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In einem bisher unbekannten Reaktionstyp ergibt [η5‐CpFe(CO)2]2 mit Phosphor‐Yliden Ph3PCHR (R = H, Me, iPr) die cis‐ und trans‐Alkyliden‐Komplexe (1) bzw. (2) sowie wenig Organometall‐substituiertes Ylid (3). Analoge Mangan‐Komplexe reagieren mit Methylenphosphoran überraschenderweise unter ortho‐Acylierung einer der P‐Phenylgruppen.

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containing by-product, for which we have proposed the structure 18,19-didehydrocobyrinic acid c-amide (3)I2I.Such a compound is of interest in its metal-free, non-amidated form as the connecting link between metal-free and cobalt-containing corrinoids in the biosynthesis of vitamin Bl2I2]. We have now unambiguously proved the structure of the hexamethyl ester of (3) as dicyano-18,19-didehydrocobyrinic acid hexamethyl ester c-amide (4).The absorption spectrum of (4) shows the typical a,o,ystructure of cobalt-containing corrinoids. The substantial bathochromic shift of the main absorption bands (369-396 nm, 545-590 nm, and 584-626 nm) compared to those of the heptamethyl ester of dicyanocobyrinic acid ("cobester") indicates a lengthening of the corrin chromophore.The presence of the same peripheral groups as in the hexamethyl ester of (2) follows from the 'H-NMR spectrum, which contains the signals of the protons of one amide group and of six methoxy-and eight angular methyl groups. An AB system (J=16.7 Hz) appears at 6=3.47/ 3.38 which is assigned to the methylene group of a [q5-CpFe(CO)z]s + Ph3P=CH

Fig. 2. Qualitative energy diagram for the double bond isomerization process in (3j (Q is reaction coordinate). The X-ray structural analysis of (3) [S] has yielded a planar 4-membered ring having bond lengths 133.9 (C-C) and 159.7 pm (C-C). corresponding to the right hand structural formula. the "adiabatic" or "nonvertical" value E?'. In fact the smooth descent of the quenching curve for Er (S) values < 120 kJ/mol is characteristic of a "nonvertical" energy transfer process [']. For example, for E+= E;O= 120 kJ/mol the dashed curve in Figure 1 would be expected. Moreover, it is striking that the plateau region of quenching rates for energy-rich sensitizers is reached at k,, z lo9 M -' s -', which is approximately an order of magnitude less than the normally observed diffusion controlled limit. This presumably arises from the steric screening of the n-system by the alkyl substituents in (3), since an exchange mechanism for the transfer of triplet energy'''] requires the chromophores to overlap. It should be noted that fluorenone and 2,3-dimethylnaphthoquinone (4) (Er=213 and 220 kJ/mol, resp.; m r * ) are quenched by (3) at a rate close to the diffusion controlled limit ( k , , = 9 x lo9 and 7 x lo9 M -' s -' , resp.). This may be a consequence of the increased electron affinityl" bl and/or the modest spacial requirements of the carbonyl groups.Based on a semi-classical model for the triplet energy transfer process in solution, Balzani er aLIY1 recently suggested a general relationship between k,, and E$O (S) which makes use of several spectroscopic, kinetic and thermodynamic parameters and which satisfactorily describes both "vertical" and "nonvertical" quenching processes. Balzanr's function (equation 29 in f91) was fitted to the measured values of k,," ' I by application of the method of least squares1I2l. The solid line shown in Figure 1 was obtained with the following values for the adj~stable1'~l parameters; E;O (3) = 50-t 5 kJ/ mol, k;"=(l.l kO.1) x lo9 s -' (rate constant for the energytransfer in an activated encounter complex) and AG* (0) = 1700 i 300 cm -I [free energy of activation for a thermoneutral energy transfer from (3S) to (3)]. From AG'(0) and E;O (3), a lower limit of 120 kJ/mol for E; (3) can be estimated. A decrease in the energy transfer rates of the high-energy sensitizers is barely observable (picene and coronene, Fig. l ) , although the energy of the T2-state of (3) is very high (ca. 300 kJ/mol above TI)[". An "inverted region", in the sense of the Marcus theory (equation 10 in [ ' I ) , accordingly does not appear even in this rather sensitive case.Conclusions: Using flash spectroscopic sensitization of the cyclobutadiene derivative (3), we were able to observe a short-lived (240 ns) intermediate which absorbed at wavelengths less than 400 rim. This species was assigned to the lowest triplet state of (3). From the observed relation between the rate constant k,, and the triplet energy E;O (S) of the sensitizers used, the following estimates for the adiabatic and vertical triplet energ...