Compounds of Zinc - Complexes

Complexes

The most common structure of zinc complexes is tetrahedral which is clearly connected with the fact that the octet rule is obeyed in these cases. Nevertheless, octahedral complexes comparable to those of the transition elements are not rare. Zn2+ is a class A acceptor in the classification of Ahrland, Chatt and Davies, and so forms stronger complexes with the first-row donor atoms oxygen or nitrogen than with second-row sulfur or phosphorus. In terms of HSAB theory Zn2+ is a hard acid.

In aqueous solution an octahedral complex, 2+ is the predominant species. Aqueous solutions of zinc salts are mildly acidic because the aqua-ion is subject to hydrolysis with a pK_aof around 5, depending on conditions.

2+ + + H+

Hydrolysis explains why basic salts such as basic zinc acetate and basic zinc carbonate, Zn₃(OH)₄(CO₃).H₂O are easy to obtain. The reason for the hydrolysis is the high electrical charge density on the zinc ion, which pulls electrons away from an OH bond of a coordinated water molecule and releases a hydrogen ion. The polarizing effect of Zn2+ is part of the reason why zinc is found in enzymes such as carbonic anhydrase.

No fluoro complexes are known, but complexes with the other halides and with pseodohalides, - and 2- can be prepared. The case of the thiocyanate complex illustrates the class A character of the zinc ion as it is the N-bonded isomer, 2-in contrast to 2- which is S-bonded. Being a class-A acceptor does not preclude the formation of complexes with sulfur donors, as is shown by zinc dithiophosphate and the zinc finger complex (below).

The acetylacetonate complex, Zn(acac)₂ is interesting. As the ligand is bidentate a tetrahedral structure might be expected. However, the compound is in fact a trimer, Zn₃(acac)₆ in which each Zn ion is coordinated by five oxygen atoms in a distorted trigonal bipyramidal structure. Other 5-coordinate structures can be engineered by choosing ligands which have specific stereochemical requirements. For example, terpyridine, which is a tridentate ligand forms the complex . Another example would involve a tripodal ligand such as Tris(2-aminoethyl)amine. The compound zinc cyanide, Zn(CN)₂, is not 2-coordinate. It adopts a polymeric structure consisting of tetrahedral zinc centres linked by bridging cyanide ligands. The cyanide group shows head to tail disorder with any zinc atom having between 1 and 4 carbon atom neighbours and the remaining being nitrogen atoms. These two examples illustrate the difficulty of sometimes relating structure to stoichiometry.

A coordination number of 2 occurs in the amide Zn(NR1R2)₂ (R1=CMe₃, R2=SiMe₃); the ligand is so bulky that there is not enough space for more than two of them.