Here for example: xyz coordinate for Ir-atom that is included in Ir-complex. The first question: what does the bold number in the parentheses mean?
Ir1 Ir 0.18461(3) 0.28575(3) 0.160923(15) 0.04728(11) Uani 1 1 d . . .
The second question: I observed similar parentheses beside bond length in some papers, for example Ir-C 2.179 (10), what does it mean also?
It is referred to as the standard uncertainty (su) or estimated standard deviation (esd) for that particular bond or coordinate. For example, 1.542 (2) Angstroms can be read as 1.542 +/- 0.002 Angstroms
The way it is often used is in determining whether two bonds are really (or significantly) different. So, for example, if a C-C bond in two similar compounds has been measured by X-ray crystallographic techniques as being 1.541 (2) in one, and 1.596 (4) in the other, a question might be "are those bonds really very different?" (and if so, is that difference caused by the differences in the two compounds, an extra methyl or amino or whatever, or maybe how the molecules pack).
The convention is that two bonds are said to be 'significantly different' if the differences in their lengths is greater than three times the weighted standard deviation (wsd):
wsd=((su1)2+(su2)2)1/2
Which for the above example is:
wsd = ((0.002)2+(0.004)2)1/2 = 0.0045
Therefore: 3 x wsd = 0.0134
The difference between 1.541 and 1.596 is 0.055
And: 0.055 > 0.0134
So the two bonds are 'significantly' different (and you might reasonably make a note of the difference when writing a paper).
Dear Dr. Morad
The question should be "what does the bold number in the parentheses mean?"
It is something to do with the accuracy limit of values in question
do not care about this, this is just a matter of uncertainty in the measurements
It is referred to as the standard uncertainty (su) or estimated standard deviation (esd) for that particular bond or coordinate. For example, 1.542 (2) Angstroms can be read as 1.542 +/- 0.002 Angstroms
The way it is often used is in determining whether two bonds are really (or significantly) different. So, for example, if a C-C bond in two similar compounds has been measured by X-ray crystallographic techniques as being 1.541 (2) in one, and 1.596 (4) in the other, a question might be "are those bonds really very different?" (and if so, is that difference caused by the differences in the two compounds, an extra methyl or amino or whatever, or maybe how the molecules pack).
The convention is that two bonds are said to be 'significantly different' if the differences in their lengths is greater than three times the weighted standard deviation (wsd):
wsd=((su1)2+(su2)2)1/2
Which for the above example is:
wsd = ((0.002)2+(0.004)2)1/2 = 0.0045
Therefore: 3 x wsd = 0.0134
The difference between 1.541 and 1.596 is 0.055
And: 0.055 > 0.0134
So the two bonds are 'significantly' different (and you might reasonably make a note of the difference when writing a paper).
These are standard uncertainties/ estimated standard deviations for the bonds.
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