fac-

https://doi.org/10.1351/goldbook.F02313
An affix used in names to designate three groups occupying the corners of the same face of an octahedral coordination sphere, not now generally recommended for precise nomenclature purposes.
Sources:
Blue Book, p. 464 (http://www.acdlabs.com/iupac/nomenclature/)
Red Book, 3rd ed., p. 245 (https://doi.org/10.1515/pac-2014-0718)