Single-particle behavior at<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>N</mml:mi><mml:mo>=</mml:mo><mml:mn>126</mml:mn></mml:mrow></mml:math>: Isomeric decays in neutron-rich<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">Pt</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>204</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>

The four proton-hole nucleus $^{204}\mathrm{Pt}$ was populated in the fragmentation of an $E/A=1$ GeV $^{208}\mathrm{Pb}$ beam. The yrast structure of $^{204}\mathrm{Pt}$ has been observed up to angular momentum $I=10\ensuremath{\hbar}$ by detecting delayed $\ensuremath{\gamma}$-ray transitions originating from metastable states. These long-lived excited states have been identified to have spin-parities of ${I}^{\ensuremath{\pi}}=({10}^{+})$, $({7}^{\ensuremath{-}})$, and $({5}^{\ensuremath{-}})$ and half-lives of ${T}_{1/2}=146(14) \mathit{ns}$, $55(3)\ensuremath{\mu}$s, and $5.5(7)\ensuremath{\mu}$s, respectively. The structure of the magic $N=126$ $^{204}\mathrm{Pt}$ nucleus is discussed and understood in terms of the spherical shell model. The data suggest a revision of the two-body interaction for $N=126$, $Z&lt;82$, which determines the evolution of nuclear structure toward the r-process waiting point nuclei.