File:HS BH3 summary.PNG

2019-05-17T12:28:29Z

Hs5017: Hs5017 uploaded a new version of File:HS BH3 summary.PNG

File:HS BH3 summary.PNG

2019-05-17T12:27:37Z

Hs5017: Hs5017 uploaded a new version of File:HS BH3 summary.PNG

InorganicGaussian 01327311

2019-05-17T12:26:09Z

Hs5017: /* Optimisation */

=BH3 Molecule=

== Optimisation ==
'''B3LYP/6-31G(d,p) level'''

[[File:HS BH3 summarytable.PNG]]

<pre>
Item Value Threshold Converged?
Maximum Force 0.000004 0.000450 YES
RMS Force 0.000003 0.000300 YES
Maximum Displacement 0.000017 0.001800 YES
RMS Displacement 0.000011 0.001200 YES
</pre>

Frequency analysis log file [[Media:HARUKA BH3 FREQ 631G DP EDITED.LOG]]

<pre>
Low frequencies --- -1.1800 -1.0028 -0.0055 4.1927 11.0182 11.0637
Low frequencies --- 1162.9912 1213.1792 1213.1819
</pre>

<jmol><jmolApplet>
<title>test molecule</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>HARUKA BH3 FREQ 631G DP EDITED.LOG</uploadedFileContents>
</jmolApplet></jmol>

[File:MODIAGRAM OF BH3]

==Comparison of MO diagrams: Gaussian vs LCAO==

Energy splitting between 2a’ and 1e’ is expected to be larger than the splitting between 1e’ and 1a2’' (ΔE2a’ - 1e’ > ΔE1e’ - 1a2'') from observation of MO diagram produced from LCAO (Linear Combination of Atomic Orbitals). However energy calculations of MOs on Gaussian indicates the opposite: 1e' and 1a2'' has a larger splitting of 0.28474 au compared to 0.16175 au between 2a’ and 1e’.
The AO or FO that is closer to the bonding/anti-bonding MO of concern has a dominant contribution to the MO. Therefore, we would expect from the diagram that the dominant contribution to the 3a1’' MO is from the B2s that is closer to it energetically than the a1’ H3 FO. However, it can be observed from the MO picture taken from gaussian that the lobes are larger on the H atoms (green) rather than B (red).

These two differences given above indicate the flaw of the LCAO theory that the energy positioning of the AOs and FOs are merely qualitative. The energetic similarity or dissimilarity between FOs determine the magnitude of energy splitting between the bonding and antibonding MOs, as well as the dominant contribution to the MO. Therefore qualitative energy positioning of the FOs likewise implies only a vague understanding of these MO properties. Calculations must be done to accurately determine the energy positioning of the orbital.

==IR analysis==

[[File:HS BH3 IR.PNG|600px|centre|thumb|IR spectrum of a BH3 molecule]]

Why do we only see 4 peaks when there are 6 vibrational modes?
From the table above, we see 6 vibrational modes as expected from the 3N-6 rule. However, only 4 peaks are seen on the IR as modes 2 and 3, and 4 and 5 are degenerate which leaves 5 distinguishable vibrational modes. Mode 4 (2582.29 cm-1) is non-existent from the IR spectrum as the symmetric B-H stretch does not result in a dipole change as seen from the displacement vectors.

[[File:HS BH3 IR table.PNG|centre|thumb|Vibrational frequencies and intensities of a BH3 molecule]]

[[File:HS BH3 mode4.PNG|centre|thumb|IR inactive symmetric BH stretching mode]]

=NH3=

== Optimisation ==
'''B3LYP/6-31G(d,p) level'''

[[File:HS NH3 summary.PNG]]

<pre>
Item Value Threshold Converged?
Maximum Force 0.000006 0.000450 YES
RMS Force 0.000004 0.000300 YES
Maximum Displacement 0.000016 0.001800 YES
RMS Displacement 0.000011 0.001200 YES
</pre>

Frequency analysis log file [[Media:HS NH3 FREQOPT.LOG]]

<pre>
Low frequencies --- -0.0137 -0.0027 0.0007 7.0783 8.0932 8.0937
Low frequencies --- 1089.3840 1693.9368 1693.9368
</pre>

