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We have first studied the selective CO substitution reactions of heteronuclear tetrahedral clusters of the type (µ 3 -S)FeMCo(CO) 8 (η 5 -C 5 H 4 R) (M ) Mo, W) with cyclohexyl isocyanide CyNC. It was found that while clusters (µ 3 -S)FeMCo(CO) 8 (η 5 -C 5 H 4 R) (M ) Mo, W; R ) MeCO) react with 1 equiv of CyNC in THF at room temperature to give mono-CyNC-substituted products (µ 3 -S)FeMCo(CO) 7 (CyNCSimilarly, tri-CyNC-substituted derivatives (µ 3 -S)FeMoCo(CO) 5 (CyNC) 3 (η 5 -C 5 H 4 R) (3a, R ) MeCO; 3b, R ) MeO 2 C) are produced by reaction of clusters (µ 3 -S)FeMoCo(CO) 8 (η 5 -C 5 H 4 R) (R ) MeCO, MeO 2 C) with 3 equiv of CyNC in 37% and 39% yields. The combined use of spectroscopic techniques (IR, 1 H and 59 Co NMR, and 57 Fe Mo ¨ssbauer spectroscopy) with crystal X-ray diffraction analyses for 1a,b and 2c,d has established that the CyNC ligand in 1a,b is bonded to M (M ) Mo, W), the two CyNC ligands in 2a-e are bound to M (Mo, W) and Fe, and the three CyNC ligands in 3a,b are attached to Mo, Fe, and Co, respectively. Thus, the metallosite selectivity toward CyNC in the reaction of this type of cluster system decreases obviously in the order of η 5 -RC 5 H 4 (CO) 2 M (M ) Mo, W) > Fe(CO) 3 > Co(CO) 3 .

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