This Perspective discusses the synthesis and reactivity of low-coordinate transition metal diaryl complexes. The development of sterically demanding ligands, in particular m-terphenyls is facilitating the isolation of complexes featuring two-coordinate open-shell transition metal centres. These coordinatively unsaturated complexes are now being investigated for their reaction chemistry, which is not only contributing to the elucidation of the structure and bonding within these systems but is also leading to the formation of novel compounds.

The search for extremely sterically demanding monodentate amide ligands to access main group complexes in low-coordination numbers and highly reactive bonding modes is an area of intense research interest. The recent development of three classes of sterically demanding, monodentate amide ligands – the m-terphenyl anilides [N(R){C₆H₃Ar₂-2,6}]⁻ (Ar = aryl, R = H, methyl, silyl), substituted carbazol-9-yl and the extremely bulky amides [N(R)(Ar′)]⁻ (Ar′ = 2,6-{C(H)Ph₂}-4-R′-C₆H₂, R = silyl, aryl, silyloxy, R′ = alkyl) is facilitating the isolation of stable species with new coordination modes for the main group elements. These compounds are of fundamental importance not only from the investigation of their structure and bonding, but also the investigation of their reactivity highlights the potential for small molecule activation chemistry under mild conditions and applications in catalysis. This review reports on the recent developments for these compounds with emphasis on their synthesis, structure and reactivity.

In this Perspective, we will highlight selected examples of transition metal complexes with low coordination numbers whose high reactivity has been exploited in catalysis and the activation of small molecules featuring strong bonds (N₂, CO₂, and CO).

Carbon monoxide is a key C₁ feedstock for the industrial production of hydrocarbons, where it is used to make millions of tonnes of chemicals, fuels, and solvents per annum. Many transition metal complexes can coordinate CO, but the formation of new C−C bonds in well-defined compounds from the scission and subsequent coupling of two or more CO moieties at a transition metal centre remains a challenge. Herein, we report the use of low-coordinate iron(II) complexes for the selective scission and homologation of CO affording unusual squaraines and iron carboxylates at ambient temperature and pressure. A modification of the ligand framework allows for the isolation and structural characterisation of a proposed metallacyclic Fe(II) carbene intermediate. These results indicate that, with the appropriate choice of supporting ligands, it is possible to cleave and homologate carbon monoxide under mild conditions using an abundant and environmentally benign low-coordinate, first row transition metal.

The redox processes associated with BODIPY analogues are studied by electrochemical and spectroscopic methods revealing a characteristic profile for the persistent BODIPY radical and quenching of fluorescence upon reduction.