= Solution
A reasonable product is \b[sodium facial <trimetaphosphate>–<molybdenum> tricarbonyl], with the <anion>
$$
\boxed{\mathrm{[Mo(CO)_3(\kappa^3O,O',O''\!\text{-}P_3O_9)]^{3-}},\qquad \text{gas}=3\mathrm{CO}.}
$$
One terminal O from each <phosphorus> binds Mo; the three O donors occupy one octahedral face, while the three carbonyls occupy the opposite face. The ring bridges remain P–O–P bridges and are not replaced by Mo–P bonds. Mo remains Mo(0), $d^6$, and $6+3(2)+3(2)=18$ <electrons>. Overall,
$$
\mathrm{Na_3P_3O_9+Mo(CO)_6\longrightarrow Na_3[Mo(CO)_3(P_3O_9)]+3CO}.
$$
<acetonitrile> can transiently replace CO during heating, but it is not required in the final six-coordinate proposal. The same <anion> has been structurally characterized with a different counterion: that primary study independently supports the facial O$_3$/CO$_3$ connectivity, without establishing identical solubility or yield for the sodium salt.<Synthesis and characterization of the trimetaphosphate molybdenum tricarbonyl anion [(P3O9)Mo(CO)3]3- as its tris(bis(triphenylphosphine)iminium) salt by Christopher R. Clough and others>[pp. 195–196, Scheme 2 and Fig. 1; §4.3 for Mo(CO)6 in refluxing acetonitrile]{ref}
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