The s-?* Electronic Transition of the Di en Trinuclear Complexes Ru(E)(E')(CO)2(iPr-DAB): a Resonance Raman, electronic Absorption, Emission and Density Functional study (E=Me, SnPh3, M(CO)5, E'=M(CO)5; M=Mn,Re; iPr-DAB=N,N'-Diisopropyl-1,4-diaza-1,3-but)