Crystal Structures of [K(2.2.2‐crypt)] 4 [Pb 9 Mo(CO) 3 ] — Isolation of the Novel Isomers [(η 5 ‐Pb 9 )Mo(CO) 3 ] 4– beside [(η 4 ‐Pb 9 )Mo(CO) 3 ] 4–

The reaction of [Mo(CO) 3 (MeCN) 3 ] with K 4 Pb 9 in the presence of 2.2.2‐cryptand (2.2.2‐crypt) has been investigated. The transition metal complex [K(2.2.2‐crypt)] 4 [(η 5 ‐ Pb 9 )Mo(CO) 3 ] ( 1 ), which contains a Pb 9 4– Zintl anion coordinated by Mo in an η 5 ‐fashion, is isolated for the first time and characterized by single‐crystal X‐ray structure analysis. The 10 metal atoms in the [(η 5 ‐ Pb 9 )Mo(CO) 3 ] 4– anion form a bicapped square antiprism, with the Mo(CO) 3 unit occupying a waist vertex position. The IR spectra of single crystals, as well as of the crude product, show clearly that only the [(η 5 ‐ Pb 9 )Mo(CO) 3 ] 4– ion is formed. In contrast, the reaction of [Mo(CO) 3 (Mes)] with K 4 Pb 9 under the same reaction conditions gives a mixture of compound 1 and its isomer [K(2.2.2‐crypt)] 4 [(η 4 ‐ Pb 9 )Mo(CO) 3 ] ( 2 ). Compounds 1 and 2 were isolated without additional solvent molecules. The Pb–Mo, Pb–Pb, and Mo–C bond lengths and Mo–C–O bond angles in the η 4 ‐ and η 5 ‐isomers are compared and the packing of compounds 1 and 2 is discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)