Chemistry of phosphido-bridged dimolybdenum complexes. Part 3. Reinvestigation of the reaction between [Mo2(η-C5H5)2(CO)6] and P2Ph4; X-ray structures of [Mo2(η-C5H5)2(µ-PPh2)2(CO)2], [Mo2(η-C5H5)2(µ-PPh2)2(µ-CO)], and trans-[Mo2(η-C5H5)2(µ-PPh2)2O(CO)]