= Solution
The <frustrated Lewis pair> captures carbon dioxide cooperatively: the phosphine lone pair attacks its carbon while a <carbonyl> oxygen coordinates to aluminium. A five-membered ring closes along
$$
\boxed{\mathrm{P-C_\alpha-Al-O-C_{CO_2}-P}.}
$$
The new $\mathrm{C_{CO_2}}$ atom is bonded to P, double-bonded to one O and single-bonded to the O bound to Al. P becomes phosphonium-like and the O–Al end completes a zwitterionic carboxylate-like adduct; the original alkene remains intact.
\b[A direct P–C bond to the captured CO$_2$ carbon] accounts for its large $^{13}$C–$^{31}$P coupling. The carbonyl-like chemical shift is consistent with a captured, bent CO$_2$ unit. Reversibility follows from cleavage of the new P–C and Al–O contacts to regenerate the separated donor and acceptor.
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