Uncovering Multiple Metal–Metal Bonding in a Tetranuclear Fluoride Rhenium Cluster or the Curious Case of {[Ni(H2O)6](NH4)4}[Re4F18]·4H2O

Abstract

The compound {[Ni(H2O)6](NH4)4}[Re4F18]·4H2O (1) has been isolated and characterized by single‐crystal X‐ray diffraction, Raman spectroscopy and computational methods. The compound 1 crystallizes in the triclinic space group P1 (a = 6.6620(9) Å, b = 9.7121(13) Å, c = 11.2007(15) Å, α = 67.644(2)°, β = 83.827(2)°, γ = 84.366(2)°). In 1, the [Re4F18]6– anion is composed of four Re(III) atoms bridged by two µ‐bridging fluorine atoms and two di‐µ‐bridging fluorine atoms. In [Re4F18]6–, the shortest Re–Re distance (i.e., 2.3631(6) Å) is characteristic of the presence of a Re=Re double bond; the Re–F distances range from 1.854(6) to 2.008(5) Å. Theoretical calculations confirm the presence of a weak Re=Re double bond; the Mayer–Mulliken bond order of the doubly bridged Re atoms is 1.8097 and the natural localized molecular orbital bond order is 1.8751. The Raman spectrum of 1 has been measured and shows a series of bands in the region 167–1449 cm–1 that have been attributed to Re–F and Ni–O stretches, Re4F18 ring breathing and NH4+ bending and libration modes.