Covalent character of ionic bond

NOTE ON SOURCE NUMBERING: the source PDF prints this section's heading number as '5.8.7', which does not fit the chapter's own numbering scheme (it directly follows 5.7 Dipole moment, and the source PDF separately prints an unrelated section correctly headed '5.8 Resonance' right after this one) — an apparent misprint or extraction artifact, so this section is renumbered '5.8' here for a coherent sequence (and the following Resonance section is renumbered '5.9'), disclosed rather than silently guessed at. Even a bond classified as 'ionic' can carry significant PARTIAL COVALENT CHARACTER, and compounds with such partial covalent character can show properties more typical of covalent compounds — for instance LiCl, despite being an ionic compound, is noticeably MORE soluble in organic solvents than in water, a distinctly covalent-like behaviour. Fajans put forward three rules for predicting when an ionic bond gains more of this covalent character. Rule 1: a SMALLER cation and/or a LARGER anion gives greater covalent character — e.g. LiCl is more covalent than NaCl (smaller cation), and LiI is more covalent than LiCl (larger anion). Rule 2: a HIGHER charge on the cation gives greater covalent character — e.g. covalent character decreases along the series $\text{Al}^{3\oplus}(\text{Cl}^-)_3 > \text{Mg}^{2\oplus}(\text{Cl}^-)_2 > \text{Na}^{\oplus}\text{Cl}^-$. Rule 3: for cations of the SAME size and SAME charge, one with an outer electronic configuration of the $s^2p^6d^{10}$ type (e.g. Cu⊕, $1s^22s^22p^63s^23p^63d^{10}$) has GREATER polarising power — and hence gives more covalent character — than one with an outer $s^2p^6$ configuration (e.g. Na⊕, $1s^22s^22p^6$), because d elec …