Sodium formate bond lengths

We have described the different types of primary bonds, but how do these bonds form in the first place What is it that causes a sodium ion and a chloride ion to form a compound, and what is it that prevents the nuclei from fusing together to form one element These questions all lead us to the topics of intermolecular forces and bond formation. We know that atoms approach each other only to a certain distance, and then, if they form a compound, they will maintain some equilibrium separation distance known as the bond length. Hence, we expect that there is some attractive energy that brings them together, as well as some repulsive energy that keeps the atoms a certain distance apart. 

The predicted configuration for the carboxylate ion group is that with the angle O—C=0 equal to 125°16 and each C—O bond length equal to 1.27 A. (The single-bond and double-bond lengths are 1.41 A and 1.21 A, respectively.) The experimental values lie close to these for example, for the formates of sodium, calcium, strontium, and barium the average values are 125.5° 1° and 1.25 0.01 A, respectively.81... 

Experimental evidence for the ecjuivalence of the two carboxylate oxygens comes from X-ray crystallographic studies on sodium formate. Both carbon-oxygen bonds are 127 pm in length, midway between the C=0 bond (120 pmi and C—O bond (134 pm) of formic acid. An electrostatic potential map of the formate ion also shows how the negative charge (red) is dispersed equally over both o.xygens. 

Problem 18.5. How do you account for the fact that the three carbon-oxygen bonds in CaCOs have the same length, and that this length (1.31 A) is greater than that found in sodium formate ... 

Carboxylate ions. Ions R-COO of carboxylic acids are symmetrical with two equal C-0 bonds (1 26 A) and O-C-0 bond angle in the range 125-130°. In sodium formate, for example, the ion is reported to have C-0, 1-27A, and O-C-0, 124 (4)°, and bond lengths close to 1-25A have been found in alkaline-earth formates. 

Physical data support the importance of resonance in carboxylate ions. In formic acid molecules, the two carbon-oxygen bonds have different lengths. But in sodium formate, both carbon-oxygen bonds of the formate ion are identical, and their length is between those of normal double and single carbon-oxygen bonds. 

The reaction of NaN03 with Na20 at 570 K leads to the formation of Na3N04 (sodium orthonitrate), and K3NO4 may be prepared similarly. X-ray diffraction data confirm that the [N04] ion is tetrahedral with N—O bond lengths of 139 pm, consistent with single bond character. 

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