We begin with a brief historical review of the development of our understanding of the normal ordering of nd orbitals of a transition metal interacting with ligands, the most common cases being three below two in an octahedral environment, two below three in tetrahedral coordination, and four below one in a square-planar environment. From the molecular orbital construction of these ligand field splittings evolves a strategy for inverting the normal order: the obvious way to achieve this is to raise the ligand levels above the metal d's; that is, make the ligands better Lewis bases. However, things are not so simple, for such metal/ligand level placement may lead to redox processes. For 18-electron octahedral complexes one can create the inverted situation, but it manifests itself in the makeup of valence orbitals (are they mainly on metal or ligands?) rather than energy. One can also see the effect, in small ways, in tetrahedral Zn(II) complexes. We construct several examples of inverted ligand field systems with a hypothetical but not unrealistic AlCH3 ligand and sketch the consequences of inversion on reactivity. Special attention is paid to the square-planar case, exemplified by [Cu(CF3)4](-), in which Snyder had the foresight to see a case of an inverted field, with the empty valence orbital being primarily ligand centered, the dx2-y2 orbital heavily occupied, in what would normally be called a Cu(III) complex. For [Cu(CF3)4](-) we provide theoretical evidence from electron distributions, geometry of the ligands, thermochemistry of molecule formation, and the energetics of abstraction of a CF3 ligand by a base, all consistent with oxidation of the ligands in this molecule. In [Cu(CF3)4](-), and perhaps more complexes on the right side of the transition series than one has imagined, some ligands are σ-noninnocent. Exploration of inverted ligand fields helps us see the continuous, borderless transition from transition metal to main group bonding. We also give voice to a friendly disagreement on oxidation states in these remarkable molecules.

A DFT computational study and a structural analysis of the coordination of arenes to transition metals in low hapticity ( 1 and  2 ) modes have been developed, including a pseudosymmetry analysis of the molecular orbitals and the introduction of a hapticity map that makes evident the different degrees of intermediate hapticities.Calculations on [Pt II L 3 (C 6 H 6 )] model complexes reveal a preference for the  2 mode, while the  1 coordination is found to be a low energy transition state for a haptotropic shift. The attachment of the arene to a side group that is coordinated to the metal introduces geometrical constraints which result in hapticities intermediate between one and two. Comparison of the  1 arene complexes with benzonium cations shows that in the former case the bonding to the metal involves essentially the  system of the arene, affecting only slightly the delocalized nature of the carbon-carbon bonds. This behavior is in sharp contrast with the frequently found  1 coordination of Cp that involves  bonding and full dearomatization of the ring.2

A qualitative analysis of the distortions that operate on the π system of bridging arenes with anionic character is presented and substantiated by computational studies at the density functional B3LYP and CASSCF levels. The observed effects of bonding to two metal atoms and of the negative charge are an expansion of the arene ring due to the partial occupation of π* orbitals, an elongation or compression distortion accompanied by a loss of the equivalence of carbon-carbon bonds due to a Jahn-Teller distortion of the arene dianions, and a ring puckering due to a second-order Jahn-Teller distortion that may appear independently of the existence of the first-order effect. The workings of the orbital mixing produced by these distortions have been revealed in a straightforward way by a pseudosymmetry analysis of the HOMOs of the distorted conformations. The systems studied include Li(I) and Y(III) adducts of benzene, as well as trimethylsilyl-substituted derivatives in the former case. An analysis of the structural data of a variety of purported di- and tetraanionic arene ligands coordinated to transition metals in several bridging modes has reproduced the main geometrical trends found in the computational study for the benzene and trimethylsilyl-substituted benzene dianions, allowing a classification of the variety of structural motifs found in the literature.

While double bonds are known for transition metals of Groups 9 and 10 as well as for boron and p-block elements of Groups 14-16, Zn sits in a small region of the periodic table with no well-characterized double bonds. A qualitative reasoning indicates that zero-valent zinc has the potential to form Zn=Zn double bonds. A computational study in search for complexes that might showcase this new bond type is presented here.

Chromium-chromium quintuple bonds seem to be approaching the lower limit for their bond distances, and this computational density functional theory study tries to explore the geometrical and electronic factors that determine that distance and to find ways to fine-tune it via the ligand choice. While for monodentate ligands the Cr-Cr distance is predicted to shorten as the Cr-Cr-L bond angle increases, with bridging bidentate ligands the trend is the opposite, since those ligands with a larger number of spacers between the donor atoms favor larger bond angles and longer bond distances. Compared to Cr-Cr quadruple bonds, the quintuple bonding in Cr2L2 compounds (with L a bridging bidentate N-donor ligand) involves a sophisticated mechanism that comprises a positive pyramidality effect for the σ and one π bond, but a negative effect for one of the δ bonds. Moreover, the shorter Cr-Cr distances produce a mismatch of the bridging ligand lone pairs and the metal acceptor orbitals, which results in a negative correlation of the Cr-Cr and Cr-N bond distances in both experimental and calculated structures.