Boranes are ubiquitous in chemistry and most commonly utilised for their Lewis acidic character. The established dogma is that boranes (BY3) are strong Lewis acids towards fluoride, with the derived fluoroborates, [F-BY3] -, being highly stable towards loss of fluoride.1 Many of the most widely used boranes, such as BX3 (X = halide) and B(C6F5)3, are indeed strong Lewis acids towards fluoride and form robust fluoroborates, 2 with [BF4] -being an archetypal weakly coordinating anion. Furthermore, boranes such as B(C6F5)3, and even HBR2, are increasingly applied in defluorinative functionalisation of fluorocarbons, with fluoride abstraction by the borane to form a fluoroborate anion a key step (Figure ). Thus using fluoroborates to transfer fluoride to carbon electrophiles appears counter intuitive. However, by controlling the relative Lewis acidity of the carbon and boron electrophiles it is possible to effect fluoride transfer from fluoroborates to carbon electrophiles. One classic example is [BF4] - reacting as a stoichiometric fluoride source in the Balz-Schiemann reaction, but this requires a highly reactive Aryl + electrophile. To expand the utility of fluoroborates in nucleophilic fluorinations it is highly desirable to: (i) use substoichiometric fluoroborate and stoichiometric MF, i.e. use boranes as MF phase transfer catalysts; (ii) fluorinate carbon electrophiles less reactive than e.g. Aryl + . To expand the electrophile scope amenable to fluorination with fluoroborates requires an understanding of the factors controlling the fluoride ion affinity (FIA) of boranes, thereby enabling its rational modulation. Analysis of calculated FIA values reveals that borane fluorophilicity can be attenuated by: (i) the presence of significant B=Y multiple bond character; (ii) reducing the positive charge localised at boron using less electron withdrawing substituents, and (iii) increasing the pyramidalisation energy at boron. The first two points combined explains the trend in the fluoride affinity of the simple boranes: BF3 (most Lewis acidic, FIA = 258 kJmol -1 ) >> BMe3 (FIA = 132 kJmol -1 ) > B(OH)3 (FIA = 106 kJmol -1 , FIA values relative to Me3Si + ). Despite the facile ability to tune fluoride affinity at boron there are no reports, to the best of our knowledge, that utilise low FIA boranes as catalysts for MF phase transfer fluorination. Due to the importance of fluorinated molecules in pharmaceuticals and agrochemicals 7 and the attractive nature of using metal fluoride (MF) salts and simple boranes to effect nucleophilic fluorination, we sought to: (i) demonstrate that low fluoride affinity boranes can be used as MF phase transfer catalysts and (ii) develop the structure activity relationships key to enabling this reactivity.