The structure of some transition metal fluorides

The structures of the solid fluorides MF₂, MF₃, MF₄ and MF₅, in which M has the coordination number 6 and belongs to the 3$\text{d}$, 4$\text{d}$- and 5$\text{d}$-periods and the Vb to VIII groups, can be divided into 3 types: (a) cubic close packing ($\text{ccp}$) of F with an MFM bridging angle of 180°; (b) hexagonal close packing ($\text{hcp}$) with an MFM-bonding angle of 132°; (c) intermediate packing between (a) and (b). The linear bridging is assumed to be a consequence of π-back bonding (or charge transfer) between $\text{p}$F-orbitals and $\text{d}$-orbitals of the metal. If such bonding is not possible then $\text{hcp}$ with the bridging angle of 132° will result. Weaker π-interactions give the intermediates (c).