Capped Trigonal Prismatic Molecular Geometry¶

A seven-coordinate molecular shape consisting of a central atom, six ligands forming a trigonal prism, and a seventh capping one rectangular face, ideally with C2v symmetry.

Version: v1 · 2026-09-28 · History
Domain-specific #: 8337
Domain group: Natural Sciences
Origin domain: Chemistry & Materials Science
Subdomains: Coordination Chemistry, Stereochemistry → Chemistry & Materials Science
Aliases: Monocapped Trigonal Prismatic Molecular Geometry, TPRS-7

Core Idea¶

The geometry begins with a trigonal prism: two triangular ligand faces joined by three rectangular faces. A seventh ligand caps one rectangular face around the central atom, creating an idealized C2v seven-coordinate polyhedron.

Real structures need not display exact symmetry. Classification depends on coordination connectivity, face topology, angles, and relative distortion from alternative heptacoordinate ideals such as the pentagonal bipyramid and capped octahedron.

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The Capped Tent Shape

Picture a tent-shaped block made from two triangles joined by three flat rectangle sides, with an atom in the middle and six atoms at the corners. Now stick a seventh atom out over the middle of one rectangle side, like a bump. Some molecules arrange their atoms in this seven-point shape.

Prism With a Cap

Some molecules have one atom in the middle with seven atoms attached around it. One shape they can make starts with a trigonal prism, which has two triangle faces joined by three rectangle faces, with an attached atom at each of the six corners. Then a seventh atom sits over the middle of one rectangle face, like a cap. Real molecules are often a little bent, so scientists compare them to this perfect shape and to other seven-atom shapes to decide which fits best.

Rectangle-Capped Trigonal Prism

Capped trigonal prismatic geometry is an idealized arrangement for a central atom with seven ligands. Start with a trigonal prism, which has two triangular faces of ligands joined by three rectangular faces. A seventh ligand caps one of the rectangular faces, giving an ideal shape with C2v symmetry. Real molecules rarely show the exact symmetry, so chemists classify a structure by comparing its connections, faces, and angles against this shape and the other seven-coordinate ideals, the pentagonal bipyramid and the capped octahedron. Capping a rectangular face is what separates it from the capped octahedron, where the extra ligand caps a triangular face.

The capped trigonal prism is one of the idealized polyhedra used for seven-coordinate centers. Its base is a trigonal prism of six ligands, two triangular faces joined by three rectangular faces, and the seventh ligand caps one rectangular face, yielding an ideal polyhedron of C2v symmetry. Because actual complexes seldom have exact symmetry, assigning this geometry is a matter of classification rather than an assertion of ideal angles: one examines coordination connectivity, face topology, bond angles, and the relative distortion from alternative heptacoordinate ideals, namely the pentagonal bipyramid and the capped octahedron. The face that is capped is diagnostic, rectangular here versus triangular in the capped octahedron.

Structural Signature¶

Sig role-phrases:

Central atom — Anchors the coordination polyhedron. It is center. Counterfactual: Seven unrelated atoms do not define molecular geometry.
Six prismatic ligands — Form two aligned triangular faces and three rectangular sides. It is base polyhedron. Counterfactual: Without the prism skeleton the capped label changes.
Seventh cap ligand — Occupies a site above one rectangular prism face. It is distinguishing vertex. Counterfactual: Capping a triangular face gives another topology.
Seven coordination bonds — Establish direct nearest-neighbor coordination. It is connectivity. Counterfactual: Close nonbonded atoms should not be counted automatically.
Ideal C2v symmetry — Provides the reference geometry and equivalent-site pattern. It is idealization. Counterfactual: Real complexes may distort while retaining topology.
Competing seven-coordinate shapes — Supply alternatives assessed by angles and continuous-shape measures. It is classification frame. Counterfactual: Coordination number alone cannot decide the shape.

What It Is Not¶

It is not any seven-coordinate complex.
It is not a pentagonal bipyramid.
It is not a capped octahedron.
Exact C2v symmetry is an ideal, not mandatory in every distorted instance.
Closest near-miss. A capped octahedron also has seven vertices but starts from an octahedral six-ligand scaffold; face topology and angular pattern distinguish it.

Scope of Application¶

Coordination chemistry. Classifies heptacoordinate complexes.
Structural chemistry. Compares ligand arrangements.
Crystallography. Interprets measured atomic positions.
Molecular modeling. Tracks distortion and shape interconversion.

