 Solution

chromium is the characteristic assignment, giving Cr(I)–Cr(I) with a formal quintuple bond. Each formamidinate is monoanionic, so $2x-3=-1$ gives $x=+1$. chromium(I) is $d^5$ and the pair supplies ten d electrons. Relative to a metal–metal $z$ axis, the idealized bonding combinations are $d_{z^2}$ sigma, $d_{xz},d_{yz}$ pi and $d_{xy},d_{x^2-y^2}$ delta:End-on sigma overlap gives the largest splitting, pi overlap a smaller splitting, and delta overlap the weakest. A useful qualitative ordering places bonding sigma lowest, then the two pi combinations, then the two delta combinations; all are occupied. Their antibonding partners are empty, with the weakly split delta-star levels nearest their bonding counterparts. The actual three-bridge ligand field can mix and split symmetry-compatible levels, so the diagram is a bond-axis model rather than five perfectly degenerate metal d levels in the molecular complex.

The identified lantern-type anion has an experimentally determined Cr–Cr distance near $1.740$ Å; reduction of its nine-bonding-electron precursor supplies the tenth electron. This corroborates the model more specifically than citing “short bonds” in an unrelated framework.^[ref]