) and 2(2-) could be reversibly generated, despite the presence of metal-halide bonds. DFT calculations of A((195)Pt) and g tensor components confirm that the singly occupied MOs of the monoanionic species have mainly pi(bpym) character with nonnegligible platinum d orbital participation. The assignments of the electronic absorption and emission and resonance Raman spectra for both complexes are supported by DFT calculations.