Heteroleptic compounds the molecular structures of fluorinated methanes and methylsilanes

8 Heteroleptic compounds the molecular structures of fluorinated methanes and methylsilanes 

Up till now we have described compounds of the Group 2,12,13, and 14 elements in which all the ligands have been equal. Such compounds are referred to as homoleptic. Compounds with different ligands like CH2F2 are referred to as heteroleptic. 

The C-F bond distances in the fluorinated methanes are found to decrease monotonically with increasing number of F atoms attached to the C atom, from 138.3(1) pm in CH3F to 131.5(1) pm in CF4. The difference between one member of the series and the next is, however, small. This is particularly true in regard to the last two members of the series, CHF3 and CF4. Can we be sure that the difference is real The best way to answer this question is to calculate the estimated standard deviation of the difference between them. If we have two parameters Pa and determined with estimated standard deviations equal to a A and ctr respectively, the e.s.d. of the difference between them is given by 

The difference between the two bond distances is 1.3 pm, while its e.s.d. is 0.35 pm. Since the difference is more than three times larger than the e.s.d., the probability that it is in fact zero is less than 1% and may consequently be ignored. Comparison of the other C-F bond distance differences along the series with their e.s.d.s, indicate that they are all real. 

The four C-H bond distances in Table 13.5 differ by less than 1 pm. All the differences are smaller than three times their e.s.d. The data in the table thus provide no evidence for variation of C-H bond distances with the number of F atoms. 

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