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1,2-Bis(dimethylphosphino)ethane has the connectivity $\mathrm{M}{\mathrm{e}}_2\mathrm{PC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{PM}{\mathrm{e}}_2$. Chelation through both phosphorus donors contributes four electrons in ionic electron counting.

A carbon-bound group $-\mathrm{C}(\mathrm{O})\mathrm{R}$, formally anionic and normally a two-electron donor in ionic electron counting. It differs from a neutral carbonyl ligand $\mathrm{CO}$.

An allyl ligand can bind as ${\eta }^3$ ${\mathrm{C}}_3{\mathrm{H}}_5^{-}$, donating four electrons in ionic electron counting. Protonation can release an alkene and make coordination sites available.

A coordination ligand assigned a negative formal charge in ionic electron counting. Distinguish the assigned charge from its electron donation and from the actual distributed charge in the complex.

A boryl ligand attaches through boron as an X-type ligand. In ionic electron counting it is formally anionic and contributes two electrons. It is distinct from a neutral borylene and from attachment to an entire borazine pi face.

A bridging carbonyl ligand spans more than one metal. A conventional two-metal bridging CO contributes one electron to each metal in neutral electron counting. Its stretching band often lies below terminal-carbonyl bands because of stronger C–O weakening, but assignment should also use composition and structural evidence.

A bridging ligand attaches to two or more metal centers. The symbol $\mu$ records bridging; its electron donation must be apportioned consistently between the metals in electron counting.

A bromide ligand is coordinated $\mathrm{B}{\mathrm{r}}^{-}$, conventionally an anionic two-electron donor in ionic electron counting.

A chloride ligand is coordinated $\mathrm{C}{\mathrm{l}}^{-}$. It is an anionic two-electron donor in ionic electron counting, with occupied orbitals that can contribute pi donation.

A bound $\mathrm{C}{\mathrm{N}}^{-}$ ligand, usually C-bound in the strongly pi-accepting coordination mode used in elementary spectrochemical comparisons. It is an anionic two-electron donor in ionic electron counting.

The cyclopentadienyl ligand is based on ${\mathrm{C}}_5{\mathrm{H}}_5^{-}$. In its common ${\eta }^5$ mode it binds through five ring carbons and donates six electrons in ionic electron counting. Ring substitution can create distinct NMR environments.

Electron counting records metal valence electrons plus ligand donations using a declared convention. In ionic counting, subtract metal oxidation state from its group number, then add donor electrons; in neutral counting, combine neutral metal and ligand fragments consistently. An Fe–Fe single bond contributes one electron to the count at each metal.

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 complex in which a metal is bound between two cyclopentadienyl-type rings, commonly with hapticity ${\eta }^5$ at each ring. In ionic electron counting, each unsubstituted cyclopentadienyl anion donates six electrons.

A metal-metal bond joins two metal centers directly through overlap of their atomic orbitals. A single bond contributes one electron to the local count at each metal in conventional electron counting. Bridging ligands can accompany direct bonding, but a short bridged separation alone does not establish its bond order.

An electron-pair interaction represented as donation from a metal-base fragment into an orbital of a metal-acid fragment. electron counting must avoid adding a new pair twice to an already occupied donor orbital.

A metal-bound methyl group, formally CH3-minus in ionic electron counting; it is commonly an X-type ligand.

Primary study in Organometallics, volume 19, pages 4536–4543. Its abstract identifies the open carbaalane with two $C=CHPh$ groups and reports their hydroalumination during conversion to a capped aluminium cube. This establishes that the unsaturated groups are real, but the abstract alone does not establish which electron counting convention the examination intended.

A coordinated oxide, formally ${\mathrm{O}}^{2-}$. A terminal $\mathrm{M}=\mathrm{O}$ unit is conventionally counted as an $X_2$ ligand giving four electrons in ionic electron counting. Additional oxygen-to-metal pi donation is an orbital-bonding issue and must not be silently mixed with another counting convention.

triphenylphosphine is a good sigma donor and a substantially poorer pi acceptor than CO. Replacing CO by this phosphine ligand increases electron density at chromium and reduces competition among carbonyl ligands for pi backbonding. Backbonding into each remaining CO therefore strengthens, their internal bond force constants fall, and both remaining carbonyl stretching frequencies are expected to shift downward relative to the parent tricarbonyl.

The two remaining carbonyl ligands have coupled approximately symmetric and antisymmetric stretches; both are generally infrared active in this geometry. One should not treat the spectral change as simply deleting one of the original two bands. The substituted complex still has $6+6+2(2)+2=18$ valence electrons. No numerical frequency shift follows from electron counting alone.