Which carbenes with only carbon and hydrogen substituents generally have triplet ground states?

In chemistry, carbenes that possess only carbon or hydrogen atoms attached to the divalent carbene carbon atom generally exhibit triplet ground states.1

This occurs because, in these specific carbenes, the triplet state is typically the most stable form, whereas carbenes with nitrogen, oxygen, or halogen substituents usually favor singlet ground states. Notable examples of carbenes with triplet ground states include:1

• Methylene (H—C—H)1
• Phenylmethylene (C₆H₅—C—H)1
• Diphenylmethylene (C₆H₅—C—C₆H₅)1
• Propargylene (HC≡C—C—H)1

In the triplet state, the two nonbonding electrons of the carbene occupy different orbitals and possess parallel spins, which results in a magnetic moment. This configuration is distinct from the singlet state, where the two nonbonding electrons occupy the same orbital with paired spins.1