I would be curious to hear the answers to this, too.  Please post some thoughts here, in addition to e-mailing Claude.

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.)