The 6H-type hexagonal perovskite Ba3BiIr2O9 undergoes an unusual first-order phase transition on cooling through 74 K marked by a 1% negative thermal expansion and a dramatic drop in magnetic susceptibility.[1] Explanations for this have focused on magnetic spin-ordering in face-sharing Ir2O9 octahedral dimers. However, direct evidence has proven elusive due to the high neutron absorption and steep magnetic form factor of Ir combined with the geometrically frustrated arrangement of the dimers. Indirect evidence for a magnetic spin-gap opening has come from inelastic neutron scattering studies of the isostructural compound Ba3BiRu2O9,[2] which shows a similar but weaker transition, but the interpretation is ambiguous, as is the analogy because Ru (group VII) and Ir (group IX) are not isoelectronic. Here we will present and discuss new evidence from 193Ir/99Ru synchrotron Mössbauer spectroscopy and polarised neutron diffuse scattering that suggest a change in 4d/5d orbital configuration within the dimers, rather than their spin states for a fixed configuration, drives these transitions; i.e., they are better described in terms of spin-crossover than magnetic ordering. Our analysis incorporates data from the structurally and chemically related 3C-type Ba2BiIrO6, 12L-type Ba4BiIr3O12, and 8H-type Ba2BiRu2O9,[3] and points to new phases and opportunities for new physics in other 4d and 5d analogues on which we are working.