Antiferromagnetic complexes with metal—metal bonds XX. Synthesis, molecular structure, electronic structure and magnetic properties of the cyclopentadienyl-alkoxide complexes Cp2Cr2(μ-OR)2[OCCo3(CO)9] and Cp2Cr2(OR)2(μ-OR)2 (R = CMe3)

Interaction of the binuclear complex Cp2Cr2(μ-OCMe3)2 (I) with Co2(CO)8 in the molar ratio 11 in benzene at room temperature yields the cluster Cp2Cr2(μ-OCMe3)2[OCCo3(CO)9] (II) (space group P21/n, a 10.377(4), b 17.879(7), c 18.115(7) A, β 101.39(3)°, V 3294.7 A3, Z = 4, T − 100°C). One of the chromium atoms of the Cp2Cr2(μ-OCMe3)2 fragment (CrCr 2.766(1), CrOCMe3 1.950(3) A) in complex II is bonded to the oxygen atom of the μ3-CO group of the cluster fragment Co3(μ3-CO)(CO)9 (CrO 1.988(3), CO 1.247(6) A, CoCo 2.483(1)–2.489(1) A), which is an analogue of the alkoxide group. Compound II is treated as a Cp2Cr2(OR)(μ-OCMe3)2 complex with chromium atoms of different oxidation states; its properties are compared with the characteristics of the binuclear tetraalkoxide Cp2Cr2(OCMe3)2(μ-OCMe3)2 (III) which is formed along with the well-known cluster Cp4Cr4Te4 in the reaction of I with tellurium. Complex III has been characterized by an X-ray diffraction study (space group Pna21, a 22.4539(12), b 11.7514(6), c 10.7303(5) A, V 2831.3 A, Z = 8, T 20°C). Both CrIII atoms of the binuclear fragment Cp2Cr2(μ-OCMe3)2(Cr...Cr 3.004(3) A, CrO(average) 2.000(9) A) in III are bonded to the terminal alkoxide group (CrO(average) 1.852(8) A). Complexes I–III are antiferromagnetic. Calculations of the MOs for I–III compared with the well-known complex Cp2Cr2(OMe)2(NO)2 have been carried out by the extended Huckel method.