PHOTOAQUATION OF [CR(EN)(NH3)4]3+, CIS-[CR(]N)2(NH3)2]3+ AND TRANS-[CR(EN)2(NH3)2]3+, AND [CR(EN)3]3+ - RELATIVE LABILITIES OF ETHYLENEDIAMINE AND AMMONIA LIGANDS

The total amine aquation yield and the free ammonia yield have been measured for photolysis of [Cr(en)x(NH3)6-2x]3+ complexes at 366 and 436 nm and at 10°C in acidic aqueous solutions. For both the bis(ethylenediamine) complexes and the mono(ethylenediamine) complex the ethylenediamine yields were larger than the statistical expectations. The enhancement may involve the steric strain in the ethylenediamine ring; for the bis complexes the rate constant ratio for ethylenediamine to ammonia photoaquation is 2.83, while for the mono(ethylenediamine) complex the analogous ratio is 1.64. The systems provide unequivocal evidence for the importance of factors other than spectroscopic parameters in determining ligand labilities from excited states. © 1979, American Chemical Society. All rights reserved.