<jmol><jmolApplet>
<title>test molecule</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>HS NH3 FREQOPT.LOG</uploadedFileContents>
</jmolApplet></jmol>

=NH3BH3=

== Optimisation ==
'''B3LYP/6-31G(d,p) level'''

[[File:HS summary NH3BH3.PNG]]
<pre>
Item Value Threshold Converged?
Maximum Force 0.000233 0.000450 YES
RMS Force 0.000083 0.000300 YES
Maximum Displacement 0.000981 0.001800 YES
RMS Displacement 0.000369 0.001200 YES
</pre>

Frequency analysis log file [[Media:HS NH3BH3 OPT.LOG]]

<pre>
Low frequencies --- -0.0329 -0.0117 -0.0055 10.3790 10.3868 38.9662
Low frequencies --- 265.6129 634.4283 639.2421
</pre>

<jmol><jmolApplet>
<title>test molecule</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>HS NH3BH3 OPT.LOG</uploadedFileContents>
</jmolApplet></jmol>

==Dissociation energy analysis==
E(NH3)= -56.55777 au

E(BH3)= -26.61532364 au

E(NH3BH3)= -83.22468857 au

ΔE=E(NH3BH3)-[E(NH3)+E(BH3)]= (-83.22469 au) - [(-56.55777 au) + (-26.61532 au)]=-0.05160 au=-135.47580 kJ mol-1

The C-N dative bond can be said that it is weak. This conclusion is based of the fact that O-O is known as a weak bond due to the lone pair repulsion of the closely residing lone pairs. Even a O-O single bond has a bond enthalpy of 146 kJ mol-1. This can be understood from the poorer energy overlap between the sp3 hybrids of B and N to make the single bond, as N is more electronegative than B making their sp3 orbitals much more tightly bound to the N centre. Furthermore, as the s-character of the hybrids involved in bonding decreases, the lesser the extent of stabilisation as the orbitals are loosely bound to the central atoms.

=NI3=

== Optimisation ==
'''B3LYP/GEN level'''

need input file

[[File:HS NI3 summary.PNG]]

<pre>
Item Value Threshold Converged?
Maximum Force 0.000096 0.000450 YES
RMS Force 0.000050 0.000300 YES
Maximum Displacement 0.001084 0.001800 YES
RMS Displacement 0.000616 0.001200 YES
</pre>

Frequency analysis log file [[Media:HS NI3 GENOPT 3V FREQ.LOG]]

<pre>
Low frequencies --- -12.7232 -12.7172 -6.4215 -0.0039 0.0189 0.0620
Low frequencies --- 101.0767 101.0775 147.4581
</pre>

<jmol><jmolApplet>
<title>test molecule</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>HS NI3 GENOPT 3V FREQ.LOG</uploadedFileContents>
<script>frame 1.16</script>
</jmolApplet></jmol>

Bond length of N—I was found to be 2.18404Å. Notice that it is substantially longer than the bond length of N—H (1.01798Å). This is due to the much diffuse orbital of the iodine atom, as it is from period 5.

=Mini Project: Ionic Liquids=

==Optimisation of [N(CH3)4]+==

'''B3LYP/6-31G(d,p) level'''

[[File:HS N complex summary.PNG]]

<pre>
Item Value Threshold Converged?
Maximum Force 0.000067 0.000450 YES
RMS Force 0.000017 0.000300 YES
Maximum Displacement 0.000252 0.001800 YES
RMS Displacement 0.000081 0.001200 YES
</pre>

Frequency analysis log file [[Media:HS NTD FREQOPT MO.LOG]]

<pre>
Low frequencies --- -0.0010 -0.0009 -0.0004 22.7104 22.7104 22.7104
Low frequencies --- 189.1568 292.9980 292.9980
</pre>

<jmol><jmolApplet>
<title>test molecule</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>HS NTD FREQOPT MO.LOG</uploadedFileContents>
</jmolApplet></jmol>

== Optimisation of [P(CH3)4]+==

'''B3LYP/6-31G(d,p) level'''

[[File:HS P complex summary.PNG]]

