J . Phys. Chem. 87 (1983) 3620. 1 550 Fig. I. Correlation between the C=C stretching frequencies and absorption maxima in the model compounds 1, 2a, 3a, 4a, and 5 .It should be noted that protonation of the dimethylamino groups in the vicinity of the polyene skeleton in 2-5 does not significantly change the position of the C=N stretching frequency (Table 1). This result is consistent with the observed insensitivity of vCZN to A,,, in visual pigments''].Our results reveal that a positive charge in the vicinity of the retinal polyene decreases charge delocalization, causing an increase in the C=C frequency. However, an external positive charge near alkylideneammonium ion (5) increases the charge delocalization and decreases bond alternation, thus causing a shift of the C=C stretching mode to a lower frequency. It is evident that external positive charge affects not only the excited state but also the ground state.The external point-charge model for bacteriorhodopsin assumes interaction with a negative charge in the vicinity of the ring moiety of the retinal skeleton. Here, we have investigated the effect of a positive charge, which is expected to give an opposite effect. However, the observation that a positive charge influences the absorption maximum and the C=C stretching frequency of a protonated retinal Schiff base supports the external point-charge model for bacteriorhodopsin and also our assumption that the different C=C stretching frequencies found in bacteriorhodopsin, visual pigments, and their photochemically induced intermediates may originate from through-space interaction with external negative charges. The importance of S(Cys)-metal bonds in a variety of metalloproteins has created a genera1 interest in the thiolate chemistry of biologically essential trace elements, one of which is vanadium. Investigations on the problem of Vnitrogenase"s2] have furnished evidence of a possible vanadium-sulfur coordination in proteins. The anion [VS(edt)2]2e 1 (edt'" = ethane-1,2-dithiolate) and the salt Na2[VO(edt)2] 8 MeOH 2 mentioned in the title, as well as the anion [VO(edt)z]zo 3 occurring in solutions of 2 and the polynuclear complexes [V2(edt),lZe 4 and [FezVS4(SPh),I3 'I3], could be useful as reference compounds for the identification of vanadium-sulfur centers in metalloproteins. On reaction of VS;' with [V2(edt)4]2e or with an excess of edt2@ in methanol, 3 is formed as main product together with the thiovanadyl complex [VS(edt),12@ 1, which crystallizes as (Ph,P),[VS(edt),]. MeOH 5 in the form of brownish-red columns along with green Na2[VO(edt),].8 MeOH 2 after addition of Ph4PBr14'. 3 is also formed on oxidation of 4 with Na2S2 in methanol, or from [VO(a~ac)~] (acac" = acetylacetonate) by ligand exchange reaction[3c1.Besides [VS(acen)] 6 (acen" = N,N'-ethylenebis(acety1acetonylideneamine) d i a n i~n ) '~] , the mononuclear anion 1 is the second thiovanadyl complex whose structure could be determined ; furthermore, 1 is the first pentacoordinated vanadium complex with a complete sulfur ligand sphere and...