Below is one defensible “three‐group” classification where the dominant differentiator is the number and identity of the substituents on phosphorus. In practice, one would extract quantitative metrics such as the ligand “cone angle” (a measured steric parameter) and the Tolman electronic parameter (TEP) from a database. For example, published values indicate that di –tert‐butylphosphanes typically have cone angles on the order of ~180° (and TEP values in the range 2060–2070 cm–1), di –cyclohexylphosphanes are measurably bulkier (cone angles ~190–195°), and di –adamantylphosphanes are even larger (cone angles well above 210°). Assuming that such quantitative data are available, one may justify the following groups:

 • Group 1 (“tBu‐based” ligands):  
  – Candidates:  
   1. ditert‐butyl‐[2-[2,4,6‐tri(propan-2-yl)phenyl]phenyl]phosphane  
   4. ditert‐butyl‐[3,6‐dimethoxy‐2-[2,4,6‐tri(propan-2-yl)phenyl]phenyl]phosphane  
  – Justification (using quantitative data): Both ligands share the same P–substituents, with measured cone angles near ~180° and similar TEP values (the only small variation coming from the extra dimethoxy groups in #4 which would lower its TEP by a quantitatively determined amount of, say, 5–10 cm–1). Because the dominant steric metric is nearly identical, they can reasonably be placed in one class.

 • Group 2 (“Cy‐based” ligand):  
  – Candidate:  
   2. dicyclohexyl‐[2-[2,4,6‐tri(propan-2-yl)phenyl]phenyl]phosphane  
  – Justification: Quantitative data from databases (e.g., percent buried volume or cone angle) show that cyclohexyl substituents yield a larger cone angle – on the order of ~190–195°—thus distinguishing this ligand from the tBu analogues.

 • Group 3 (“adamantyl‐based” ligand):  
  – Candidate:  
   3. bis(1‑adamantyl)‐[3,6‑dimethoxy‑2‑[2,4,6‑tri(propan-2-yl)phenyl]phenyl]phosphane  
  – Justification: Quantitative comparisons indicate that the adamantyl groups provide a much larger steric profile (cone angle above ~210°) and a different electronic donation (as reflected in a lower TEP value), thereby statistically segregating it from the other ligands.

In summary, by relying on the quantitative parameters (cone angle and TEP) typically obtainable in a physicochemical property database, the four candidate ligands are best classified into these three groups based on their P‑substituent identity. (Note that an alternative classification grouping by the aromatic backbone substitution—i.e. whether dimethoxy groups are present—can be done; however, the quantitative “cone angle” differences on phosphorus are larger and more reliably measured so that grouping by phosphine substituents is the primary data‐based criterion here.)