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The number of donor atoms from one ligand in coordination chemistry bonded to a particular metal centre. Unlike hapticity, it counts distinct donor sites rather than a contiguous bonded fragment.

A ligand in coordination chemistry is an atom, ion or molecule attached to a central atom through one or more donor sites. This use is distinct from a protein-binding ligand. A neutral CO ligand and an anionic chloride ligand can both donate two electrons in ionic electron counting.

A dissociative substitution begins with slow CO loss,followed by capture of the incoming ligand in coordination chemistry. A more electron-releasing $L$ increases metal-to-CO pi backbonding in the six-coordinate reactant, stabilizing it and strengthening its M–CO attachments. Removing CO sacrifices a useful pi acceptor from that electron-rich center. Thus the activation free energy for CO dissociation often rises and the substitution slows. Within an otherwise matched series, $k_d\propto \exp [-\Delta G^{‡}/(RT)]$ captures this sensitivity. The rate depends on the relative energies of reactant and dissociation transition state, so the trend is conditional: large changes in steric hindrance, geometry or mechanism can override the electronic effect.

Disconnect the two new bonds of the isoxazolidine as a nitrone [3+2] cycloaddition. Suitable achiral partners are $\mathrm{PhCH}={\mathrm{N}}^{+}(\mathrm{Ph}){\mathrm{O}}^{-}$ and the trans enone $(2-\mathrm{pyridyl}\mathrm{N}-\mathrm{oxide})\mathrm{CO}-\mathrm{CH}=\mathrm{CHPh}$.

A practical catalyst family is Cu(II) with an enantiopure bis(oxazoline) Ligand in coordination chemistry, for example a Cu(OTf)${}_2$–BOX complex. The enone can bind through its ketone oxygen and pyridine N-oxide oxygen, giving an O,O-chelated assembly. Chelation both activates the conjugated double bond and fixes its orientation in the chiral pocket. Highly enantioselective cycloadditions of this substrate class with Cu(II)–BOX complexes are experimentally established.^[ref]

The nitrone carbon bonds to the enone $\alpha$ carbon and the nitrone oxygen bonds to the $\beta$ carbon. Thus the carbonyl-bearing carbon lies between the two phenyl-bearing ring carbons, as in the target. The trans enone gives the required trans relationship of its original acyl and phenyl substituents. Selection between endo/exo dipole approaches gives the remaining relative stereochemistry; the sterically preferred chelated transition-state arrangement places the two ring phenyl substituents on the same face and the acyl group on the opposite face, matching the drawing.

One enantiomer of the BOX Ligand in coordination chemistry exposes one enone face; its mirror image reverses the product's absolute stereochemistry. The exact Ligand in coordination chemistry substituents, metal counterion and temperature require optimization. The proposal identifies an experimentally supported catalyst platform and its stereochemical control elements, rather than claiming that an arbitrary BOX complex must give a specified numerical enantiomeric excess.

The four shortest contacts are the three Cu–N bonds and Cu–OR bond. They define an approximately square equatorial set. The two remaining Cu–O contacts are longer and define a strongly elongated axial direction, as expected for the Jahn-Teller effect of $d^9$ Cu(II).

Choose $z$ along the long contacts and $x,y$ towards the short equatorial donors. The singly occupied, predominantly metal magnetic orbital is then $d_{x^2-y^2}$, with substantial Ligand in coordination chemistry admixture. All three pyrazine nitrogen donors lie in this equatorial set, so both ladder directions have strong metal–Ligand in coordination chemistry orbital overlap. Neighboring copper centers communicate through comparable pyrazine superexchange pathways. Virtual electron transfer between half-filled orbitals stabilizes the antiparallel arrangement, giving antiferromagnetic interactions in both directions with comparable magnitudes. The axial oxygen donors are much less effective exchange routes.

Let $L^{-}=\mathrm{P}{\mathrm{h}}_2\mathrm{P}(\mathrm{O})\mathrm{CH}=\mathrm{C}(\mathrm{tBu})\mathrm{N}{\mathrm{H}}^{-}$, with its O and N donors available for lithium coordination. A plausible symmetric tetramer has a Li${}_4$O${}_4$ cubane-type core, with alternating Li and phosphoryl O vertices and an additional enamide N donor attached to each Li. The O,N-chelating Ligand in coordination chemistry can be arranged equivalently around the aggregate, consistent with one ${}^7$Li environment. This is a connectivity model, not a uniquely reconstructed diffraction structure.

The initially formed nitrile-addition product has an acidic $\mathrm{C}{\mathrm{H}}_2$ group next to the phosphine oxide. Transfer of one of these protons to the nitrile-derived N gives a delocalized lithiated enamide, reinforced by O,N coordination and organolithium aggregation. The organic framework becomes $\mathrm{P}(\mathrm{O})-\mathrm{CH}=\mathrm{C}(\mathrm{tBu})-\mathrm{NHLi}$.

Assign ${\delta }_H=4.82$ to the vinylic CH and ${\delta }_H=3.25$ to NH. Each integrates to one proton, as required after hydrogen migration. Their singlet appearance does not mean that the original carbon has become a freely rotating, equivalent-proton methylene group.

Tetramethylethylenediamine, a bidentate nitrogen-donor Ligand in coordination chemistry.