Crystal structures of [K(2.2.2-crypt)]₄[Pb₉Mo(CO) ₃] - Isolation of the novel isomers [(η⁵-Pb ₉)Mo(CO)₃]^4- beside [η⁴-Pb ₉Mo(CO)₃]^4-

The reaction of [Mo(CO)₃(MeCN)₃] with K ₄Pb₉ 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[(η⁵-Pb₉)-Mo(CO)₃] (1), which contains a Pb₉^4- Zintl anion coordinated by Mo in an η⁵-fashion, is isolated for the first time and characterized by single-crystal X-ray structure analysis. The 10 metal atoms in the [(η⁵-Pb₉)Mo(CO)₃]^4- anion form a bi-capped 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 [(η⁵-Pb₉)Mo(CO) ₃]^4- ion is formed. In contrast, the reaction of [Mo(CO)₃(Mes)] with K₄Pb₉ under the same reaction conditions gives a mixture of compound 1 and its isomer [K(2.2.2-crypt)]₄[(η⁴-Pb₉)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 η⁴- and η⁵-isomers are compared and the packing of compounds 1 and 2 is discussed.