Syntheses, structures and spectroscopic properties of a novel series of metalmetal bonded complexes Ru(E)(E′)(CO)2(iPrDAB): (E Br, E′ Mn(CO)5; E SnPh3, E′ Mn(CO)5, Re(CO)5, CO(CO)4; E Me, E′ Re(CO)5; E E′ Mn(CO)5, Re(CO)5; iPrDAB N,N′-diisopropyl-1,4-diaza-1,3-butadiene)