<pre>
Item Value Threshold Converged?
Maximum Force 0.000048 0.000450 YES
RMS Force 0.000016 0.000300 YES
Maximum Displacement 0.000256 0.001800 YES
RMS Displacement 0.000162 0.001200 YES
</pre>

Frequency analysis log file [[Media:HS PTD 6-31G FREQ OPT TIGHT.LOG]]

<pre>
Low frequencies --- -0.0022 -0.0016 0.0030 50.8737 50.8737 50.8738
Low frequencies --- 187.9725 213.0220 213.0220
</pre>

 Note that the low frequencies list a range of over ±20~30 cm-1. To improve the accuracy of the optimisation, a tight optimisation was done, however yielded the same results. Please refer to Fredrick (Monday demonstrator) for clarification if needed. 

<jmol><jmolApplet>
<title>test molecule</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>HS PTD 6-31G FREQ OPT TIGHT.LOG</uploadedFileContents>
</jmolApplet></jmol>

===Charge analysis of [N(CH3)4]+ and [P(CH3)4]+ complex===

The charge analysis was done with a fixed colour range of 1.667 (green) to -1.060 (red) for both molecules. These values are the charge extremes of the P complex and were used as the values to fix the colour range for both molecules for two reasons: to normalise the charge comparisons for the two molecules by colour, and to maximise the colour gradient between the charges.

[[File:HS ionicliquids charge both.PNG | 800 px |centre| thumb | A charge distrubution analysis for [N(CH3)4]+ (left) and [P(CH3)4]+ (right)]]

{| class="wikitable" border="1"
|+ Charges on atoms for [N(CH3)4]+ complex
!Atom!! Charge
|-
| N || -0.295
|-
| C || -0.485
|-
| H || 0.269
|}

{| class="wikitable" border="1"
|+ Charges on atoms for [P(CH3)4]+ complex
!Atom!! Charge
|-
| P || 1.667
|-
| C || -1.060
|-
| H || 0.298
|}

It can be seen from the diagrams that [P(CH3)4]+ (right) has a significantly greater charge disparity between the central metal ion and the rest of the complex. P has a charge of 1.667 as compared to -0.295 on N. This can be attributed to the electronegativity differences with the directly bonded C atom. C atom's electronegativity (2.5) is less than the electronegativity of N (3.0). This means that the C-N bond will be negatively polarised towards the N atom resulting in the negative charge of the N central atom. In contrast, P has a lower electronegativity (2.2) than C atom, hence resulting in the positive polarisation towards the P centre. Nitrogen has a greater stabilisation ability of negative charges from its energetically low lying orbitals, that P lacks being in period 3.

It is interesting to see that despite C-P has a smaller electronegativity difference compared to C-N, it has a greater polarisation than the [N(CH3)4]+ complex. This is explained by the greater degree of polarisation for the longer M-L bond (metal-ligand) is supported by the longer bond length of P-Me (1.81653Å) than N-Me (1.50956Å).

===Formal Charge analysis for [N(CH3)4]+===

[[File:HS formalcharge Ncomplex.png|200px|right|thumb|Lewis structure for [N(CH3)4]+]]

What does the "formal" positive charge on the N represent in the traditional picture? On what atoms is the positive charge actually located for this cation?

In the traditional picture the formal charge (+) is shown to be singularly located on the N atom. This means that the positive charge on the N atom is solely responsible for the +1 charge of the cation. The formal charges can be understood when a Lewis dot structure is drawn for the complex. The dative bond from the N centre to one of the methyl ligand causes the formal charge to reside on the N.

However from the charge analysis above, we see that the positive charge does not actually reside on the N or the C atom, but rather the H atoms around the periphery of the complex. This indicates that the positive charge is spread around the whole ion. In contrast, the positive charge is concentrated on the P centre for the [P(CH3)4]+ complex. The charge distribution is an important aspect to consider when assessing the viscosity of a liquid crystal. Complexes with a diffuse charge distribution such as [N(CH3)4]+ would interact through attractive coulombic interactions between its counter-ion making it hard for ions to flow past each other smoothly. In contrast, because the positive charge on the [P(CH3)4]+ complex strongly resides on the central atom, the counter-anion is sterically more hindered to interact with the positive centre, therefore making it less viscous.

