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
