Which of the following compounds has a P - P Bond?

To determine which compound has a phosphorus-phosphorus (P-P) bond, we need to examine the structure of each option. A P-P bond means a direct bond between two phosphorus atoms, not connected through an oxygen atom or any other bridge.

Starting with option A: $$(HPO_3)_3$$. This compound is metaphosphoric acid, which exists as a cyclic trimer. The structure consists of a ring where each phosphorus atom is bonded to three oxygen atoms: two bridging oxygen atoms (connecting to adjacent phosphorus atoms) and one terminal oxygen atom (which is part of an OH group). The phosphorus atoms are connected via P-O-P bridges, but there is no direct P-P bond. Therefore, option A does not have a P-P bond.

Moving to option B: $$H_4P_2O_6$$. This compound is hypophosphoric acid. First, let's verify the oxidation state of phosphorus. Assign oxidation states: hydrogen is +1, oxygen is -2. The molecule has four hydrogen atoms, two phosphorus atoms, and six oxygen atoms. Setting up the equation: $$4 \times (+1) + 2 \times \text{(oxidation state of P)} + 6 \times (-2) = 0$$. Solving: $$4 + 2x - 12 = 0 \rightarrow 2x - 8 = 0 \rightarrow 2x = 8 \rightarrow x = +4$$. Each phosphorus has an oxidation state of +4. The structure of hypophosphoric acid is $$H_2O_3P-PO_3H_2$$, where there is a direct P-P single bond. Each phosphorus atom is also bonded to three oxygen atoms: two are part of OH groups, and one has a double bond character (or is a terminal oxygen). Thus, there is a direct P-P bond in this compound. Therefore, option B has a P-P bond.

Now, option C: $$H_4P_2O_7$$. This compound is pyrophosphoric acid. Oxidation state calculation: $$4 \times (+1) + 2 \times \text{(oxidation state of P)} + 7 \times (-2) = 0$$. Solving: $$4 + 2x - 14 = 0 \rightarrow 2x - 10 = 0 \rightarrow 2x = 10 \rightarrow x = +5$$. Each phosphorus has an oxidation state of +5. The structure consists of two phosphate units connected by an oxygen bridge: $$(HO)_2OP-O-PO(OH)_2$$. This is a P-O-P linkage, with no direct P-P bond. Therefore, option C does not have a P-P bond.

Finally, option D: $$H_4P_2O_5$$. This compound is sometimes called pyrophosphorous acid or isohypophosphoric acid. Oxidation state calculation: $$4 \times (+1) + 2 \times \text{(oxidation state of P)} + 5 \times (-2) = 0$$. Solving: $$4 + 2x - 10 = 0 \rightarrow 2x - 6 = 0 \rightarrow 2x = 6 \rightarrow x = +3$$. Each phosphorus has an oxidation state of +3. The structure of this compound is often represented as having a P-O-P bridge, similar to pyrophosphoric acid, but with different bonding due to the lower oxidation state. Specifically, it is $$(HO)_2P-O-P(OH)_2$$, where the phosphorus atoms are connected by an oxygen atom. There is no direct P-P bond; instead, it has a P-O-P linkage. Therefore, option D does not have a P-P bond.

After evaluating all options, only option B ($$H_4P_2O_6$$) has a direct P-P bond. Hence, the correct answer is Option B.