Examples of a new class of zwitterionic, ''right-hand side'' merocyanines containing a cyanodicyanomethylidenedihydrofuran electron acceptor have been prepared. As well as allowing for the facile synthesis of these chromophores, our synthetic methodology enables considerable variation in both the donor moiety as well as the extent of conjugation between the donor and acceptor systems. As expected, all of these ''right-hand side'' systems are negatively solvatochromic, with the difference between l max (polar vs. nonpolar solvents) increasing with the extent of conjugation. In accord with expectations, hyper-Raleigh scattering (HRS) measurements confirm that molecules with the greatest conjugation pathway (for e.g. 21c, 23c) have the largest first hyperpolarisabilites, b o . In addition, our HRS evaluation indicates that the 4-quinolinylidene donor nucleus is superior to both the 4-pyridinylidene and benzothiazolylidene systems. The figures of merit, m (calc) .b o(measured) , that we obtain for some of these compounds, are of a similar magnitude to the best ''left hand side'' examples reported in the literature. In order to demonstrate the versatility of our synthetic technique, representative polymer-tetherable derivatives of these compounds have been prepared, as have the corresponding TDI-based polyurethanes.

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The oxocomplexes were prepared from aqueous solutions of molybdenum- (VI) and -(V) and the Iigands. With hydrogen sulphide some of the complexes gave complexes in which bridging oxide ligands had been replaced by sulphide. The complexes are diamagnetic. We report their i.r. and electronic spectra and discuss their structures, the effect on their spectra and other properties of different donor atoms (N, 0, S), and their relevance as models for molyb d e n urncontaining enzymes.COMPLEXES of molybdenum with L-cysteine and other amino-acids and with organic sulphur compounds are of interest as possible models for molybdenum-containing enzymes, e.g., xanthine oxidase,l nitrogenases,2 and nitrate reductases.3 Recently we published a preliminary account of the first molybdenum-cysteine complex, the structure of which has been determined by a singlecrystal X-ray diffraction study.5 Subsequently, Melby reported the isolation of the cysteine complex and similar complexes with esters of L-cysteine. Here we give a full acount of our molybdenum-cysteine complex and related complexes with a-alanine, CH3*CH(NH3+)CO2-, p-alanine, (NH3+) CH2-CH,*C02-, 2-mercap toethylamine, HS*CH,*-CH2*N H,, L-cyst eine methyl ester , H S* CH,*CH (N H,) 0 -CO,Me, 2-hydroxyethanethiol, HS*CH,-CH,.OH, and L-cysteine, HS*CH2*CH(NH3+)*C02-. One object of the work was to study the effect, if any, on the properties of the coniplexes of varying the donor atoms, N, 0, and S. We report also reactions of the complexes with hydrogen sulphide in which bridging oxide ligands are replaced by sulphide. Complexes containing MoS2Mo groups are likely to be of particular interest in the context of niolybdenum-containing enzymes.2 EXPERIMENTAL Di-p-0x0-bis [di-p-hydroxop-(u-alaninato) -dioxohydroxowzolybdenunz (VI) oxomoZybdenunz(v~)], [Mo40, (OH) ,(CH,*CH-(NH,).CO,},] (I) .-TWO procedures were med. ( a ) An excess of freshly prepared silver molybdate, Ag2Mo0, (5.5 g) was added to a hot solution of a-alanine (2.1 g) in 111hydrochloric acid (20 ml). The mixture was boiled and