Methylenebis(phosphinic acid), CH$\sb2$(PHO(OH)) $\sb2$, or H$\sb2$pcp, forms a binuclear complex with platinum, Pt$\sb2$(pcp)$\sb4\sp{4-}$, which is luminescent. The green emission, 510 nm, has a long lifetime in the solid, 6 $\mu$s, and a short one in solution, 55 ns. The energy of emission depends on the ligand more than on the Pt-Pt separation in these compounds. The short solution lifetime is due to fast nonradiative decay. The decay is related to a conformational change involving flipping of CH$\sb2$ groups from one side to the other of the PPtPtP plane. The temperature dependence is fit by a function containing a discontinuity at the solvent freezing point, where conformational change stops. A tetrameric palladium compound that analyzes as Pd$\sb4$($\mu$-Cl)$\sb4$($\mu$-Hpcp)$\sb4$ is easily made and luminesces red at 77 K. The $\sp{31}$P and $\sp{195}$Pt nmr spectra of X$\sb2$Pt$\sb2$(pcp)$\sb4\sp{4-}$, X$\sb2$Pt$\sb2$(pop)$\sb4\sp{4-}$, Pt$\sb2$(pcp)$\sb4\sp{4-}$, and Pt$\sb2$(pcp)$\sb4\sp{4-}$ were completely simulated. A Karplus relation was observed for $\sp3$J(PPtPtP) couplings. $\sp1$J(PtPt) can be determined from either the $\sp{195}$Pt or the $\sp{31}$P nmr spectrum
