Factors responsible for the crystallization of multiple addition products of C-60 With Vaska-type iridium complexes have been examined. A 10-fold excess of Ir(CO)Cl(PMe(3))(2) reacted with a benzene solution of C-60 over a period of 7 days to give the double addition product, C-60{Ir(CO)Cl(PMe(3))(2)}(2).2C(6)H(6). Black parallelepipeds form in the orthorhombic space group Cmcm (No. 63) with a = 20.188(4) Angstrom, b = 16.042(3) Angstrom, and c = 18.517(4) Angstrom, at 123 K with Z = 4. Refinement (on F-2) of 3641 reflections and 225 parameters with 73 restraints yielded R = 0.065 (for 2506 reflections with I > 2.0 sigma(I)) and wR2 = 0.142. Reaction of a 10-fold excess of Ir(CO)Cl(PEt(3))(2) with a saturated benzene solution of C-60 resulted in the gradual formation of yet another crystalline double addition product, C-60{Ir(CO)Cl(PEt(3))(2)}(2).C6H6. Black needles form in the monoclinic space group I2/a (an alternate setting of No. 15) with a = 20.105(2) Angstrom, b = 12.467(1) Angstrom, c = 26.458(2) Angstrom, beta = 99.63(1)degrees at 123 K with Z = 4. Refinement (on F-2) of 4275 reflections and 242 parameters with 42 restraints yielded R = 0.073 for 3987 reflections with I > 2.0 sigma(I) and wR2 = 0.190. Both double addition products have the iridium complexes bound as usual to 6:6 ring junctions. Both also have the two complexes bound at the "para" positions at opposite ends of the fullerene. As a result of the distortions induced by the binding of the two iridium complexes, the fullerene portion is elongated by 0.40 Angstrom along the Ir---Ir axis. P-31{H-1} NMR and infrared spectroscopic studies of solution of C-60{Ir(CO)Cl(PEt(3))(2)}(2).C6H6 in 6:4 o-dichlorobenzcneltoluene indicate that there is extensive dissociation of the adducts at room temperature. At -80 degrees C there is evidence of the formation of a multiplicity of adducts which are probably the regioisomers of the double addition product. We believe that isolation of the "para" double addition products is a consequence of the low solubility of this isomer which possesses the lowest polarity and highest symmetry of all of the possible eight regioisomers of these double addition products.