Metals four-coordinate

Most freqnently, cubane or other alnminum-nitrogen cage snpermolecnles are formed throngh additional dative bonds (making the metal four-coordinate). ... 

Gold Compounds. The chemistry of nonmetallic gold is predominandy that of Au(I) and Au(III) compounds and complexes. In the former, coordination number two and linear stereochemistry are most common. The majority of known Au(III) compounds are four coordinate and have square planar configurations. In both of these common oxidation states, gold preferably bonds to large polarizable ligands and, therefore, is termed a class b metal or soft acid. 

Iron hahdes react with haHde salts to afford anionic haHde complexes. Because kon(III) is a hard acid, the complexes that it forms are most stable with F and decrease ki both coordination number and stabiHty with heavier haHdes. No stable F complexes are known. [FeF (H20)] is the predominant kon fluoride species ki aqueous solution. The [FeF ] ion can be prepared ki fused salts. Whereas six-coordinate [FeCy is known, four-coordinate complexes are favored for chloride. Salts of tetrahedral [FeCfy] can be isolated if large cations such as tetraphenfyarsonium or tetra alkylammonium are used. [FeBrJ is known but is thermally unstable and disproportionates to kon(II) and bromine. Complex anions of kon(II) hahdes are less common. [FeCfy] has been obtained from FeCfy by reaction with alkaH metal chlorides ki the melt or with tetraethyl ammonium chloride ki deoxygenated ethanol. 

Boric oxide is an excellent Lewis acid. It coordinates even weak bases to form four-coordinate borate species. Reaction with sulfuric acid produces H[B(HSO 4] (18). At high (>1000° C) temperatures molten boric oxide dissolves most metal oxides and is thus very corrosive to metals in the presence of oxygen. 

Mononuclear Carbonyls. The lowest coordination number adopted by an isolable metal carbonyl is four. The only representative of this class is nickel carbonyl [13463-39-3] the first metal carbonyl isolated (15). The molecule possesses tetrahedral geometry as shown in stmcture (1). A few transient four-coordinate carbonyls, such as Fe(CO)4, have also been detected (16). 

Four-coordinate metal complexes may have either of two different geometries (Figure 15.3). The four bonds from the central metal may be directed toward the comers of a regular tetrahedron. This is what we would expect from VSEPR model (recall Chapter 7). Two common tetrahedral complexes are Zn(NH3)42+ and C0CI42. ... 

Geometry of four-coordinate complexes. Complexes in which the central metal has a coordination number of 4 may be tetrahedral or square planar. 

Coordination number The number of bonds from the central metal to the ligands in a complex ion, 409,412t four-coordinate metal complex, 413 six-coordinate metal complex, 413-414 Copper, 412 blister, 539... 

Four-coordinate metal complex, 413 Franklin, Rosalind, 628 Frasch, Herman, 558... 

Bidentate sulphoxides, such as 1,3-bis(methylsulphinyl)propane, 1,4-bis(methyl-sulphinyl)butane and 1,2-bis(ethylsulphinyl)ethane, have been synthesized and employed as ligands toward Mn2 +, Co2 +, Ni2 +, Cu2 + and Zn2 + 204. All metal ions are bound to the ligands via the oxygen of the sulphoxide groups and are six-coordinate, as shown in Scheme 20, with the exception of Cu2+, which is four-coordinate. 

The M(dioxime-BR2)2 class of complexes 120-122 with four-coordinate metal ions in a square-planar environment has attracted attention in view of possible columnar M M interactions that may result in interesting semiconducting properties in the solid state [182]. Therefore, a series of nickel(II) complexes... 

Derived from the German word meaning devil s copper, nickel is found predominantly in two isotopic forms, Ni (68% natural abundance) and Ni (26%). Ni exists in four oxidation states, 0, I, II, III, and IV. Ni(II), which is the most common oxidation state, has an ionic radius of —65 pm in the four-coordinate state and —80 pm in the octahedral low-spin state. The Ni(II) aqua cation exhibits a pAa of 9.9. It forms tight complexes with histidine (log Af = 15.9) and, among the first-row transition metals, is second only to Cu(II) in its ability to complex with acidic amino acids (log K( = 6-7 (7). Although Ni(II) is most common, the paramagnetic Ni(I) and Ni(III) states are also attainable. Ni(I), a (P metal, can exist only in the S = state, whereas Ni(lll), a cT ion, can be either S = or S =. ... 

Organometallic porphyrin complexes containing the late transition elements (from the nickel, copper, or zinc triads) are exceedingly few. In all of the known examples, either the porphyrin has been modified in some way or the metal is coordinated to fewer than four of the pyrrole nitrogens. For nickel, copper, and zinc the 4-2 oxidation state predominates, and the simple M"(Por) complexes are stable and resist oxidation or modification, thus on valence grounds alone it is easy to understand why there are few organometallic examples. The exceptions, which exist for nickel, palladium, and possibly zinc, are outlined below. Little evidence has been reported for stable organometallic porphyrin complexes of the other late transision elements. 

The distance between the diagonal nitrogen atoms is approximately 4 A [ 12]. The distortion induced by the incorporated metal is negUgible and planar four-coordinated metallocomplexes are produced. Therefore, the metal-nitrogen dis-... 

Neutral, diamagnetic M(//) complexes with the formula Ni(R2 itc)2 are found (2). Stmctural studies for a great variety of R groups (88-93) showed a square planar coordination geometry as is expected for a four-coordinated metal in a tf configuration. 

Four-coordinate complexes exhibit lower isomer shifts than six-coordinate compounds. Metal-ligand bonds are shorter and more covalent if the coordination number is smaller because of less steric hindrance and less overlap with antibonding 2g orbitals in the case of four as compared to six bonds. 

Two other publications on Ir (73 keV) Mossbauer spectroscopy of complex compounds of iridium have been reported by Williams et al. [291,292]. In their first article [291], they have shown that the additive model suggested by Bancroft [293] does not account satisfactorily for the partial isomer shift and partial quadrupole splitting in Ir(lll) complexes. Their second article [292] deals with four-coordinate formally lr(l) complexes. They observed, like other authors on similar low-valent iridium compounds [284], only small differences in the isomer shifts, which they attributed to the interaction between the metal-ligand bonds leading to compensation effects. Their interpretation is supported by changes in the NMR data of the phosphine ligands and in the frequency of the carbonyl stretching vibration. 

The second category is the transition metal ions, all of which in Fig. 1 are six-coordinate with the exception of Pt2+ and Pd2+, which are square-planar four-coordinate (6-9). Their labilities are strongly influenced by the electronic occupancy of their d orbitals. This is illustrated by the divalent first-row transition metal ions, which should exhibit similar labilities to Zn2+ on the basis of their rM instead, however, their labilities encompass seven orders of magnitude. On a similar basis, the trivalent first-row transition metal ions might be expected to be of similar lability to Ga3+, but instead they exhibit a lability variation of 11 orders of magnitude, with Cr3 being at the... 

It should be noted that four-coordinate Cu(II) complexes do not typically favor tetrahedral coordination, so that distortions for this metal center may be more pronounced upon substitution at the 5-position. See Cotton, F. A. Wilkinson, G. Advanced Inorganic Chemistry (5th Ed.) Wiley-Interscience New York, 1988. 

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