Nomenclature and Coordination number of complexes Introduction

Nomenclature in coordination chemistry has undergone several modifications. While several of the old and trivial names are still in use for some complexes, International Union of Pure and Applied Chemistry (IUPAC) provides the modern and most acceptable system of naming complexes. The IUPAC naming system put into consideration the nature of the complex whether cationic or anionic, the number and nature of the ligands in addition to the bonding mode (bridging and non-bridging mode) accepted via the ligand. It is worthy to note that isomeric complexes can as well be identified from their nomenclature. The IUPAC naming system has many features that build it unique and extensively acceptable. 

IUPAC system of naming metal complexes

Many coordination compounds have very compound structures due to the nature of their ligands. This might provide increase to their name being long and complicated. Through IUPAC system of naming, the complication in naming isn't eliminated but the naming procedure is organized in a way that it can be simply comprehended via anybody who understands the rules behind it.  Rules for Naming Complexes

The subsequent rules must be examined in naming complexes:

Rule 1: For negative complex the positive counter ion (cation) is named first if present, followed via the complex. This is the common way of naming simple salts as well for instance NaCl Sodium Chloride. For positive complex, the complex is named 1^st before naming the negative counter ion (anion)

Instances: 

Complex                                Name

K₂[Ni(CN)₄]                Potassium tetracyanonickelate(II)

[Co(NH₃)₆]Cl₃             Hexamminecobalt (III) chloride

Rule 2: In the coordination sphere, the ligands are named before the metal in alphabetical order of the ligands' names, however the metal ion is written 1^st in formula. The coordination sphere is enclosed in square brackets in the formula.

Examples: 

Complex                                         Name

[Cu(NH₃)₄(H₂O)₂]SO₄         Tetraamminediaquacopper (II) sulphate

[Co(NH₃)₅Cl]Cl₂                  Pentaamminechlorocobalt (III) chloride

Rule 3: The number of ligands in a complex is specified via one or both of the prefixes listed below. If the name of the ligand encloses the 1^st prefix or is complicated, it is enclosed in parenthesis and the 2^nd set of prefix is used. Instances below will illustrate how such prefixes are utilized.

Complex                                                   Name

[Co(H₂NCH₂CH₂NH₂)2Cl₂]Cl          Dichlorobis (ethylenediammine) cobalt (III) chloride

[Fe(NH₄C₅-C₅H₄N)₃]Cl₂                   tris (bipyridine)iron(III) chloride

Rule 4: Anionic ligands for instance Br-, Cl-, F-, SO₄^2- etc are named with a suffix "o" while neutral ligands retained their usual name except H₂O called aqua, NH₃ called ammine. Note also that the name of the metal in an anionic complex ends with a suffix "ate"

  Ligand                                Name                      Metal                             Name in anionic compex

Br- (Bromide)                      Bromo                      Copper                            Cuprate

SO₄^2-(Sulphate)                  Sulphato                   Iron                                  Ferrate

C₂O₄^2- (Oxalate)                Oxalato                     Nickel                              Nickelate

CN- (Cyanide)                     Cyano                      Platinum                           Platinate

-SCN- (Thiocyanate)            Thiocyanato-S           Titanium                           Titanate

-SCN-(Isothiocyanate)         Thiocyanato-N            Gold                                 Aurate

Complex                             Name

K4[Fe(SCN)₆]          Potassium hexathiocyanatoferrate(II)

[Ni(C₂O₄)₂]              2-bis(oxalato)nickelate(II)

[Ti(C₂O₄)₂]               bis(oxalato)titanium(IV)

K[Au(CN)₂]             Potassium dicyanoaurate(i)  

Rule 5: For complexes through geometric isomers these isomers are specified using prefix 'cis' and "trans". For instance [PtCl₂(NH₃)₂] 

Fig: geometric isomers

Rule 6: A bridging ligand (a ligand joining two or more metal centers) is specified through a prefix "μ" and a subscript to demonstrate the number of the metal centers bridged via the ligand. Instances

Fig: bridging ligand

Coordination number of metal complexes

The coordination number of a metal ion in a compound is the number of ligand donor atoms to that the metal is straightly linked. It is determined by counting the number of the donor atoms or site directly connected to the metal. Coordination number varies from 1 to 8, though the two extremes are rare. The structure of a ligand strongly depends on the coordination number as it determines the number of spatial orientation possible in any specified complex. The diverse coordination numbers will be considered and the possible structures discussed.

Coordination Number1: complexes having coordination number one are rare and little is known of their chemistry. Coordination Number 2: The complex with coordination number 2 well established are silver complexes for instance [Ag(NH3)₂]⁺ where the electronic configuration of Ag⁺ is d¹⁰. The hybridization of Ag+ is sp through bond angle 180⁰and the possible shape is linear structure. Other examples are [Hg(CN)₂], [Au(CN)₂] etc.

 [H₃N→Ag←NH₃]+. The arrows point from the donors to the acceptor.

Coordination Number 3: Complexes through coordination number 3 are few. Metals by d¹⁰ configuration are generally found through this coordination number. The hybridization is sp² bond angle 120⁰, which provides increase to trigonal planar structure. [HgI₃]^-, [AuCl(PPh₃)₂], [Au(PPh₃)₃]⁺ and so on. Coordination number three is favored via bulky ligand that can induce stearic hindrance and prevent further coordination. 

Coordination Number 4: This is a ordinary coordination system that can provide increase to two different geometries i.e. Tetrahedral and square planar, depending on the orbitals of the central metal that received the donor pairs. Divalent ions such as Zn²⁺, Cd²⁺, Hg²⁺and Cu⁺ with d10 electronic configuration and zero crystal field stabilization energy will provide rise to tetrahedral complexes by sp³ hybridization with bond angle 1090 examples [CdCl₄]^2-, [Zn(OH)₄]^2-and [Hg(Br)₄]^2-. Likewise, few metals through d⁰ and d⁵ are recognized to form tetrahedral complexes MnO^4-, MnCl₄^2-, TiCl₄. Metals by other d-configurations have extremely bounded number of tetrahedral complexes e.g. [NiCl₄2-] and [Ni(CO)₄]with d⁸ configuration.

Fig: tetrahedral complexes

Square Planar Complexes are common with metals d8 electronic configuration. Instances are [PtCl₂(NH₃)₂], [Ni(CN)₄]^2- and PdCl₄^2-. The hybridization in such complexes is dsp² by bond angle of 90⁰.

Fig: Square Planar Complexes  

Coordination number 5: two possible structures through coordination number five are square based pyramidal and trigonal bipyramidal through the metal having sp³d hybridization. In square planar, dx²-y² orbital in the metal will receive one of the donated pairs while in trigonal bipyramidal, dz² orbital of the metal will receive one of the contributed pairs. The energy difference between the two configurations is small therefore they are interconvertible. Instances are [Fe(CO)₅] and [Cu(bipy)₂I]+, [VO(acac)₂] and [VO(SCN)₄]^2-.

Coordination Number 6: This is the most general coordination number through two possible geometries for instance octahedral and trigonal prismatic. Octahedral is the most common with metal center having sp³d² or d²sp³ hybridization through bond angle 90⁰. Instances [Cu(H₂O)₆]²⁺, [Co(en)₃]³⁺and [Fe(CN)₆]^3-. 

Fig: octahedral and trigonal prismatic

Higher coordination numbers are possible but not general for instance coordination seven [ZrF₇]^3- and[H_fF₇]^3-, coordination number eight [ZrF₈]4- and [Mo(CN)₈]^4-.

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