The dynamic behavior of the exohedral transition metal complexes of $$\mathrm{B}_{40}: {\upeta}^{6}$$ B 40 : η 6 - and $${\upeta}^{7}$$ η 7 - $$\mathrm{B}_{40}\hbox {Cr(CO)}_{3}$$ B 40 Cr(CO) 3 and $$\hbox {Cr(CO)}_{3}$$ Cr(CO) 3 - $${\upeta}^{7}$$ η 7 - $$\hbox {B}_{40}$$ B 40 - $${\upeta}^{7}$$ η 7 - $$\hbox {Cr(CO)}_{3}$$ Cr(CO) 3