Effects of steric strain on the bonding in zinc metallocenes: The structure of [(C3H7)(4)C5H](2)Zn

Zinc diiodide reacts with the potassium salts of 1,2,4-tri- and 1,2,3,4-tetra(isopropyl)cyclopentadiene to form the corresponding zinc metallocenes in high yield. Both compounds are highly air-sensitive, but [(C3H7)(4)C5H]Zn-2 is isolated as a high-melting crystalline solid, whereas [(C3H7)(4)C5H2]Zn-2 is a waxy solid that melts just above room temperature. Like other structurally characterized zincocenes, [(C3H7)(4)C5H]Zn-2 has an asymmetric ''slipped-sandwich'' structure with one eta(1)- and one approximately eta(5)-ring. The Zn-C distance for 3 the eta(1)-ring of 2.223(4) Angstrom is the longest yet observed in a zincocene. The Zn-C distances for the eta(5)-ring cover a larger than usual range (0.52 Angstrom), and include the shortest Zn-C ring bond reported in a cyclopentadienyl 2:inc complex (1.991(4) Angstrom). These extremes in structural values reflect the increased slippage of the rings in [(C3H7)(4)C5H]Zn-2, which is a consequence of the exceptional steric bulk of the [(C3H7)(4)C5H](-) anion. Variable temperature NMR data for [(C3H7)(4)C5H]Zn-2 are consistent with the existence of an asymmetric structure in solution.