Oxidative and thermal isomerization of phosphido-bridged molybdenum dicobalt clusters: x-ray crystal structures of the complexes [MoCo2(.mu.3-CC6H4Me-4)(.mu.-PPh2)(CO)6(.eta.-C5H5)] and [NEt4][MoCo2(.mu.3-CC6H4Me-4)(.mu.-PPh2)(CO)6(.eta.-C5H5)]