Q6(c). Quadruple bond and bond order 4 in [Re₂Cl₈]²-

1. Structure. Its structure was solved and the quadruple bond recognised by Cotton in 1964: the ion is two square-planar ReCl₄ units held together only by a Re–Re bond, with no bridging ligand. The Re–Re distance, 224 pm, is far shorter than in rhenium metal (about 274 pm, twice the metallic radius of 137 pm). The eight Cl atoms are eclipsed although a staggered form would reduce the Cl…Cl repulsion.
2. Orbitals. Take z along Re–Re. The d_(x²-y²) orbital of each Re points at its four Cl and is used for the Re–Cl σ bonds. The other four d orbitals overlap with those of the second Re: d_(z²)+d_(z²) along the axis → one σ bond (strongest overlap); d_(xz)+d_(xz) and d_(yz)+d_(yz) side by side → two π bonds; d_(xy)+d_(xy) face to face, overlapping in four regions → one δ bond (weakest).
3. Filling. Re(III) has 7-3=4 d electrons, eight in all. The order of the levels is σ<π<δ<δ^*<π^*<σ^*, so the eight electrons give σ²π⁴δ² with every antibonding orbital empty, and the ion is diamagnetic.
4. bond order=12(8-0)=4 a quadruple bond: one σ + two π + one δ.
5. Why eclipsed. The d_(xy) orbitals overlap only when they lie exactly face to face. Rotating one ReCl₄ unit by 45° (staggered) would make the δ overlap zero. The δ bond is weak but strong enough to lock the ion in the eclipsed form. In [Os₂Cl₈]²- (Os(III), d⁵) the two extra electrons fill δ^*; the bond order falls to 3, the δ bond is lost and the ion is nearly staggered (D_(4d)).
6. Colour. The δ–δ^* gap is small, so the δ→δ^* transition falls in the visible, and [Re₂Cl₈]²- is royal blue.

Calculations put the effective bond order nearer 3.2, because the δ bond is weak, but the formal description remains a quadruple bond.

Application. quadruple bonds are common for Mo, W, Re and Cr. Molybdenum(II) acetate, Mo₂(O₂CCH₃)₄, made from Mo(CO)₆ and acetic acid, is the usual starting material; with concentrated HCl it gives red [Mo₂Cl₈]⁴-.