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Coordination numbers and oxidation states

Determine the name of the following complexes and give the coordination number of the central metal atom.

(a) Na 2 [PtCl 6 ]

(b) K 3 [Fe(C 2 O 4 ) 3 ]

(c) [Co(NH 3 ) 5 Cl]Cl 2


(a) There are two Na + ions, so the coordination sphere has a negative two charge: [PtCl 6 ] 2− . There are six anionic chloride ligands, so −2 = −6 + x , and the oxidation state of the platinum is 4+. The name of the complex is sodium hexachloroplatinate(IV), and the coordination number is six. (b) The coordination sphere has a charge of 3− (based on the potassium) and the oxalate ligands each have a charge of 2−, so the metal oxidation state is given by −3 = −6 + x , and this is an iron(III) complex. The name is potassium trisoxalatoferrate(III) (note that tris is used instead of tri because the ligand name starts with a vowel). Because oxalate is a bidentate ligand, this complex has a coordination number of six. (c) In this example, the coordination sphere has a cationic charge of 2+. The NH 3 ligand is neutral, but the chloro ligand has a charge of 1−. The oxidation state is found by +2 = −1 + x and is 3+, so the complex is pentaaminechlorocobalt(III) chloride and the coordination number is six.

Check your learning

The complex potassium dicyanoargenate(I) is used to make antiseptic compounds. Give the formula and coordination number.


K[Ag(CN) 2 ]; coordination number two

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The structures of complexes

The most common structures of the complexes in coordination compounds are octahedral, tetrahedral, and square planar (see [link] ). For transition metal complexes, the coordination number determines the geometry around the central metal ion. [link] compares coordination numbers to the molecular geometry:

This figure contains three diagrams in black and white. The first is labeled, “Pentagonal Bipyramid.” It has 10 isosceles triangle faces, five at the top, joined at a vertex, making a point projecting upward at the top of the figure, and five below, joined at a vertex, making a point projecting downward, at the base of the figure. The second is labeled, “Square Antiprism.” It has flat upper and lower square surfaces and sides made up of 8 equilateral triangles. The sides alternate in orientation between pointing up and pointing down. The third diagram is labeled, “Dodecahedron.” It has twelve isosceles triangle faces.
These are geometries of some complexes with coordination numbers of seven and eight.
Coordination Numbers and Molecular Geometry
Coordination Number Molecular Geometry Example
2 linear [Ag(NH 3 ) 2 ] +
3 trigonal planar [Cu(CN) 3 ] 2−
4 tetrahedral( d 0 or d 10 ), low oxidation states for M [Ni(CO) 4 ]
4 square planar ( d 8 ) [NiCl 4 ] 2−
5 trigonal bipyramidal [CoCl 5 ] 2−
5 square pyramidal [VO(CN) 4 ] 2−
6 octahedral [CoCl 6 ] 3−
7 pentagonal bipyramid [ZrF 7 ] 3−
8 square antiprism [ReF 8 ] 2−
8 dodecahedron [Mo(CN) 8 ] 4−
9 and above more complicated structures [ReH 9 ] 2−

Unlike main group atoms in which both the bonding and nonbonding electrons determine the molecular shape, the nonbonding d -electrons do not change the arrangement of the ligands. Octahedral complexes have a coordination number of six, and the six donor atoms are arranged at the corners of an octahedron around the central metal ion. Examples are shown in [link] . The chloride and nitrate anions in [Co(H 2 O) 6 ]Cl 2 and [Cr(en) 3 ](NO 3 ) 3 , and the potassium cations in K 2 [PtCl 6 ], are outside the brackets and are not bonded to the metal ion.

Three structures are shown. In a, a structure is shown with a central C o atom. From the C o atom, line segments indicate bonds to H subscript 2 O molecules above and below the structure. Above and to both the right and left, dashed wedges indicate bonds to two H subscript 2 O molecules. Similarly, solid wedges below to both the right and left indicate bonds to two more H subscript 2 O molecules. Each bond in this structure is directed toward the O atom in each H subscript 2 O structure. This structure is enclosed in brackets. Outside the brackets to the right is the superscript 2 plus. Following this to the right appears 2 C l superscript negative sign. In b, a central C r atom has six N H subscript 2 groups attached with single bonds. These bonds are indicated with line segments extending above and below, dashed wedges extending up and to the left and right, and solid wedges extending below and to the left and right. The bonds to these groups are all directed toward the N atoms. The N H subscript 2 groups are each connected to C atoms of C H subscript 2 groups extending outward from the central C o atom. These C H subscript 2 groups are connected in pairs with bonds indicated by short line segments. This entire structure is enclosed in brackets. Outside the brackets to the right is the superscript 3 plus. Following to the right is 3 N O subscript 3 superscript negative sign. In c, 2 K superscript plus is followed by a structure in brackets. Inside the brackets is a central P t atom. From the P t atom, line segments indicate bonds to C l atoms above and below the structure. Above and to both the right and left, dashed wedges indicate bonds to C l atoms. Similarly, solid wedges below to both the right and left indicate bonds to two more C l atoms. This structure is enclosed in brackets. Outside the brackets to the right is the superscript 2 negative sign.
Many transition metal complexes adopt octahedral geometries, with six donor atoms forming bond angles of 90° about the central atom with adjacent ligands. Note that only ligands within the coordination sphere affect the geometry around the metal center.

For transition metals with a coordination number of four, two different geometries are possible: tetrahedral or square planar. Unlike main group elements, where these geometries can be predicted from VSEPR theory, a more detailed discussion of transition metal orbitals (discussed in the section on Crystal Field Theory) is required to predict which complexes will be tetrahedral and which will be square planar. In tetrahedral complexes such as [Zn(CN) 4 ] 2− ( [link] ), each of the ligand pairs forms an angle of 109.5°. In square planar complexes, such as [Pt(NH 3 ) 2 Cl 2 ], each ligand has two other ligands at 90° angles (called the cis positions) and one additional ligand at an 180° angle, in the trans position.

Questions & Answers

An atom or group of atoms bearing anelectrical charge such as the sodium and chlorine atoms in a salt solution.
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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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