Metal carbenes are important intermediates for many broadly applied chemical reactions such as cyclopropanations, 1 crosscouplings, 2 and olefin metathesis. A detailed understanding of the reactivity of these organometallic entities is necessary to fully exploit their catalytic potential, both to optimize known reactions and to develop new ones. First row transition metal centers are of high current interest, because of not only their earth-abundance but also their high reactivity. Early work by Miyashita and Grubbs suggested the potential of nickel carbenes generated by the fragmentation of metallacycles for olefin metathesis and cyclopropanation. However, the stabilization of such species is challenging because of the electron rich nature of nickel and the strong s-donation of the carbene fragment. Several years later, based on the conceptualization that a "L2Ni" moiety has isolobal orbitals with the CH2 fragment, the group of Hillhouse pioneered the synthetic study of nickel carbene species. They described the isolation and thorough characterization of nickel carbene complexes bearing the dtpbe ligand (bis(di-tert-butylphosphino)ethane = dtbpe) and another related rigid bidentate ligand (Figure ). To the best of our knowledge these are the only examples of isolated nickel carbenes that are neither heteroatom-stabilized (e.g., NHCs) nor incorporated in a multidentate (e.g., pincer) ligand. The dtbpe-supported nickel carbene complex undergoes nucleophilic addition to two molecules of CO2 to form an 1,1-dicarboxylate ligand, and it can transfer the carbene fragment to small molecules such as CO and ethylene to form the corresponding ketene and cyclopropane, respectively. In the case of the reaction with ethylene, a metallacyclobutane intermediate was proposed. Our group recently investigated the reactivity of pentacoordinated nickelacyclobutanes with distorted trigonal bipyramidal geometry incorporated in a pincer framework. In agreement with the early observation from Myashita that the metathesis- like fragmentation of nickelacyclobutanes could be favored by higher coordination numbers, selective [2+2] cycloreversion could be observed in one case. This contrasts with the known reactivity of square planar nickelacyclobutanes, for which no metathesis-like fragmentation has been observed. For a putative olefin-metathesis cycle based on a 5-coordinate nickelacyclobutane to be viable, the corresponding 4-coordinate Ni carbene would also need to be stable despite deviating from the isolobal paradigm outlined above.