Coordination number four

Coordination compounds of vanadium are mainly based on six coordination, in which vanadium has a pseudooctahedral stmcture. Coordination number four is typical of many vanadates. Coordination numbers five and eight also are known for vanadium compounds, but numbers less than four have not been reported. The coordination chemistry of vanadium has been extensively reviewed (8—12) (see Coordination compounds). 

The reactivity of chromium(V) and chromium(IV) species is uncertain since there are no reliable thermodynamic data, and not much can be said at present about the structure of these species. With respect to the latter some hints can be obtained from the fact that the changeover from chromium(V) to chromium(IV) or vice versa in all cases was found to be rate determining, which seems to correlate well with the conclusion of Tong and King d that Cr(VI) and Cr(V) have coordination number four, whereas Cr(IV) and Cr(III) have six. 

Equations 12, 14 and 17 require the presence of H20. Thus H20 plays an important role in promoting the catalytic activity, but can also cause deactivation. Catalysis will be more efficient when all the reactions directly involved in the catalytic cycle are faster than the side reactions subtracting active species. Deactivation is related to the requirement of the palladium centre to have a vacant coordination site to ensure high catalytic activity. However, palladium tends to achieve the usual coordination number four, for example through dimerisation. Dimerisation/deactivation can be prevented by coordination of labile ligands, like H20, which acts also as an efficient hydride source. Also deprotonation leads to dimerisation/deactivation an acid can prevent it. 

Coordination Number Four. These complex ions either have (a) a square planar geometry with the four ligands at each corner, such that the metal ion lies in-plane at the center, or (b) a tetrahedral complex where the centrally located metal ion has four ligands arranged as the hydrogen atoms in methane. Square planar complexes of... 

As the first step of the catalyzed reaction series, the aluminum hydroxide suspended in the reaction liquid forms an aluminum complex with the acid anion, aluminum having the coordination number four in this complex. 

Coordination number four is observed either in chelate complexes, such as (43) and (44), or in bridged complexes, such as [I Au(,u-Ph2PC=CPPh2)3AuI].375-378... 

The geometric shape of the fixed positions occupied by ligating atoms is the coordination polyhedron. The common coordination polyhedra are the tetrahedron, square plane, trigonal bipyramid, square pyramid, octahedron and trigonal prism, for coordination numbers four, five and six. Distortions from the idealized coordination polyhedra occur due to size requirements of ligands and electronic. effects such as the Jahn-Teller effect. Nevertheless, it is common practice to describe the overall geometry of the ligand environment as an idealized coordination polyhedron. 

For his coordination number four bond arrangements are possible, in Table XI is given the... 

Compound 43 is a polycycle that can be derived from a cube having one corner missing (Fig. 4). The distortion from the point symmetry 3m (C3v) is small, the pseudo-threefold axis passing through the Si—C bond. All atoms of the ring have the coordination number four in a tetrahedrally distorted environment ((55). The Al-N distances are almost the same [2.02(1) A], whereas the Al-C distances are slightly shorter in the direction of the threefold axis [1.95(1) A] than those that are almost perpendicular to it [1.98(1) A]. The chelates 44-48 have been shown by cryoscopy in benzene to be monomeric. Their ring structure can be deduced from NMR spectra (<5<5) (see also Section III,D). 

These are the most extensively studied class of poly(pyrazolyl)borate compounds for platinum and palladium, and rank among the most numerous and varied. The literature includes examples of M(II) and M(IV) centers in cationic, neutral, and anionic complexes, with all possible coordination numbers four to six represented moreover, many of these complexes are readily interconverted. This section will therefore be organized according to oxidation state, as the most logical approach. 

If the iminosilene is too bulky to dimerize, it often exhibits unusual ways of attaining the coordination number four for the silicon atom. For example, bis(silyl)amino-iminosilenes cyclize with simultaneous electrophilic 1,3-migration of the silyl group from the amine to the more negatively charged imine nitrogen3 (Scheme 7). 

The most important chelate types are illustrated in Figure 2.7. Compounds in column A display a metal ion with coordination number four. The coordination number six is represented by the chelates of column B and C. Rows a, b, and c cover bi-, tri- and tetraftmctional or -dentate ligands, respectively. The types Ab and the types of column B contain additional monofunctional ligands to complete the coordination numbers of four and six, respectively. 

According to reaction (a) mixed substituted platinum(0)complexes of the hitherto unknown type Ptl L1 and PtLL 2 (where L and L are phosphorus ligands) can be isolated and characterized by nmr spectroscopy. Instead of Ph3P other R3P derivatives and Ph3As can be used. Complex D is very labile. IR data give evidence for a a-coordination of the CS2ligands. With the double ylide RR N-P(S)=NR (reaction (c) ) the four-membered chelate complex E with pentavalent phosphorus of coordination number four is formed. Compound F is an example of the well characterized alkyne complexes (here with the phospha(III)azene ligand L). 

The existence of H atoms of a higher coordination number (four, five or six) is an intriguing problem. It was not until very recently that examples of such species were unequivocally established in molecular complexes246,247). In the solid state literature, however, high-coordination H atoms are well-known entities there are many examples of binary metal hydrides in which H atoms are known to occupy tetrahedral or octahedral sites in a metal lattice8). 

Both forms (110) and (111), give rise to the same equilibrium in solution. Some solvents favor the pentacoordinated form, while others favor the tetracoordinated form. As a result, there is a significant solvent dependence of the 81P n.m.r. shift, since the observed shift corresponds to the average value of the signals of the two types of phosphorus nuclei, S =ca. + 40 p.p.m. (vs. H8P04) for coordination number five, and ca. —40 p.p.m. for coordination number four, in this particular type of compound. The establishment of the equilibrium is fast in the time scale of n.m.r. 

Although in the above paragraph the silicates in particular were discussed, we meet with a corresponding, almost equally great, multiplicity with a number of other elements which can have a coordination number four with oxygen. 

Just as the three-dimensional type of structure of SiOz occurs in the felspars as a consequence of isomorphous substitution, phosphates such as A1P04 and BPO4, in which A1 and B both have a coordination number four, also have structures which correspond to those of the SiOa modifications, quartz and cristo-balite, respectively. 

The complexes of nickel and palladium, with a coordination number four mentioned above, are interesting because here a plane configuration occurs, which is inexplicable on the basis of the electrostatic theory the latter leads to a tetrahedral model. The diamagnetism also points to the formation of electron pairs in the bonding. The possibility of the occurrence of cis and trans isomers, as in the octahedral complexes, is in agreement with a plane structure, but incompatible with a tetrahedral arrangement. 

In aqueous solution metal ions are surrounded by water molecules.36 In some cases, such as the alkali ions, they are weakly bound, whereas in others, such as [Cr(H20)6]3+ or [Rh(H20)6]3+, they may be firmly bound and exchange with solvent water molecules only very slowly for the lanthanides water exchange decreases with decreasing ionic radii.37 Coordination numbers vary extensively, depending on the size of the metal ion. For example, coordination number four is common for lithium, six is most frequently found for transition metal ions 38 higher coordination numbers are not unusual for larger ions, e.g., Bi3+ can form Bi(H20)93+.39... 

The square-planar coirfiguration of platinum(n) was proven by the same methods bnt with a different chronology and a difference in interpretation of the resolntions. Three symmetrical configurations for the coordination number four are theoretically possible (see Figure 6). 

The tetrahedron, octahedron, and icosahedron are the three polyhedra formed from equivalent triangular faces, and these rigid, close packed structures dominate the stereochemistries for coordination numbers four, six, and twelve, respectively. 

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