BOND-STRETCH ISOMERS OF TRANSITION-METAL COMPLEXES - DO THEY EXIST

The existence of bond-stretch isomers of transition-metal complexes and the electronic mechanisms explaining these isomers are investigated with ab initio calculations. For (LWOCl2)+ (L = N,N',N"-trimethyl-1,4,7-triazacyclononane) complexes, only the 2A' state can be identified as a ground state. 2A" is an excited state, and the orbital crossing mechanism cannot explain the occurrence of two stable isomers. For cis-mer-MoOCl2(PR3)3 complexes, the second-order Jahn-Teller effect is too weak to cause the bond-stretch phenomenon. On both molecular systems, our calculations support the structure of the blue "isomer" as the stable compound and preclude the existence of any other isomer with a long M-O bond and similar energy.