Crystal structure ammonia complexes

Crystal structures of dinickel complexes with 0,0-bridging and 0,N-bridging (O-methylated) carbamate have been reported.2074,20 (874) produces one equivalent of ammonia upon heating in methanol/water solution.2082... 

The compounds M(NH3)2Ni(CN)4 (M = Zn or Cd), which consist of two-dimensional polymeric sheets of tetracyanonickelate ions bridged by coordinating diamminemetal(II) cations, function as host lattices for clathration of small aromatic molecules such as thiophene, furan, pyrrole or pyridine IR studies indicate the presence of hydrogen bonding between the host lattice ammonia and the aromatic guest molecules.132,133 A crystal structure determination of the related clathrate Cd(en)Ni(CN)4(pyrrole)2 has been reported.134 Similarly, the complex Cd(py)2Ni(CN)4 consists of polymeric [Cd—Ni(CN)4] layers held together by Cd-bound pyridine.135... 

Further condensation of the dinuclear species may occur either by continuing the edge-to-edge condensation, which leads to linear trinuclear structures such as 4a and 4b, or by condensation of a third chromium ion to each of the two chromium(III) centers in structure 2, yielding the cyclic structure 5. The (NH3)4Co(OH)2Co(CN)2(OH)2-Co(NH3)43+ cation (49) possesses structure 4a, and there is good evidence that the trinuclear amine complex Cr3(en)5(OH)45+ has the same structure (42). The linear trinuclear structure 4b has not been observed in crystal structures. The cyclic, trinuclear structure 5 has been observed in crystal structures of chromium(III) complexes with ammonia, tacn, and bispicam (40, 50, 51). 

The tetranuclear complex Co (OH)2Co(en)2 36 + was first prepared by Werner (6) and has subsequently been the subject of several studies (53, 54, 57,156,245 247). Its ammonia analog has already been mentioned in Section IV,A. As mentioned in Section II,B, the en tetranuclear species may exist in eight optically isomeric forms. All eight isomers are formed during the oxidation reaction and have been separated by column chromatography (57). The yields of the isomers decrease in the order A(AAA) > A(AAA) > A(AAA) > A(AAA). The X-ray crystal structures of... 

Metalloenzymes pose a particular problem to both experimentalists and modelers. Crystal structures of metalloenzymes typically reveal only one state of the active site and the state obtained frequently depends on the crystallization conditions. In some cases, states probably not relevant to any aspect of the mechanism have been obtained, and in many cases it may not be possible to obtain states of interest, simply because they are too reactive. This is where molecular modeling can make a unique contribution and a recent study of urease provides a good example of what can be achieved119 1. A molecular mechanics study of urease as crystallized revealed that a water molecule was probably missing from the refined crystal structure. A conformational search of the active site geometry with the natural substrate, urea, bound led to the determination of a consensus binding model[I91]. Clearly, the urea complex cannot be crystallized because of the rate at which the urea is broken down to ammonia and, therefore, modeling approaches such as this represent a real contribution to the study of metalloenzymes. 

K4[Ni(CN)4] also belongs to this group it is produced from potassium in liquid ammonia from normal K2[Ni(GN)4]. The former can perhaps be regarded as a complex, built up from a nickel atom and four CN ions, thus with 10 + 4 X 2 electrons around the central atom in a krypton configuration, in place of 8 + 4 X 2 as in the ordinary complex of Ni2+. One would have to expect tetrahedral sp3 bonding as opposed to the normal dsp2 bonding in K2[Ni(CN)4] the crystal structure is, however, not known. 

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