# Electron-donating trends for ligands bound to metal complexes

Imagine an inorganic complex with a bunch of terminal ligands. All else the same, would $$\ce{F-}$$ or $$\ce{Br-}$$ ligands be more electron-donating and why?

Of course, in terms of $$\mathrm{p}K_{\mathrm{a}}$$, $$\ce{F-}$$ is more basic than $$\ce{Br-}$$, predominantly due to the smaller size of the $$\ce{F-}$$ anion. I have also read in multiple places that the $$\mathrm{p}K_{\mathrm{a}}$$ trends can be used to describe Lewis acidity/basicity trends, although it's not immediately clear to me why this is true. If one takes the prior statement at face-value, then it appears that that $$\ce{F-}$$ is more Lewis basic and, by the very definition of a Lewis base, is more electron-donating than $$\ce{Br-}$$. However, why would $$\ce{F-}$$ be more electron-donating if its electronegativity is higher than $$\ce{Br-}$$? These two points seem paradoxical.

For full context, I am trying to understand why $$\ce{F-}$$ would be better at stabilizing higher oxidation states of metals in inorganic complexes than $$\ce{Br-}$$, which based on the literature appears to be related to the electron-donating strength of the ligands (with more electron-donating ligands increasing the stability of higher oxidation states).