Compounds featuring bonds between mercury and transition metals are of interest for their intriguing/ambiguous bonding and scarcely explored reactivities. We report herein the synthesis and reactivities of the new compound [(POCOP)Ni]₂Hg, [Ni₂Hg], featuring a trinuclear Ni−Hg−Ni core (POCOP=κ^P,κ^C,κ^P'-2,6-(i-Pr₂PO)₂C₆H₃). [Ni₂Hg] reacts with CO₂ to give the carbonate-bridged complex [Ni₂CO₃]. Bubbling CO gas through a solution of [Ni₂CO₃] gave its μ-CO₂ analogue [Ni₂CO₂], which itself reacts with CO₂ to give back [Ni₂CO₃], indicating that these two compounds interconvert reversibly. This implies that the formation of [Ni₂CO₃] from [Ni₂Hg] and CO₂ constitutes a reductive disproportionation of two molecules of CO₂ into CO₃²⁻ and CO. Tests showed that this process proceeds through three steps, an initial CO₂ insertion to give [Ni₂CO₂], followed by another CO₂ insertion to give the second intermediate [Ni₂C₂O₄], and the latter's decarbonylation to give [Ni₂CO₃]. Although the putative second intermediate could not be isolated, we have shown that it likely features a μ-carbonyl-carbonate rather than a μ-oxalate moiety, because the latter complex is thermally stable to decarbonylation. Reduction of [Ni₂CO₃] with excess Na/Hg regenerates [Ni₂Hg], establishing that the observed deoxygenation of CO₂ in this system can, in principle, be catalytic in the presence of excess reductant.