Left: Nebular emission metallicity, 12 + log(O/H) (Weng et al. 2023b), as a function of molecular gas mass (M_H2) for our H I–selected galaxies. Blue circles indicate CO detections, and blue leftwards arrows show 3σ upper limits for CO non-detections. The result from the stacked spectrum described in Sect. 3.2 is shown by the blue star. The solid line indicates the mass-metallicity relation from Genzel et al. (2015) for the PHIBSS sample (Tacconi et al. 2018). A positive correlation emerges between metallicity and CO detectability, with CO-detected galaxies tending to occupy the higher-metallicity regime (log Z ≳ 8.5), as found previously in many studies (e.g., Bolatto et al. 2013). Right: Histogram of emission metallicities for all galaxies, with CO detections in blue and non-detections in light blue filled histogram. The dashed orange line indicates a metallicity log Z ∼ 8.5, which sets the higher and lower metallicity regimes discussed in Sect. 5. Although the distributions overlap significantly, a two-sample Kolmogorov-Smirnov test yields a p-value of 0.004, which indicates a statistically significant difference in the underlying metallicity distributions. This reinforces the interpretation that metallicity plays a key role in governing the observability of molecular gas in H I–selected systems.