  ${\lambda}$-RhO$_2$, a prototype 4d transition metal oxide, has been prepared
by oxidative delithiation of spinel LiRh$_2$O$_4$ using ceric ammonium nitrate.
Average-structure studies of this RhO$_2$ polytype, including synchrotron
powder X-ray diffraction and electron diffraction, indicate the room
temperature structure to be tetragonal, in the space group I41/amd, with a
first-order structural transition to cubic Fd-3m at T = 345 K on warming.
Synchrotron X-ray pair distribution function analysis and $^7$Li solid state
nuclear magnetic resonance measurements suggest that the room temperature
structure displays local Rh-Rh bonding. The formation of these local dimers
appears to be associated with a metal-to insulator transition with a
non-magnetic ground state, as also supported by density functional theory-based
electronic structure calculations. This contribution demonstrates the power of
soft chemistry to kinetically stabilize a surprisingly simple binary oxide
compound.
