3.8: Ionic Bonding: Symbolizing and Naming Transition Metal Cations

Learning Objectives

• Symbolize and name transition metal cations.

Section 3.5 presented a pattern-based "charge shortcut" for predicting the charges of the ions that result when main group elements ionize.  Recall that all main group elements found within the same column in the periodic table have the same number of valence electrons, and, consequently, will form ions with the same charge.  However, this charge pattern only applies to main group elements.  Transition metals, which are the elements found in Groups 3 - 12, do not have readily-apparent reactivity patterns or trends, with one exception:  Because all of the elements in the "B-Block" of the periodic table are metals, they achieve stable electron configurations by losing electrons and, therefore, ionize to form cations. 

The remaining aspects of transition metal ionizations are relatively complex.  The electron configurations of transition metals utilize d orbitals, in addition to the s and p orbitals that were discussed in Section 2.5.  As a result, transition metals are able lose inner shell electrons, in addition to their valence electrons.  This atypical behavior allows transition metals to lose more than three total electrons, which is energetically-unfavorable for main group elements.  Additionally, most transition metals are able to achieve stable electron configurations through multiple ionization pathways.  For example, iron (Fe) can lose two electrons to produce an ion symbolized as Fe⁺².  However, iron is equally likely to ionize by losing three electrons, resulting in the formation of an Fe⁺³ cation.  As there is no consistency for the ionization of individual transition metals, developing a column-based charge trend that relates these elements to one another is impossible. 

Naming transition metal cations is also complicated by their ability ionize through multiple pathways.  In Section 3.3, cations were named by including the word "ion" after the name of the element that was ionized.  Therefore, the ion symbolized as "Fe⁺²" should be named as an "iron ion."  However, the ion symbolized as "Fe⁺³" should also be named as an "iron ion."  As stated in Section 3.7, no two chemical formulas should share a common chemical name.  Since the phrase "iron ion," could refer to either Fe⁺² or Fe⁺³, this name is ambiguous and, therefore, is not chemically-correct. 

There are two methods for distinguishing the names of transition metal cations.

1. The simpler, more modern approach, uses the IUPAC, or International Union of Pure and Applied Chemistry, system.  This method indicates the charge of a transition metal  cation by writing a corresponding Roman numeral in parentheses after the element name, but before the word "ion," in an ion name.  The charges that can result upon the ionization of transition metals, as well as their corresponding Roman numerals, are shown below in Table \(\PageIndex{1}\).  Therefore, using the IUPAC system, Fe⁺² is called the "iron (II) ion," and Fe⁺³ is named as the "iron (III) ion."  Incorporating a Roman numeral into the name of a transition metal cation removes its ambiguity and results in a chemically-acceptable name.
    

   Table \(\PageIndex{1}\):  Possible Transition Metal Charges and Their IUPAC Roman Numeral Designations

   Charge	Roman Numeral
   +1	(I)
   +2	(II)
   +3	(III)
   +4	(IV)
   +5	(V)
   +6	(VI)
   +7	(VII)

2. The second system, called the common system uses two suffixes, "-ous" and "-ic," to distinguish the names of transition metal elements.  The "-ous" suffix indicates the cation with the lesser relative cation charge, and an "-ic" suffix is used to refer to a cation with a larger charge.  In most cases, the stem, or main portion, of the elemental name used in the common system is typically derived from the element's Latin name.  For example, the Latin stem for "iron" is "ferr-."  Therefore, using the common system, Fe⁺² is called the "ferrous ion," and Fe⁺³ is named as the "ferric ion."
   
   While these names are distinctive and, therefore, are considered chemically-correct, the common system has two significant limitations.
    
   1. The common system only utilizes two suffixes and, consequently, can only distinguish the names of transition metals that have exactly two ionization pathways.  Vanadium (V) can readily lose two, three, four, or five electrons to form V⁺², V⁺³, V⁺⁴, and V⁺⁵, respectively.  The common system cannot be distinguish these ions, as no suffix is available for the ions formed through the third and fourth ionization pathways.  In contrast, the IUPAC system can be employed to unambiguously name these ions, as its Roman numerals are directly tied to the charges of the ions.  Therefore, V⁺², V⁺³, V⁺⁴, and V⁺⁵ are named as "vanadium (II) ion," "vanadium (III) ion," "vanadium (IV) ion," and "vanadium (V) ion," respectively, using the IUPAC system.
       
   2. Additionally, the suffixes in the common system indicate the relative charges of two transition metals, but do not directly specify their exact charges.  For example, the Latin stem for "copper" (Cu) is "cupr-."  Therefore, using the common system, Cu⁺¹ is called the "cuprous ion," and Cu⁺² is named as the "cupric ion."  The common names for both iron and copper use the "-ous" ending to refer to the cation with the lesser relative charge.  However, iron's lesser charge is +2, but copper's smaller charge is +1.  Finally, when iron ionizes to form a cation with a +2 charge, its common name ends in "-ous."  When copper ionizes to form Cu⁺², its common name ends in "-ic."  These inconsistencies often result in confusion, which, again, can be completely avoided by using the IUPAC system.

