The selection rule for electronic transitions states that a transition is symmetry allowed if the direct product of the initial and final states contains the totally symmetric representation (A1g in the Oh point group). Let's consider the possible d-d transitions: 1. t2g to t2g: The direct product t2g x t2g = a1g + e + t1u + t2g + t1g. This contains the totally symmetric representation (a1g), so t2g to t2g transitions are allowed. 2. eg to eg: The direct product eg x eg = a1g + t1u + t2g + eg. This also contains the totally symmetric representation (a1g), so eg to eg transitions are allowed. 3. t2g to eg: The direct product t2g x eg = t1u + t2u. This does not contain the totally symmetric representation, so t2g to eg transitions are not allowed. 4. eg to t2g: The direct product eg x t2g = t1u + t2u. This also does not contain the totally symmetric representation, so eg to t2g transitions are not allowed. So, the symmetry allowed d-d transitions for the [Ru(en)3]2+ complex ion are from t2g to t2g and from eg to eg