==LCAO diagram of occupied MOs==

[[File:HS Ligand FOs.png|centre|thumb|500px|LCAO analysis of the MOs from simplified ligand FOs]]

File:HS BH3 summary.PNG

2019-05-17T12:24:44Z

Hs5017: Hs5017 uploaded a new version of File:HS BH3 summary.PNG

2019-05-16T17:22:09Z

Hs5017:

InorganicGaussian 01327311

2019-05-16T17:17:21Z

Hs5017: /* Formal Charge analysis for [N(CH3)4]+ */

=BH3 Molecule=

== Optimisation ==
'''B3LYP/6-31G(d,p) level'''

[[File:HS BH3 summary.PNG]]

<pre>
Item Value Threshold Converged?
Maximum Force 0.000004 0.000450 YES
RMS Force 0.000003 0.000300 YES
Maximum Displacement 0.000017 0.001800 YES
RMS Displacement 0.000011 0.001200 YES
</pre>

Frequency analysis log file [[Media:HARUKA BH3 FREQ 631G DP EDITED.LOG]]

<pre>
Low frequencies --- -1.1800 -1.0028 -0.0055 4.1927 11.0182 11.0637
Low frequencies --- 1162.9912 1213.1792 1213.1819
</pre>

<jmol><jmolApplet>
<title>test molecule</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>HARUKA BH3 FREQ 631G DP EDITED.LOG</uploadedFileContents>
</jmolApplet></jmol>

[File:MODIAGRAM OF BH3]

==Comparison of MO diagrams: Gaussian vs LCAO==

Energy splitting between 2a’ and 1e’ is expected to be larger than the splitting between 1e’ and 1a2’' (ΔE2a’ - 1e’ > ΔE1e’ - 1a2'') from observation of MO diagram produced from LCAO (Linear Combination of Atomic Orbitals). However energy calculations of MOs on Gaussian indicates the opposite: 1e' and 1a2'' has a larger splitting of 0.28474 au compared to 0.16175 au between 2a’ and 1e’.
The AO or FO that is closer to the bonding/anti-bonding MO of concern has a dominant contribution to the MO. Therefore, we would expect from the diagram that the dominant contribution to the 3a1’' MO is from the B2s that is closer to it energetically than the a1’ H3 FO. However, it can be observed from the MO picture taken from gaussian that the lobes are larger on the H atoms (green) rather than B (red).

These two differences given above indicate the flaw of the LCAO theory that the energy positioning of the AOs and FOs are merely qualitative. The energetic similarity or dissimilarity between FOs determine the magnitude of energy splitting between the bonding and antibonding MOs, as well as the dominant contribution to the MO. Therefore qualitative energy positioning of the FOs likewise implies only a vague understanding of these MO properties. Calculations must be done to accurately determine the energy positioning of the orbital.

==IR analysis==

[[File:HS BH3 IR.PNG|600px|centre|thumb|IR spectrum of a BH3 molecule]]

Why do we only see 4 peaks when there are 6 vibrational modes?
From the table above, we see 6 vibrational modes as expected from the 3N-6 rule. However, only 4 peaks are seen on the IR as modes 2 and 3, and 4 and 5 are degenerate which leaves 5 distinguishable vibrational modes. Mode 4 (2582.29 cm-1) is non-existent from the IR spectrum as the symmetric B-H stretch does not result in a dipole change as seen from the displacement vectors.

[[File:HS BH3 IR table.PNG|centre|thumb|Vibrational frequencies and intensities of a BH3 molecule]]

[[File:HS BH3 mode4.PNG|centre|thumb|IR inactive symmetric BH stretching mode]]

=NH3=

== Optimisation ==
'''B3LYP/6-31G(d,p) level'''

[[File:HS NH3 summary.PNG]]

<pre>
Item Value Threshold Converged?
Maximum Force 0.000006 0.000450 YES
RMS Force 0.000004 0.000300 YES
Maximum Displacement 0.000016 0.001800 YES
RMS Displacement 0.000011 0.001200 YES
</pre>

Frequency analysis log file [[Media:HS NH3 FREQOPT.LOG]]

<pre>
Low frequencies --- -0.0137 -0.0027 0.0007 7.0783 8.0932 8.0937
Low frequencies --- 1089.3840 1693.9368 1693.9368
</pre>

