Domain: Organometallic chemistry

When a solution of the tetracarbonylchromate ion (IV), [Cr(CO)_4]^{4-} is subjected to treatment with ammonia gas in the presence of ammonium chloride, a series of ligand substitution and dimerization processes take place. These transformations ultimately result in the formation of a dinuclear complex in which two chromium centers are bridged by hydride ligands, and the overall charge of the species is retained as a dianion. Considering the electronic configuration of chromium, predict the total number of chromium–chromium (metal–metal) bonds present in this dinuclear complex.

Grading rubric:
criterion: Identifies the dimeric intermediate as Cr_2(CO)_8^{4-}.
weight: 20
criterion: Specifies ammonia gas followed by addition of ammonium chloride to generate NH_4^+.
weight: 10
criterion: Depicts the final dinuclear product as [(OC)_4Cr(μ-H)_2Cr(CO)_4]^{2-} formed from the reaction of Cr_2(CO)_8^{4-} with NH_4^+.
weight: 20
criterion: Correctly identifies that ammonia (NH_3) is evolved during formation of the final product.
weight: 20
criterion: Correctly calculates the total valence electrons (TVE) as 34 for [(OC)_4Cr(μ-H)_2Cr(CO)_4]^{2-}.
weight: 10
criterion: Applies the metal–metal bond order formula ((18 × N) − TVE) / 2 correctly to determine the number of Cr–Cr bonds.
weight: 10
criterion: Correctly concludes that the complex contains exactly one Cr–Cr bond.
weight: 10

Answer:
1 Cr-Cr bond