[vhpk]

Hi everyone,
I've got a question related to two compounds above.
As we know, BeH₂ and BH₃ are both electron-deficient and they form polymeric form with 3-centre-2-electron bond. However, in the gas phase, we could find BeH₂ molecule but can't find BH₃ but only B₂H₆. So my question is why can BeH₂ exist as electron deficient compound in gas phase while BH₃ can't ?
Can anyone help me with this? Thank you for your help

[vhpk]

Is there any help? I get stuck with this problem