<jmol><jmolApplet>
<title>test molecule</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>HS NH3 FREQOPT.LOG</uploadedFileContents>
</jmolApplet></jmol>

=NH3BH3=

== Optimisation ==
'''B3LYP/6-31G(d,p) level'''

[[File:HS summary NH3BH3.PNG]]
<pre>
Item Value Threshold Converged?
Maximum Force 0.000233 0.000450 YES
RMS Force 0.000083 0.000300 YES
Maximum Displacement 0.000981 0.001800 YES
RMS Displacement 0.000369 0.001200 YES
</pre>

Frequency analysis log file [[Media:HS NH3BH3 OPT.LOG]]

<pre>
Low frequencies --- -0.0329 -0.0117 -0.0055 10.3790 10.3868 38.9662
Low frequencies --- 265.6129 634.4283 639.2421
</pre>

<jmol><jmolApplet>
<title>test molecule</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>HS NH3BH3 OPT.LOG</uploadedFileContents>
</jmolApplet></jmol>

==Dissociation energy analysis==
E(NH3)= -56.55777 au

E(BH3)= -26.61532364 au

E(NH3BH3)= -83.22468857 au

ΔE=E(NH3BH3)-[E(NH3)+E(BH3)]= (-83.22469 au) - [(-56.55777 au) + (-26.61532 au)]=-0.05160 au=-135.47580 kJ mol-1

The C-N dative bond can be said that it is weak. This conclusion is based of the fact that O-O is known as a weak bond due to the lone pair repulsion of the closely residing lone pairs. Even a O-O single bond has a bond enthalpy of 146 kJ mol-1. This can be understood from the poorer energy overlap between the sp3 hybrids of B and N to make the single bond, as N is more electronegative than B making their sp3 orbitals much more tightly bound to the N centre. Furthermore, as the s-character of the hybrids involved in bonding decreases, the lesser the extent of stabilisation as the orbitals are loosely bound to the central atoms.

=NI3=

== Optimisation ==
'''B3LYP/6-31G(d,p) level'''

[[File:HS NI3 summary.PNG]]

<pre>
Item Value Threshold Converged?
Maximum Force 0.000096 0.000450 YES
RMS Force 0.000050 0.000300 YES
Maximum Displacement 0.001084 0.001800 YES
RMS Displacement 0.000616 0.001200 YES
</pre>

Frequency analysis log file [[Media:HS NI3 GENOPT 3V FREQ.LOG]]

<pre>
Low frequencies --- -12.7232 -12.7172 -6.4215 -0.0039 0.0189 0.0620
Low frequencies --- 101.0767 101.0775 147.4581
</pre>

<jmol><jmolApplet>
<title>test molecule</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>HS NI3 GENOPT 3V FREQ.LOG</uploadedFileContents>
<script>frame 1.16</script>
</jmolApplet></jmol>

Bond length of N—I was found to be 2.18404Å. Notice that it is substantially longer than the bond length of N—H (1.01798Å). This is due to the much diffuse orbital of the iodine atom, as it is from period 5.

=Mini Project: Ionic Liquids=

==Optimisation of [N(CH3)4]+==

'''B3LYP/6-31G(d,p) level'''

[[File:HS N complex summary.PNG]]

<pre>
Item Value Threshold Converged?
Maximum Force 0.000067 0.000450 YES
RMS Force 0.000017 0.000300 YES
Maximum Displacement 0.000252 0.001800 YES
RMS Displacement 0.000081 0.001200 YES
</pre>

Frequency analysis log file [[Media:HS NTD FREQOPT MO.LOG]]

<pre>
Low frequencies --- -0.0010 -0.0009 -0.0004 22.7104 22.7104 22.7104
Low frequencies --- 189.1568 292.9980 292.9980
</pre>

<jmol><jmolApplet>
<title>test molecule</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>HS NTD FREQOPT MO.LOG</uploadedFileContents>
</jmolApplet></jmol>

== Optimisation of [P(CH3)4]+==

'''B3LYP/6-31G(d,p) level'''

[[File:HS P complex summary.PNG]]

