Coordination Number 6

Coordination number 6 means a central metal ion is bonded to six ligands, producing an octahedral geometry with 90 degree bond angles that splits the metal's d-orbitals into the lower-energy t2g set and higher-energy eg set.

What is Coordination Number 6?

Coordination number 6 tells you how many coordinate covalent bonds a central metal ion forms with surrounding ligands, in this case six. Each ligand acts as a Lewis base and donates an electron pair to the metal (the Lewis acid), and six of these bonds almost always arrange themselves into an octahedral shape with metal-ligand angles of 90 degrees.

This is the most common coordination number for transition metal complexes, which is why you see it constantly in Gen Chem II. Classic examples include $[Fe(H_2O)_6]^{3+}$ and $[Co(NH_3)_6]^{3+}$. The octahedral arrangement isn't just about shape, it sets up how the metal's five d-orbitals interact with the ligands, which then controls the complex's color, magnetism, and stability.

Why Coordination Number 6 matters in General Chemistry II

Coordination number 6 shows up in two places in your coordination chemistry unit: naming and drawing complexes (Topic 8.1) and predicting their properties with crystal field theory (Topic 8.3). Once you know a complex is six-coordinate and octahedral, you can apply crystal field theory directly. The six ligands point along the x, y, and z axes, so the d-orbitals that point at them ($d_{z^2}$ and $d_{x^2-y^2}$, the eg set) get pushed higher in energy than the ones that point between them ($d_{xy}$, $d_{xz}$, $d_{yz}$, the t2g set). That energy gap is what lets you predict whether a complex is high-spin or low-spin, whether it's paramagnetic or diamagnetic, and what color it absorbs.

How Coordination Number 6 connects across the course

Octahedral Geometry (Unit 8)

Coordination number 6 and octahedral geometry go hand in hand. Six ligands spacing themselves as far apart as possible around one metal center is what produces the octahedral shape with its 90 degree angles.

Crystal Field Theory (Unit 8)

Crystal field theory takes the octahedral arrangement of six ligands and predicts how they split the d-orbitals into t2g and eg sets. Without knowing the coordination number is 6, you can't choose the right splitting pattern.

Coordination Number 4 (Unit 8)

Coordination number 4 gives tetrahedral or square planar complexes instead of octahedral, and the d-orbital splitting flips or changes magnitude. Comparing 4 and 6 is the fastest way to see how geometry controls electronic structure.

Color of Complexes (Unit 8)

The t2g to eg energy gap in a six-coordinate octahedral complex equals the energy of light it absorbs. That gap, set up by the six ligands, is why $[Cu(H_2O)_6]^{2+}$ looks blue and other octahedral complexes show their characteristic colors.

Is Coordination Number 6 on the General Chemistry II exam?

On problem sets and exams, you'll be given a complex like $[Co(NH_3)_6]^{3+}$ and asked to identify the coordination number, state the geometry (octahedral), and draw the t2g/eg splitting diagram. Expect to count ligands, assign the metal's oxidation state, fill in d-electrons as high-spin or low-spin, and then predict magnetism (paramagnetic vs diamagnetic) or color. Multiple-choice questions often hinge on connecting "six ligands" to "octahedral" to "two d-orbital sets," so practice running that chain quickly.

Coordination Number 6 vs Coordination Number 4

Coordination number 6 gives an octahedral complex where the eg orbitals sit above the t2g orbitals. Coordination number 4 gives a tetrahedral complex (where the splitting is inverted and smaller) or a square planar one (with a more complex four-level pattern). The number of ligands decides which splitting diagram you use, so don't default to octahedral splitting for a four-coordinate complex.

Key things to remember about Coordination Number 6

• Coordination number 6 means six ligands are bonded to one central metal ion, almost always giving an octahedral geometry with 90 degree bond angles.

• Octahedral six-coordinate complexes split the d-orbitals into a lower-energy t2g set (three orbitals) and a higher-energy eg set (two orbitals).

• Six-coordinate is the most common arrangement for transition metals, seen in complexes like [Fe(H2O)6]3+ and [Co(NH3)6]3+.

• The size of the t2g to eg energy gap determines whether the complex is high-spin or low-spin and what color it appears.

• To analyze any six-coordinate complex, find the metal's oxidation state, count its d-electrons, then fill the t2g and eg orbitals to predict magnetism and color.

Frequently asked questions about Coordination Number 6

What does coordination number 6 mean in chemistry?

It means a central metal ion has formed six coordinate covalent bonds to surrounding ligands. Six ligands arrange into an octahedral shape with 90 degree metal-ligand angles, like in [Co(NH3)6]3+.

Is coordination number 6 always octahedral?

Yes, in nearly every case you'll meet in Gen Chem II. Six ligands repel each other into the positions farthest apart, which is the octahedral arrangement, so you can safely treat coordination number 6 as octahedral.

How is coordination number 6 different from coordination number 4?

Six ligands give octahedral geometry with eg above t2g, while four ligands give either tetrahedral (inverted, smaller splitting) or square planar geometry. The number of ligands decides which crystal field splitting diagram you apply.

Why do six-coordinate complexes split into t2g and eg orbitals?

The six ligands point along the x, y, and z axes. The eg orbitals (dz2 and dx2-y2) point directly at the ligands and get pushed to higher energy, while the t2g orbitals point between them and stay lower.

Why is coordination number 6 so common for transition metals?

Transition metals have available d-orbitals and the right size to accommodate six ligands without crowding, forming stable octahedral complexes. That stability is why ions like Fe3+ and Co3+ favor six ligands.