Table 4.

Selected axial pairwise steric exchange energies^a ΔE_X from NBO analysis for interactions between U 6p/6s and ligand valence NLMOs for the dominant interactions shown in Fig. 2a

	NLMO	ΔEX
Compound	Steric Interactions	(1)	(2)	(3)
${\left[\mathrm{O}-{\mathrm{U}}^{\mathrm{VI}}-\mathrm{O}\right]}^{2+}$	U 6pσ ↔ O 2ps	37.6	–	–
(1) :O ≡ U ≡ O:b	U 6s ↔ O 2ps	17.4	–	–
(2) O≣U ≡ O:	U 6pσ ↔ O 2sp	12.3	–	–
(3) :O ≡ U≣O	U 6s ↔ O 2sp	0.9	–	–
	U 6pπ ↔ O 2πd	3.5	–	–
O−UVI−C	U 6pσ ↔ C 2ps	9.6	8.0	9.1
(1) :C ≡ U ≡ O:c	U 6s ↔ C 2ps	5.6	4.8	5.6
(2) C≣U ≡ O:	U 6pσ ↔ C 2sp	10.2	12.8	12.7
(3) :C ≡U≣O	U 6s ↔ C 2sp	3.3	4.0	2.9
	U 6pπ ↔ C 2πd	0.3	0.3	0.3
	U 6pσ ↔ O 2ps	36.2	36.4	40.0
	U 6s ↔ O 2ps	12.2	12.3	12.9
	U 6pσ ↔ O 2sp	9.7	9.3	1.5
	U 6s ↔ O 2sp	0.2	0.1	0.5
	U 6pπ ↔ C 2πd	3.6	3.6	3.6
${\left({\mathrm{R}}^{\mathrm{a}}\right)}_3{\mathrm{N}}_{\mathrm{amine}}-{\mathrm{U}}^{\mathrm{VI}}-{\mathrm{N}}_{\mathrm{nitride}}$	U 6pσ ↔ Nnitride 2ps	34.0	32.3	23.5
(1) ${\left({\mathrm{R}}^{\mathrm{a}}\right)}_3\mathrm{N}$: U ≡ N:c	U 6s ↔ Nnitride 2ps	11.5	12.8	8.5
(2) ${\left({\mathrm{R}}^{\mathrm{a}}\right)}_3\mathrm{N}-\mathrm{U}\equiv \mathrm{N}$:	U 6pσ ↔ Nnitride 2sp	9.1	10.0	18.5
(3) ${\left({\mathrm{R}}^{\mathrm{a}}\right)}_3\mathrm{N}$: U≣N	U 6s ↔ Nnitride 2ps	1.3	0.4	4.3
	U 6pπ ↔ Nnitride 2πd	2.3	2.3	2.3
	U 6pσ ↔ Namine 2sp	8.4	8.6	8.8
	U 6s ↔ Namine 2sp	4.2	3.3	4.1

DFT/B3LYP calculations. Linear structures for the triatomics.

^aEnergies in kcal mol⁻¹.

^bThe dominant structure for uranyl is O ≡ U ≡ O⁺².

^cThis is not a major resonance structure for CUO or ${\left({\mathrm{R}}^{\mathrm{a}}\right)}_3\mathrm{N}-{\mathrm{U}}^{\mathrm{VI}}-\mathrm{N}$; it was included for direct comparison with O≡U≡O⁺².

^dData for one of the two equivalent U 6p_π ↔ ligand_π pairwise steric energies is shown.