<pre>
Item Value Threshold Converged?
Maximum Force 0.000048 0.000450 YES
RMS Force 0.000016 0.000300 YES
Maximum Displacement 0.000256 0.001800 YES
RMS Displacement 0.000162 0.001200 YES
</pre>

Frequency analysis log file [[Media:HS PTD 6-31G FREQ OPT TIGHT.LOG]]

<pre>
Low frequencies --- -0.0022 -0.0016 0.0030 50.8737 50.8737 50.8738
Low frequencies --- 187.9725 213.0220 213.0220
</pre>

 Note that the low frequencies list a range of over ±20~30 cm-1. To improve the accuracy of the optimisation, a tight optimisation was done, however yielded the same results. Please refer to Fredrick (Monday demonstrator) for clarification if needed. 

<jmol><jmolApplet>
<title>test molecule</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>HS PTD 6-31G FREQ OPT TIGHT.LOG</uploadedFileContents>
</jmolApplet></jmol>

===Charge analysis of [N(CH3)4]+ and [P(CH3)4]+ complex===

The charge analysis was done with a fixed colour range of 1.667 (green) to -1.060 (red) for both molecules. These values are the charge extremes of the P complex and were used as the values to fix the colour range for both molecules for two reasons: to normalise the charge comparisons for the two molecules by colour, and to maximise the colour gradient between the charges.

[[File:HS ionicliquids charge both.PNG | 800 px |centre| thumb | A charge distrubution analysis for [N(CH3)4]+ (left) and [P(CH3)4]+ (right)]]

{| class="wikitable" border="1"
|+ Charges on atoms for [N(CH3)4]+ complex
!Atom!! Charge
|-
| N || -0.295
|-
| C || -0.485
|-
| H || 0.269
|}

{| class="wikitable" border="1"
|+ Charges on atoms for [P(CH3)4]+ complex
!Atom!! Charge
|-
| P || 1.667
|-
| C || -1.060
|-
| H || 0.298
|}

It can be seen from the diagrams that [P(CH3)4]+ (right) has a significantly greater charge disparity between the central metal ion and the rest of the complex. P has a charge of 1.667 as compared to -0.295 on N. This can be attributed to the electronegativity differences with the directly bonded C atom. C atom's electronegativity (2.5) is less than the electronegativity of N (3.0). This means that the C-N bond will be negatively polarised towards the N atom resulting in the negative charge of the N central atom. In contrast, P has a lower electronegativity (2.2) than C atom, hence resulting in the positive polarisation towards the P centre. Nitrogen has a greater stabilisation ability of negative charges from its energetically low lying orbitals, that P lacks being in period 3.

It is interesting to see that despite C-P has a smaller electronegativity difference compared to C-N, it has a greater polarisation than the [N(CH3)4]+ complex. This is explained by the greater degree of polarisation for the longer M-L bond (metal-ligand) is supported by the longer bond length of P-Me (1.81653Å) than N-Me (1.50956Å).

===Formal Charge analysis for [N(CH3)4]+===
What does the "formal" positive charge on the N represent in the traditional picture? On what atoms is the positive charge actually located for this cation?

[[File:HS formalcharge Ncomplex.png|200px|right|thumb|Lewis structure for [N(CH3)4]+]]

In the traditional picture the formal charge (+) is shown to be singularly located on the N atom. This means that the positive charge on the N atom is solely responsible for the +1 charge of the cation. The formal charges can be understood when a Lewis dot structure is drawn for the complex. The dative bond from the N centre to one of the methyl ligand causes the formal charge to reside on the N.

However from the charge analysis above, we see that the positive charge does not actually reside on the N or the C atom, but rather the H atoms around the periphery of the complex. This indicates that the positive charge is spread around the whole ion. In contrast, the positive charge is concentrated on the P centre for the [P(CH3)4]+ complex. The charge distribution is an important aspect to consider when assessing the viscosity of a liquid crystal. Complexes with a diffuse charge distribution such as [N(CH3)4]+ would interact through attractive coulombic interactions between its counter-ion making it hard for ions to flow past each other smoothly. In contrast, because the positive charge on the [P(CH3)4]+ complex strongly resides on the central atom, the counter-anion is sterically more hindered to interact with the positive centre, therefore making it less viscous.