= Solution
In the <chromium> complex, Cr(0) is $d^6$ and receives substantial donation from four <carbonyl ligands>. Its occupied orbitals can give <pi backbonding> into the boron-rich acceptor orbitals of the B$_2$N$_2$ ring, while the <nitrogen>-rich occupied ring orbitals donate to the <metal>. This favors binding across the ring face, approximately $\eta^4$, with all four ring atoms participating. A nearly planar ring maintains the p-orbital alignment needed for that interaction and accounts for comparable Cr–B and Cr–N distances. A four-<electron> ring donor also gives the useful count $6_d+8_{\mathrm{CO}}+4_{\mathrm{ring}}=18$.
Ti(IV) in TiCl$_4$ is $d^0$ and strongly <electron> deficient. It is a good acceptor for the two <nitrogen> <lone pairs>, but has no filled $d$ set for comparable backbonding into the boron acceptors. \b[<chromium> therefore favors a face-bound pi complex, while <titanium> favors N,N sigma coordination.] Donation makes the <nitrogen> environments more nearly tetrahedral and permits the <2,4-Diazadiboretidine> ring to pucker. The borons are not directly bound in the same way, explaining $d(\mathrm{Ti}\cdots\mathrm B)>d(\mathrm{Ti{-}N})$. The distinction follows from <electron> availability, rather than simply calling every short <metal>–ring contact a separate ordinary <sigma bond>.
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