The microcrystalline organometallic coordination polymer [Ru(2){&mgr;(2):&mgr;(2):eta(2)-O(2)PMe(2)}(2)(CO)(4)](n)() which results from the oxidative addition of dimethylphosphinic acid to triruthenium dodecacarbonyl has been structurally characterized by X-ray powder diffraction, lambda = 1.149 49(1) Å, at 295 and 50 K. At room temperature the crystallites have a monoclinic unit cell with the space group C2/c with lattice constants a = 18.0792(3) Å, b = 9.0626(2) Å, c = 10.0372(2) Å, beta = 112.107(1) degrees, and Z = 4; the final refinement of 52 variables converged to R(p)(), R(wp)(), R(F)(), and R(F)()()2 of 8.2, 10.8, 4.6, and 8.3%, respectively, for data collected between 4 and 60 degrees (2theta). At 50 K the phase is described by a triclinic unit cell, space group P&onemacr;, and is characterized by the lattice constants a = 9.8637(6) Å, b = 8.9290(6) Å, c = 9.8870(5) Å, alpha = 115.051(3) degrees, beta = 108.587(5) degrees, gamma = 92.015(5) degrees, and Z= 2; the final refinement of 102 variables converged to and R(p)(), R(wp)(), R(F)(), and R(F)()()2 of 8.3, 11.4, 1.5, and 3.0%, respectively, for data collected between 3 and 74 degrees (2theta). The transition between the two crystalline phases has been determined by differential scanning calorimetry to occur at circa 220 K, and the most pronounced difference in the environment of the chains, as determined by variable-temperature IR spectroscopy, is in the rho(PCH(3)) modes for the bridging dimethylphosphinate ligands.