Despite these limitations, the common system is still fairly prevalent, particularly in the health sciences.  Table \(\PageIndex{2}\) provides the IUPAC and common names of several transition metal cations.

Table \(\PageIndex{2}\):  IUPAC and Common Names of Selected Transition Metal Cations

Element	Stem	Charge	IUPAC Name	Common Name
chromium	chrom-	+2	chromium (II) ion	chromous ion
		+3	chromium (III) ion	chromic ion
iron	ferr-	+2	iron (II) ion	ferrous ion
		+3	iron (III) ion	ferric ion
cobalt	cobalt-	+2	cobalt (II) ion	cobaltous ion
		+3	cobalt (III) ion	cobaltic ion
nickel	nickel-	+2	nickel (II) ion	nickelous ion
		+3	nickel (III) ion	nickelic ion
copper	cupr-	+1	copper (I) ion	cuprous ion
		+2	copper (II) ion	cupric ion
tin	stann-	+2	tin (II) ion	stannous ion
		+4	tin (IV) ion	stannic ion
gold	aur-	+1	gold (I) ion	aurous ion
		+3	gold (III) ion	auric ion
mercury	mercur-	+1	mercury (I) ion	mercurous ion
		+2	mercury (II) ion	mercuric ion
lead	plumb-	+2	lead (II) ion	plumbous ion
		+4	lead (IV) ion	plumbic ion

Finally, remember that these systems are only used for elements, like the transition metals, that can ionize to form more than one cation.  Recall that the main group elements can only follow one ionization pathway.  Therefore, the incorporation of a Roman numeral into a main group cation name is unnecessary, as the charge of its corresponding cation is unambiguous.  For example, in Section 3.3, the ion symbolized as "Ca⁺²" was correctly named as a "calcium ion," not a "calcium (II) ion," because calcium will only ionize to form a cation with a +2 charge.

Example \(\PageIndex{1}\)

Use the IUPAC system to write an unambiguous ion name for Au⁺³.

Solution

The IUPAC system specifies the charge of a transition metal cation by writing a corresponding Roman numeral in parentheses after the element name, but before the word "ion," in an ion name.  The element symbolized as "Au" is named "gold."  The superscript in the ion symbol indicates that this ion of gold has a charge of +3, which is represented by a Roman numeral (III) in the IUPAC system.  Therefore, the IUPAC name of Au⁺³ is the gold (III) ion.

Exercise \(\PageIndex{1}\)

Use the IUPAC system to write an unambiguous ion name for each of the following ions.

1. Cr⁺²
2. Pb⁺⁴

Answer a

The IUPAC system specifies the charge of a transition metal cation by writing a corresponding Roman numeral in parentheses after the element name, but before the word "ion," in an ion name.  The element symbolized as "Cr" is named "chromium."  The superscript in the ion symbol indicates that this ion of chromium has a charge of +2, which is represented by a Roman numeral (II) in the IUPAC system.  Therefore, the IUPAC name of Cr⁺² is the chromium (II) ion.

Answer b

The element symbolized as "Pb" is named "lead."  This ion of lead has a charge of +4, which is represented by a Roman numeral (IV) in the IUPAC system.  Therefore, the IUPAC name of Pb⁺⁴ is the lead (IV) ion.

Example \(\PageIndex{2}\)

Write the ion symbol for a nickel (II) ion.

Solution

The Roman numeral within the ion name is an IUPAC system designation that specifies the charge of a transition metal cation.  The element named "nickel" is symbolized as "Ni".  The Roman numeral (II) in the ion name indicates that this ion of nickel has a charge of +2, and this information is incorporated into the ion symbol as a superscript on the corresponding elemental symbol.  Therefore, the ion named nickel (II) ion is symbolized as Ni⁺².

Exercise \(\PageIndex{2}\)

Write the ion symbol for each of the following ions.

1. Mercury (II) ion
2. Cobalt (III) ion

Answer a

The Roman numeral within the ion name is an IUPAC system designation that specifies the charge of a transition metal cation.  The element named "mercury " is symbolized as "Hg".  The Roman numeral (II) in the ion name indicates that this ion of mercury has a charge of +2, and this information is incorporated into the ion symbol as a superscript on the corresponding elemental symbol.  Therefore, the ion named mercury (II) ion is symbolized as Hg⁺².

Answer b

The element named "cobalt" is symbolized as "Co".  The Roman numeral (III) in the ion name indicates that this ion has a charge of +3.  Therefore, the ion named cobalt (III) ion is symbolized as Co⁺³.