t h e AI, as has been shown for similar complexes 141. No noticeable exchange between R' on the Al and R on the Ti occurs before relatively slow reduction reactions take place.The methyl group is a weaker donor than the ethyl. This direrence may cause great changes in transition metal complexes[6? 7J. In (20) R' = CzH5 is more effective than R' = CH3 in (2b) against the delocalization of the unpaired electron of the Ti111: consequently there is a lower spin density at the A1 nucleus with R' = C~H S (lines in the spectra are closer together). The donor ligands on the A1 thus influence the electronic distribution in the whole complex; this influence is also to be expected for the TiIv species, and has in fact been detected. The higher the donor ability of R' the higher the electron density o n the Ti, and the higher the instability of the Ti-R bond. Such behavior is well illustrated by the reduction rate curves shown in Fig. 2, for the systems ( f a ) + (2n) (uppermost curve) and (fc) + (26) (middle curve). This result confirms the hypothesis [51 that the stability of a carbonmetal bond can be varied at will by varying the ligands. The 5 10 t ( h ) + Fig. 2. w : from the magnetic susceptibility. 0 @: from the ESR spectra (two series of experiments).
Concentration of Ti(in) as a function of time.observed higher stability of the Ti-CH3 bond in the system ( l a ) + (2b) (Fig. 2, lowest curve) can be interpreted according to the Mulliken model of the CH3 group 181. This model considers the methyl as a triply bonded -C -H3 species, and "H3" as a heteroatom. The two bonding TC-MO's perpendicular to the axis of -C =H3 (and the corresponding antibonding MO's) have the right symmetry to match with the d-x-Ti orbitals (d,,, dyz). This convenient configuration is obviously not available in the case of other alkyls.The photochemical isomerization of cis-9,1O-dihydro-9,10naphthalenedicarboxylic anhydride (la) t o an anhydride with the bicyclo[4.2.2]deca-2,4,7,9-tetraene structure 111 disclosed a rearrangement that has since been demonstrated also for the parent hydrocarbon ( I b ) [21, the ester ( f c ) (31, and the compounds (Id) and ( I e ) 141. The isolation andlor detection ofthe tetracyclo[4.4.0.02~~o.O5,7]deca-3,8-diene intermediates (2d) 141, ( 2 e ) 141, and (2b) 151 suggested that the rearrangement took the course shown in Scheme 1. (b), R = H (C), R = CO2CHs If), R = CO2H Scheme 1.The bicyclo[4.2.2]deca-2,4,7,9-tetraene photoisomer obtained from (la) should then have structure (3~). in which the anhydride group is attached to one of the isolated double bonds. However, chemical degradation of the photoisomer showed that it has structure ( 5 2 ) with the anhydride group attached to the diene system. (a), R -R = -C -O -C -(c), R = C02CH3 (f), R = COzH II II 0 0 Scheme 2.On catalytic hydrogenation over platinum in glacial acetic acid the photoanhydride gives a hexahydro compound [m.p.from which cis-l,4-cyclohexanediacetic acid [61 (identified by means of the dimethyl ester prepared with diazomethane) is formed on ozonol...
