Pushing the Limits of Delta Bonding in Metal–Chromium Complexes with Redox Changes and Metal Swapping

Into the metalloligand Cr­[N­(<i>o-</i>(NCH₂P­(^<i>i</i>Pr)₂)­C₆H₄)₃] (<b>1</b>, CrL) was inserted a second chromium atom to generate the dichromium complex Cr₂L (<b>2</b>), which is a homobimetallic analogue of the known MCrL complexes, where M is manganese (<b>3</b>) or iron (<b>4</b>). The cationic and anionic counterparts, [MCrL]⁺ and [MCrL]⁻, respectively, were targeted, and each MCr pair was isolated in at least one other redox state. The solid-state structures of the [MCrL]^+,0,– redox members are essentially the same, with ultrashort metal–metal bonds between 1.96 and 1.74 Å. The formal shortness ratios (<i>r</i>) of these interactions are between 0.84 and 0.74 and are interpreted as triple to quintuple metal–metal bonds with the aid of theory. The trio of (d–d)¹⁰ species [Cr₂L]⁻ (<b>2</b>^<b>red</b>), MnCrL (<b>3</b>), and [FeCrL]⁺ (<b>4</b>^<b>ox</b>) are <i>S</i> = 0 diamagnets. On the basis of MCr bond distances and theoretical calculations, the strength of the metal–metal bond across the (d–d)¹⁰ series increases in the order Fe < Mn < Cr. The methylene protons in the ligand are shifted downfield in the ¹H NMR spectra, and the diamagnetic anisotropy of the metal–metal bond was calculated as −3500 × 10^–36, −3900 × 10^–36, and −5800 × 10^–36 m³ molecule^–1 for <b>2</b>^<b>red</b>, <b>3</b>, and <b>4</b>^<b>ox</b> respectively. The magnitude of diamagnetic anisotropy is, thus, affected more by bond polarity than by bond order. A comparative vis–NIR study of quintuply bonded <b>2</b>^<b>red</b> and <b>3</b> revealed a large red shift in the δ⁴ → δ³δ* transition energy upon swapping from the (Cr₂)²⁺ to the (MnCr)³⁺ core. Complex <b>2</b>^<b>red</b> was further investigated by resonance Raman spectroscopy, and a band at 434 cm^–1 was assigned as the CrCr bond vibration. Finally, <b>4</b>^<b>ox</b> exhibited a Mössbauer doublet with an isomer shift of 0.18 mm/s that suggests a primarily Fe-based oxidation to Fe­(I)