ine, are observed. No molecular ion is formed at 180°C by the relatively labile (5).
The linear relationships between the a-band wave numbers of the absorption spectra, and the Os"/Os"'-redox poply not only to the tetrahydrothiophene derivatives (l), (2), (5) and (12)16"1 but also to the 1-methylimidazole derivatives (6), (l), (10) and (8). A certain n-acceptor capacity can therefore be ascribed to the tetrahydrothiophene ligands. This result, together with the a-donor capacity of the imidazole sysresults in a "push-pull" stabilization of (I), and also accounts for the greater chemical stability of (1) relative to (2). In view of this inherent stability of the trans-arrangement of thioether and imidazole donors, cytochrome c represents a good choice by Nature.
A further detail of the cytochrome model, the difference AE = E, -E2 between the metal redox potential E, of the corresponding symmetrical bis(imidazo1e) and E2 of the unsymmetrical thioether/imidazole systems, is reproduced by the osmochrome system. For the heme-octapeptide obtained by partial hydrolysis of cytochrome c, AE= -160 mV['], for the "tail-porphyrin", AE= -167 mVfZ1, for the mesoheme with covalently bonded N/N-or S/N-donors, AE = -147 mV[*"], and for the osmochromes ( 6) and (1). AE= -180 mV18b1. By comparison of cytochromes b5 and c one finds for the latter AE= -235 rnVf9]. This larger value may be attributed to a special protein-effect. tentials shown in Figure 3 ("bathochromic rule"15b.6b,6c1 ) ap-Procedure A solution of (5)1'*] (64 mg, 7 mmol) in 11 cm3 THF/1-methylimidazole (10: 1) is prepared under an argon atmosphere, warmed for 2 h at SO " C and concentrated in a stream of argon. The residual solution is allowed to stand under argon until crystallization occurs. The black, analytically pure needles of (1) which precipitate (51 mg, 65%) are suction filtered, washed with methanol/water (4:l) and dried at 40"C/10-4 torr.