= Solution
Replace the two chlorides on one ring phosphorus $P_1$ with the two ends of the diiron unit. \b[$D$ has a spiro $P_1\mathrm{Fe_2}$ triangle], with an Fe–Fe bond, a $P_1$–Fe bond to each metal and four terminal CO ligands on each Fe. The other two phosphorus atoms retain their $\mathrm{PCl_2}$ groups. Its composition is $\mathrm{N_3P_3Cl_4Fe_2(CO)_8}$. In neutral <electron counting>, each metal has $8_{\mathrm{Fe}}+8_{4\mathrm{CO}}+1_{\mathrm{Fe-P}}+1_{\mathrm{Fe-Fe}}=18$.
\b[$E$ has a triangular Fe$_3$ cluster, one bridging CO, and an adjacent ring N$\to$Fe bond.] The two original metals remain bonded to $P_1$ and to one another, and each bonds to the added third Fe. Each Fe bears three terminal CO ligands; one additional <bridging carbonyl ligand> spans the original Fe–Fe edge. A <nitrogen> directly adjacent to $P_1$ donates its <lone pair> to the third Fe. The formula is therefore $\mathrm{N_3P_3Cl_4Fe_3(CO)_{10}}$, satisfying $8+5-3=10$ retained CO ligands. The original diagram above gives all metal–metal and metal–ring connections.
The counts are an especially useful structural check. Each original Fe has $8+6_{3\mathrm{CO}}+1_{\mu\mathrm{CO}}+1_{\mathrm{P}}+2_{2\mathrm{Fe-Fe}}=18$. The added Fe has $8+6_{3\mathrm{CO}}+2_{\mathrm{N}}+2_{2\mathrm{Fe-Fe}}=18$. Omitting <nitrogen> coordination leaves the third Fe two <electrons> short; omitting the bridging CO leaves both original metals one <electron> short.
In $D$, the two untouched phosphorus sites are equivalent: an $A_2X$ <phosphorus-31 nuclear magnetic resonance> system gives their doublet with integral 2, and the unique $P_1$ triplet with integral 1. The two equivalent nuclei do not split each other. In $E$, coordination of one adjacent <nitrogen> distinguishes the two sides of the ring, producing three inequivalent phosphorus sites. Each couples to the other two and gives a doublet of doublets in the first-order limit; equal couplings or strong coupling could distort this simple pattern.
The high-frequency <infrared spectroscopy> bands belong to terminal carbonyl stretching modes. The extra low-frequency band is diagnostic of a strongly backbonded <bridging carbonyl ligand>. The number of IR bands counts allowed, possibly overlapping <normal modes>, not individual CO ligands. Formula, <18-electron rule>, NMR equivalence and IR together select this model; the historical experimental work independently established diiron spiro rings and triiron products containing both P–Fe and N–Fe bonds.<Iron-Containing Metallophosphazenes and Their Clusters Derived from Chlorophosphazenes and Organometallic Dianions by Harry R. Allcock and others>[1984, pp. 4966–4977, diiron compound 2, triiron compound 3, and conversion with <iron> carbonyls]{ref}
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