Neutral L-type ligation of Tl(I) to transition metals

Determine whether monovalent thallium Tl(I) can form neutral two-electron L-type Tl(I)→transition-metal coordination bonds and, if so, construct and rigorously characterize isolable Tl(I)–transition-metal complexes that satisfy CBC L-type criteria (unchanged metal oxidation state upon coordination and a two-electron dative bond from Tl(I)), distinguishing these from Z-type reverse-dative M→Tl interactions or purely metallophilic contacts.

Background

Within Group 13, low-valent Al(I), Ga(I), and In(I) fragments are established as neutral two-electron L-type donors to transition metals, enabling distinctive heterometallic bonding and reactivity. In contrast, thallium(I) exhibits a pronounced inert-pair effect that disfavors neutral donation, and most characterized Tl(I)–transition-metal assemblies are better described as Z-type reverse-dative interactions or metallophilic contacts.

The authors explicitly note that a convincing demonstration of neutral L-type Tl(I)→transition-metal coordination has not yet been achieved, framing a concrete unresolved question. Establishing such ligation would require synthesis and definitive electronic-structure evidence that Tl(I) acts as a neutral two-electron donor without altering the formal oxidation state of the metal center, thereby expanding Group-13 L-type chemistry to the heaviest congener.