Clarity¶

Report central atom, directly coordinated ligands, geometry determination, prism and cap assignment, bond lengths and angles, symmetry, disorder, uncertainty, and comparison with competing continuous-shape measures.

Manages Complexity¶

A simple seven-neighbor count branches into several polyhedral topologies whose distinction persists under moderate distortion but can become continuous and ambiguous.

Abstract Reasoning¶

Establish direct coordination number.
Map six ligands to a trigonal-prism scaffold.
Locate the cap over a rectangular face.
Quantify distortion and symmetry.
Compare all common seven-coordinate alternatives.

Knowledge Transfer¶

The label transfers between compounds only at the local coordination-polyhedron level; ligand identity, bonding, dynamics, and reactivity cannot be inferred from shape alone.

Examples¶

Canonical¶

In a heptafluorometalate ion, six fluorides outline a trigonal prism and the seventh lies over a rectangular face around the metal center.

Mapped back: center → metal; prism → six F ligands; cap → seventh F; coordination → 7; reference → C2v.

Applied / In Practice¶

Seven ligands with five in an equatorial pentagon and two axial occupy a pentagonal bipyramid, not a capped trigonal prism.

Mapped back: coordination → 7; skeleton → pentagonal bipyramid; cap → absent.

Structural Tensions¶

T1 — Ideal Symmetry versus Molecular Distortion. The named C2v polyhedron is a reference while ligand and crystal effects lower observed symmetry.

Diagnostic: Does connectivity/topology still favor the reference under a shape measure?

T2 — Discrete Label versus Continuous Geometry. Heptacoordinate shapes can interconvert continuously and ambiguous cases resist a categorical name.

Diagnostic: How far is the observed structure from each competing ideal?

Structural–Framed Character¶

Capped Trigonal-Prismatic Geometry is structural as a seven-vertex coordination polyhedron and chemically framed by bonding and distortion.

Structural Core vs. Domain Accent¶

The core is central carrier, six-vertex prism, cap site, and topology. Chemistry supplies ligand bonding, observed structures, symmetry, and energetic alternatives.

This entry presupposes Classification.

Approved root. No reviewed parent entails this exact coordination geometry.
Related — trigonal prism, pentagonal bipyramid, capped octahedron, coordination number, and molecular symmetry. They provide scaffold and alternatives.

Relationships to Other Abstractions¶

Current abstraction Capped Trigonal Prismatic Molecular Geometry Domain-specific

Parents (1) — more general patterns this builds on

Capped Trigonal Prismatic Molecular Geometry presupposes Classification Prime

Capped Trigonal Prismatic Molecular Geometry presupposes Classification because the geometry category depends on classifying seven-coordinate arrangements by an ideal capped trigonal prism.

Hierarchy path (1) — routes to 1 parentless root

Capped Trigonal Prismatic Molecular Geometry → Classification

Neighborhood in Abstraction Space¶

Capped Trigonal Prismatic Molecular Geometry sits in a moderately populated region (41^st percentile for distinctiveness): it has near-neighbors but no dense thicket of look-alikes.

Family — Molecular Structure & Interaction Models (20 abstractions)

Nearest neighbors

Metal–Ligand Multiple Bond — 0.90
Octahedral Molecular Geometry — 0.88
Constraint (Computational Chemistry) — 0.87
Prism graph — 0.87
Flory–Huggins Solution Theory — 0.87

Computed from structural-signature embeddings · 2026-10-08

Not to Be Confused With¶

Pentagonal bipyramid. Tell: Uses a five-member equator and two axes.
Capped octahedron. Tell: Adds a cap to an octahedral scaffold.
Bicapped trigonal prism. Tell: Has eight rather than seven coordinated ligands.
Molecular conformation. Tell: Usually concerns torsional arrangement within connectivity rather than central coordination polyhedra.

References¶

Frozen Wikipedia discovery revision: https://en.wikipedia.org/wiki/Capped_trigonal_prismatic_molecular_geometry (revision 1360664304).

The frozen Wikipedia revision is discovery provenance. The retained source set was reviewed for identity, formal or operational relation, and scope. The encyclopedia's structural synthesis is bounded to those claims; a thin authority surface is recorded as a nonblocking source-strengthening repair rather than concealed.