Bis(hydroxyalkylcyclopentadienyl)-containing mixed molybdenum-iridium clusters Mo2Ir2-(CO)(10){eta -C5H4(CH2) OH}(2) [x = 2 (1), 10] react with alkyl or aryl 1,omega -diisocyanates OCNRNCO [R = (CH2)(y) (y = 4, 6, 12), trans-1,4-cyclohexyl, or 4-C6H4CH2-4-C6H4] to form oligourethanes with transition metal clusters in the oligomer backbone. Characterization of the cluster-containing oligourethanes is aided by spectral comparison with model cluster diurethanes Mo2Ir2(CO)(10){eta -C5H4(CH2)(2)OC(O)NHRH}2 [R = (CH2)(y) y = 4 (6), 6, 12], trans-1,4-cyclohexyl prepared from reaction between the cluster diol I and alkyl isocyanates HRNCO. The precursor diol cluster 1 and cluster diurethane 6 have been characterized by single-crystal X-ray diffraction studies. The extent of polymerization has been assessed by gel permeation chromatography, with little dependence on diisocyanate precursor linker R but strong dependence on alkylcyclopentadienyl linker length (CH2), suggesting that the steric influence of the bulky dimolybdenum-diiridium cluster core and co-ligands is the most important factor governing extent of polymerization. Supporting this assessment, the considerably more sterically hindered precursor 2-butyn-1,4-diol-substituted cluster Mo2Ir2{mu (4)-eta (2)-C-2(CH2OH)(2)}(CO)(8)(eta -C5H5)(2) fails to react to any significant extent with 1,6-diisocyanatohexane.