  A new honeycomb lattice iridate (La,Na)IrO$_3$ ($\approx$ LaNaIr$_2$O$_6$) is
successfully synthesized from the spin-orbit coupled Mott insulator
Na$_2$IrO$_3$ by replacing the interlayer Na$^+$ ions with La$^{3+}$ ions.
(La,Na)IrO$_3$ shows a finite Sommerfeld term in heat capacity and a $-\ln T$
dependence of resistivity, indicating a realization of a metallic state driven
by a Mott collapse. Furthermore, crystal structure analysis reveals the
formation of Ir zig-zag chains with metal-metal bonding, increasing kinetic
energy resulting in the Mott collapse. This observation would be due to a Mott
collapse induced in a $J_{\mathrm{eff}} = 1/2$ spin-orbit coupling Mott
insulator with an Ir honeycomb lattice by topochemical control of the ionic
configuration.
