= Solution
\b[<ammonia> and <cyclohexylamine> are <sigma-donor ligands>; <chloride> and <acetate> are sigma- and <pi-donor ligands>.] The <nitrogen> <lone pair> of each <amine> points toward the <metal> and forms the donor interaction. These saturated <amines> have no low-lying conjugated acceptor system that would make them conventional <pi-acceptor ligands>.
<chloride> uses a <lone pair> for <sigma donation> and has additional occupied $p$ orbitals perpendicular to the bond that can donate with pi symmetry. An oxygen-bound <acetate> similarly has occupied oxygen-centred orbitals available for sigma and <pi donation>, although its carboxylate <resonance in chemistry> changes their energies and distribution. Neither should be classified as a pi acceptor merely because its bonding can be drawn with multiple bonds or <resonance in chemistry> contributors. \b[None of these four is a standard sigma-donor/<pi-acceptor ligand>.]
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