Metal‐ligand cooperativity (MLC) had a tremendous impact on d‐block metal‐mediated bond activation and homogeneous catalysis. Is this concept translatable to the elements of the p‐block? Are there analogies already at hand? In this review, we describe contributions in which a p‐block element (group 13–15) and its ligand (surrounding molecular framework) operate synergistically in a substrate activation or a catalytic cycle. This activity is termed element‐ligand cooperativity (ELC), in correspondence to MLC. After the concepts of p‐block low‐valent states and frustrated Lewis pairs mimicking the ambiphilic reactivity of transition metals, the spatial proximity of nucleophilic and electrophilic reaction sites and a small HOMO‐LUMO gap in a p‐block ELC complex might offer yet another approach. Selected examples shall illustrate common reactivities of ELC, disclose conceptual analogies with d‐block MLC, and outline shortcomings in this field.

Tetracoordinate aluminum­(III) anions are generally tetrahedral and non-Lewis acidic. Herein, we describe the first planar tetracoordinate aluminum anion. The anion is isolated in its free form, representing a new “anti-van’t Hoff/Le Bel” case. Despite its negative charge, it remains Lewis acidic and permits the formation of various unique adducts, e.g., a dianionic p-block metal hydride. Turning an inert spectator anion into a Lewis acid by planarization verifies theoretical predictions made 40 years ago, ultimately connects “anti-van’t Hoff/Le Bel” structure with reactivity, and pushes further the field of geometrically enhanced p-block element reactivity.

Anionic hydridosilicates are highly reactive and strong hydride donors. In contrast, calix[4]pyrrole hydridosilicate is an entirely water-stable, anionic silicon hydride, which does not show hydridic reactivity. However, it still acts as an electron donor and enables the detection of a single electron transfer process in the reduction chemistry with hydridosilicates. Most important, these unusual properties are imparted by the unique planar structure of its elusive parent neutral silanesubstantiating the effect of planar tetracoordinate silicon for the first time.

Metal‐ligand cooperativity (MLC) had a remarkable impact on transition metal chemistry and catalysis. By use of the calix[4]pyrrolato aluminate, [1]−, which features a square‐planar AlIII, we transfer this concept into the p‐block and fully elucidate its mechanisms by experiment and theory. Complementary to transition metal‐based MLC (aromatization upon substrate binding), substrate binding in [1]− occurs by dearomatization of the ligand. The aluminate trapps carbonyls by the formation of C−C and Al−O bonds, but the products maintain full reversibility and outstanding dynamic exchange rates. Remarkably, the C−C bonds can be formed or cleaved by the addition or removal of lithium cations, permitting unprecedented control over the system's constitutional state. Moreover, the metal‐ligand cooperative substrate interaction allows to twist the kinetics of catalytic hydroboration reactions in a unique sense. Ultimately, this work describes the evolution of an anti‐van't Hoff/Le Bel species from their being as a structural curiosity to their application as a reagent and catalyst.

Our Earth's crust is covered with compounds of silicon(IV). In each, tetracoordinated silicon(IV) arranges its four substituents in a tetrahedral fashion, while other environments are unknown. This work describes the isolation and properties of a molecular complex with square-planar silicon(IV). The flattened structural motif provokes a range of features that primes the second most abundant element for new applications in catalysis and materials science.