Group 13 Metals as L-Type Ligands for Transition Metals

TL;DR

This paper reviews low-valent Group 13 elements as L-type ligands for transition metals, analyzing their bonding, reactivity, and design principles for creating heterometallic platforms for catalysis.

Contribution

It provides a unifying descriptor-based framework for understanding and designing Group 13 M(I) donors as ligands in transition-metal complexes.

Findings

Aluminium exhibits the strongest sigma-donation among Group 13 M(I) donors.

Gallylene and indylene show decreasing sigma-donor strength: Ga > In.

Tl(I) has limited L-type behavior and does not convincingly engage in neutral Tl->TM coordination.

Abstract

Low-valent Group 13 fragments can serve as neutral two-electron L-type metalloligands to transition-metal (TM) centers, enabling heterometallic M-TM platforms with bonding and reactivity patterns distinct from classical CO, phosphine, and carbene ligation. This chapter develops a unifying, descriptor-based view of aluminylene Al(I), gallylene Ga(I), and indylene In(I) donors, and contrasts them with the limited L-type behavior of Tl(I). We map synthetic gateways to isolable M(I) donors, analyze their sigma-donation/pi-acceptance profiles, and extract periodic design rules in which the sigma-donor strength decreases Al > Ga > In, whereas Tl(I) has not yet been convincingly shown to engage in neutral L-type Tl->TM coordination. Borderline cases that blur L-, X-, and Z-type classifications are also examined to clarify descriptors and guide consistent usage across the series. This contribution links ligand sterics/electronics, ambiphilicity at M(I), and the chosen TM fragment to guide the rational design of M-TM platforms that harness Group-13 M(I) donors for small-molecule activation and cooperative catalysis.