Metal−Metal Bonding in M<sub>2</sub>Cl<sub>6</sub>(H<sub>2</sub>PCH<sub>2</sub>PH<sub>2</sub>)<sub>2</sub>, M<sub>2</sub>Cl<sub>6</sub>(PH<sub>3</sub>)<sub>4</sub>, and M<sub>2</sub>Cl<sub>10</sub><sup>4-</sup> (M = Cr, Mo, W) Edge-Shared Dimer Systems
