Submitted by Claude Mertzenich / Luther College on Tue, 03/16/2010 - 15:56

Why is cis-[Co(en)2Cl2]+ purple and the trans isomer green?  Thanks for any thoughts you might have on that.  Please email me at  Thanks.

Maggie Geselbracht / Reed College
I would be curious to hear the answers to this, too.  Please post some thoughts here, in addition to e-mailing Claude.
Tue, 03/16/2010 - 17:27 Permalink
Nancy Williams / Scripps College, Pitzer College, Claremont McKenna College
My colleague, Andy Zanella, an accomplished Cobalt hand, had this to say: I could resort to the usual hand-waving about different symmetries, but Messrs. Cotton, Wilkinson, Murillo, and Bochmann (6th ed., p. 824, 825)provide a few more details. Compared to the hexaammine complex, for both cis and trans, the 1T1g state is split into two states such that the lower energy transition (from A1g) is smaller for the trans compared to the cis, while the tran's higher transition is at larger energy than for the cis. (Of course, the same argument should also apply to the corresponding tetraammine complexes.)
Tue, 03/16/2010 - 20:52 Permalink