PHOTODISSOCIATION OF S-BENZYL-L,2-DIPHENYL-1,2-ETHYLENEDITHIOLATO)METAL (NI,PD,PT) COMPLEXES

(S-benzyl-1,-diphenyl-1,2-ethylenedithiolato)metal (Ni,Pd,Pt) (1a-c) c omplexes are known to undergo photochemical reactions that produce the corresponding neutral free metal dithiolene M(S(2)C(2)Ph(2))(2) (3a-c ) complexes in solution. The reaction mechanisms of these photoreactio ns were examined by means of EPR and UV/vis absorption techniques. In the first step of the reaction the photodissociation of a C-S bond occ urs to yield one benzyl radical and the monobenzyl dithiolene complex radical (2a-c). Both radicals were observed by time-resolved electron paramagnetic resonance (TREPR). The benzyl radical was also trapped by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl). Analysis of the TREPR s ignals shows that the radicals are produced from the excited triplet s tates of 1a-c within 100 ns. From the analyses of time-resolved and st eady state EPR data, and absorption spectra, the intermediate complex radicals (2a-c) were assigned as [M(S(2)C(2)Ph(2)){S-2(CH(2)Ph)C(2)Ph( 2)}] (M = Ni, Pd, Pt). Spin densities on the central metals were deter mined from their anisotropic g values and hyperfine coupling constants by comparing the data with those of [M(S(2)C(2)Ph(2))(2)](-) A correl ation between the spin density and the decay time of the intermediate complexes (2a-c) was pointed out. The second step of the reaction was found to be the dissociation of a second benzyl substituent, which occ urs in the dark. The reaction rate was discussed in terms of the spin density (rho(M)) on the metal and the S-C(benzyl) bond order (rho(s) r ho(c))(1/2) of the intermediate radicals, 2a-c.