# Naming phosphonium cations in ionic liquids

In Rennie et al., J. Phys. Chem. Lett., 2013, the following cation and anion structures are given in Figure 1. Figure 1(f) is captioned as butyltrimethylphosphonium, which seems to match the given structure. However, in the caption, the abbreviation $$\text{P}_{2225}$$ is given, which seems strange:

Rennie et al. repeat this in their Supporting Information:

But, other authors, such as Tsuchida et al., J. Mol. Liq., 2018, use $$\text{P}_{2225}$$ as an abbreviation for triethyl-n-pentylphosphonium, which makes sense if the subscript numbers represent the number of carbon atoms in the substituent alkyl groups:

Does it seem to you that there is a typo in the Rennie et al. paper? Or, what are the standard abbreviations for phosphonium cations?

Your notation is correct for $$\ce{P2225}$$.