Coordination compounds tetrahedral complexes

As in olefin coordination compounds, acetylene complexes may have trigonal, square-planar, octahedral, trigonal-bipyramidal, and tetrahedral structures. In four-coordinate square-planar complexes, the acetylene molecule is perpendicular to the plane which comprises the remaining ligands and the central atom, while in trigonal and... 

What one characteristic of the metal in a coordination compound or complex ion primarily determines the geometry of the species Specifically, why is [Zn(CN)4] tetrahedral while [Ni(CN)4] is square planar, and why is [CuCb] trigonal bipyramidal while [MnCU] " is square pyramidal ... 

Simple nickel salts form ammine and other coordination complexes (see Coordination compounds). The octahedral configuration, in which nickel has a coordination number (CN) of 6, is the most common stmctural form. The square-planar and tetrahedral configurations (11), iu which nickel has a coordination number of 4, are less common. Generally, the latter group tends to be reddish brown. The 5-coordinate square pyramid configuration is also quite common. These materials tend to be darker in color and mostiy green (12). 

Cobalt exists in the +2 or +3 valence states for the majority of its compounds and complexes. A multitude of complexes of the cobalt(III) ion [22541-63-5] exist, but few stable simple salts are known (2). Werner s discovery and detailed studies of the cobalt(III) ammine complexes contributed gready to modem coordination chemistry and understanding of ligand exchange (3). Octahedral stereochemistries are the most common for the cobalt(II) ion [22541-53-3] as well as for cobalt(III). Cobalt(II) forms numerous simple compounds and complexes, most of which are octahedral or tetrahedral in nature cobalt(II) forms more tetrahedral complexes than other transition-metal ions. Because of the small stabiUty difference between octahedral and tetrahedral complexes of cobalt(II), both can be found in equiUbrium for a number of complexes. Typically, octahedral cobalt(II) salts and complexes are pink to brownish red most of the tetrahedral Co(II) species are blue (see Coordination compounds). 

In class I compounds (or complexes) the two sites are very different from each other and the valences are strongly localized. The properties of the complex are the sum of the properties of the constituting ions. The optical MMCT transitions are at high energy. The compounds are insulators. Here are some examples [60, 97]. In GaCl2, or Ga(I)[Ga(III)Cl4] there are dodecahedrally coordinated Ga(I) ions with Ga-Cl distances of 3.2-3.3 A and tetrahedrally coordinated Ga(III) ions with Ga-Cl distance 2.2 A. In [Co(III)(NH3)6]2- Co(II)Cl4)3 there are low-spin, octahedrally coordinated Co(III) ions and high-spin, tetrahedrally coordinated Co(II) ions. For our purpose this class is not the most interesting one. 

Square-planar zinc compounds predominate with these ligand types as would be predicted. This is in contrast to the prevalence of tetrahedral or distorted tetrahedral geometries for four-coordinate species that have been discussed thus far. Zinc porphyrin complexes are frequently used as building blocks in the formation of supramolecular structures. Zinc porphyrins can also act as electron donors and antenna in the formation of photoexcited states. Although the coordination of zinc to the porphyrin shows little variation, the properties of the zinc-coordinated compounds are extremely important and form the most extensively structurally characterized multidentate ligand class in the CSD. The examples presented here reflect only a fraction of these compounds but have been selected as recent and representative examples. Expanded ring porphyrins have also... 

The boron atom has tetrahedral coordination in pyridine complexes of 1,3,2,5-dioxaboraphosphorinane sulfides and selenides. These compounds exist as a mixture of conformers and stereoisomers [Eq. (86)]. In some cases it appeared possible to isolate three individual stereoisomers of one substance. The chemical shift in 31P NMR spectra was shown to be stereospecific. 

Tin has been observed in both valences +2 and 4 in which the —4 state represents the Xe configuration in which formally four electrons have been accepted (5s25p6). The sp3 hybridization to form tetrahedral covalent compounds can be discussed if one of the 5s electrons is promoted into the 5p orbital. When 5d orbitals are added to the hybridization, tin(IV) compounds with higher coordination numbers (of 5, 6, 7, 8) are formed. In general, tin(IV) is known to form compounds or complexes in which it adopts the coordination numbers 4 and 6, although compounds with numbers 2, 3, 5, 7 and 8 are... 

Square-planar complexes of platinum(II) and palladium(II) have been known for a long time the comparatively simple unit cells of compounds such as K2PdCl4, K2PtCl4, and Pd(NH3)4Cl2H20 led to early elucidation of the structures (257) and they all contain square-planar ions. The simple halides PdCl2 and Pt,Cl2 (71) consist of chains in which the metal is bonded from the corners of a square. Nickel chloride, on the other hand, has a layer lattice in which the nickel is octahedrally coordinated, and in the halide complexes the coordination is tetrahedral, as described in Section IV,B. 

Complexes containing the MoO + core are by far the most common in Mo chemistry. These complexes are usually six-coordinate although some four-coordinate compounds are known. The latter are tetrahedral or pseudotetrahedral and related to MoO -. Examples include Mo02S -, Mo02Se2-, Mo02Br2 and Mo02(R2NO)2. An example of a five-coordinate complex now exists.73... 

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